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2004 Elsevier Ltd. All rights reserved.

Energy 30 (2005) 41–71

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An evaluation of the appropriateness of using overallthermal transfer value (OTTV) to regulate envelope energy

performance of air-conditioned buildings

F.W.H Yik �, K.S.Y Wan

Department of Building Services Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, HongKong SAR, China

Received 18 March 2003

Abstract

This paper inquires into whether overall thermal transfer value (OTTV) is an appropriate buildingenvelope energy performance index for use in regulatory control. First, a historical review of the use ofOTTV in American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) Stan-dard 90 is presented, followed by a review of more recent work on its further development and appli-cation. The major deficiencies of OTTV are then discussed, and simulation study results meant tohighlight the impacts of such deficiencies are presented. The study embraced air-conditioned office build-ings and air-conditioned high-rise residential buildings in Hong Kong. Results of this study clearly showthat the OTTV calculated with the use of pre-calculated coefficients may not truly reflect the thermal per-formance of a building envelope. Therefore, a second thought should be given to the use of OTTV inbuilding energy codes.# 2004 Elsevier Ltd. All rights reserved.

1. Introduction

In the early 1970s, the oil crisis awakened industrialised countries to the fact that economicdevelopment can be highly vulnerable to instabilities in imports of energy resources. In additionto reducing reliance on imported fuels, environmental protection and sustainable developmentare nowadays the major impetus of energy conservation initiatives. Since buildings are a domi-nant energy consumer in modern cities, energy use in buildings has become a policy issue inmany regimes worldwide [1–3].

F.W.H. Yik, K.S.Y. Wan / Energy 30 (2005) 41–7142

The American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE)originated in 1975 the use of overall thermal transfer value (OTTV) as a thermal performanceindex for the envelope of air-conditioned buildings [4]. Thereafter, many countries includedassessment of OTTV in their building energy codes [5–11]. OTTV was considered a better per-formance index than thermal transmittance (U-value) because it takes into account the impactof direct sun on the envelope of mechanically cooled buildings [12]. With all other things beingequal, lowering the OTTV of a building should lead to less envelope heat gain and thus lesscooling energy use. Despite that ASHRAE has ceased using OTTV in its Standard 90 [13] since1989, use of OTTV in building energy codes continues outside the US, including those launchedin the 1990s [11,14]. Much effort has also been made to enhance the method for OTTV calcu-lation in Asia, including in Singapore and Hong Kong. The divergence between the US and theAsian countries in the use of OTTV warrants a more thorough evaluation of the appropriate-ness of using OTTV to regulate energy performance of building envelopes.

2. The introduction and abandonment of OTTV in ASHRAE Standard 90

Using OTTV to quantify the energy performance of envelopes of air-conditioned buildingswas first introduced by ASHRAE in Standard 90-75 [4], which was revised later into Standard90A-1980 [15]. In the latter, OTTV was explicitly defined as ‘the maximum thermal transfer per-missible into the building through its walls or roof, due to solar heat gain and outdoor–indoortemperature difference’, to be determined using Eqs. (1) and (2) below. The compliance criterionfor the OTTV of roofs was a constant value of 26.8 W/m2, but that for OTTV of walls wouldvary with the latitude of the building site.

OTTVw ¼ ðUw � Aw � TDEQÞ þ ðAf � SF � SCÞ þ ðUf � Af � DTÞðAw þ AfÞ

(1)

OTTVR ¼ ðUR � AR � TDEQÞ þ ð434:7 � AS � SCÞ þ ðUS � AS � DTÞðAR þ ASÞ

(2)

where OTTVw is OTTV of a wall, W/m2; OTTVR is OTTV of a roof (�26.8), W/m2; Uw, Uf,UR and US are thermal transmittance of the opaque part of a wall, a fenestration, the opaquepart of a roof and a skylight, respectively, W/m2 v

C; Aw, Af, AR and AS are area of the opaquepart of a wall, a fenestration, the opaque part of a roof and a skylight, respectively, m2; TDEQ

is equivalent temperature difference for the opaque part of a wall or a roof,vC; SC is shading

coefficient of a fenestration or a skylight; SF is solar factor, W/m2; DT is temperature differencebetween exterior and interior design conditions,

vC.

As Eqs. (1) and (2) show, OTTV represents the total of three major components of envelopeheat gain: conduction through opaque parts of walls and roofs; solar transmission through win-dows and skylights; and heat transfer through windows and skylights due to outdoor–indoortemperature difference. This resembles how envelope heat gains are determined in the CLTD/CLF design cooling load calculation method first introduced in 1977 by ASHRAE [16].In the review for updating Standard 90A-80, the exterior envelope criteria were considered

too simplistic to properly account for the interactions of the envelope with the complex energy

43F.W.H. Yik, K.S.Y. Wan / Energy 30 (2005) 41–71

flow within commercial buildings [17]. One major criticism was about the use of the equivalenttemperature difference (TDEQ) in the OTTV equation to account for the thermal storage effectsof envelope elements [18]. The impact of the building envelope on cooling energy use is depen-dent on climate, building operation schedule, and three characteristics of the perimeter wall: theorientation, thickness and position of the insulation relative to the mass [19]. Furthermore, thecontribution of conduction to the cooling energy is not as consistent as other heat gains, such assolar and lighting, and it can be very small and can be either positive or negative. Conductionloss can occur during some of the cooling hours for some buildings located at high latitudes[17]. The envelope performance requirements were also considered restrictive, as the envelopeand the HVAC systems were treated independently, and the few number of factors consideredlimited design flexibility [20–24].The use of OTTV was finally abandoned since ASHRAE launched Standard 90.1-1989—

Energy Efficient Design of New Buildings Except New Low-rise Residential Buildings [13].Instead, the prescriptive criteria put limits to the percentage of fenestration relative to the grossexternal wall area; the thermal transmittance (U-value) of envelope elements and fabric elementsseparating conditioned and unconditioned spaces; and the thermal resistances of slabs-on-gradeand walls below grade. The permissible limits were dependent on the local weather conditions;shading coefficient of fenestration; characteristics of shading device; and heat capacitance ofwall and position of insulation [13,24,25].Alternative compliance paths had also been introduced in Standard 90.1, which included per-

formance-based criteria that were based on the cumulative heating and cooling energy flux toallow trade-offs among different envelope assemblies, and the Energy Cost Budget approach,which offered even greater flexibility for meeting the Standard requirements. In 1992, the USEnergy Policy and Conservation Act required every state in the US to certify, before October1994, its energy codes would meet or exceed the requirements of the ASHRAE Standard 90.1-1989 [3,26,27]. Ten years later, a new version of ASHRAE Standard 90.1-1999 [28] was issued.Since then, Standard 90.1 will be re-issued on regular three-year cycles, for incorporatingchanges resulting from continuous maintenance proposals from the public. The latest version ofStandard 90.1 has been published in 2001 [29].

3. Use of OTTV for regulating building energy performance in Asian countries

Among Asian countries, Singapore was the first to have regulatory control over the OTTV ofexternal walls of air-conditioned buildings (since 1979). Details of the control were stipulated ina Singaporean Standard [5,30]. Four years later, the Handbook was revised [5], which includedthe introduction of a new standard on OTTV for roofs with skylights and a new method fordetermining the coefficients in the OTTV equation to account for the effects of external shadingdevices at exterior walls. Turiel et al. [31,32] reviewed the OTTV standard and showed that theterm for solar gain through windows in the OTTV formulation understated, but the conductionterms for walls and windows exaggerated the rates of heat transfer through the respective paths.They recommended the OTTV formulation be revised into a single term equation, accountingonly for the solar heat gain.


Later, Chou and Lee [33,34] reviewed both the OTTV formulation in the Standard as well asthat proposed by Turiel et al. In their attempt to derive an OTTV equation for Singaporeanbuildings, they defined OTTV as ‘the annual heat gain of the air-conditioned spaces in a build-ing from the envelope during both air-conditioned and non-air-conditioned periods, averagedover the total air-conditioned hours throughout the year and normalised by the envelope areaenclosing such spaces’, as Eq. (3) depicts. This was based on the consideration that the heatgain would ultimately contribute to the cooling load on air-conditioning systems [35].

OTTV ¼ Total heat gain through building envelope

Total air-conditioned hours� Envelope areaðW=m2Þ (3)

Note that Eq. (3) is for evaluation of OTTV based on predicted heat gains from detailedcomputer simulations. The OTTV predictions can then be used in regression analyses or othermethods to evaluate the coefficients (TDEQ, SF and DT) in an OTTV equation similar to Eqs. (1)and (2) for practical applications. However, the meanings of OTTV and the coefficients TDEQ,SF and DT are now different from those defined originally in ASHRAE Standard 90-75. Chouand Lee’s OTTV definition has become widely accepted, but still not universal. They did findfrom their results that the Standard formulation required revision, but also pointed out that theequation proposed by Turiel et al. tended to overemphasise the effects of measures for reducingsolar heat gains.In July 2000, the envelope and roof thermal transfer values (ETTV and RTTV) were intro-

duced to replace the respective OTTVs in the earlier version of the Singaporean Standard [36].The new ETTV and RTTV formulae are basically modified versions of the original OTTV for-mulation based on more rigorous computer simulations but are assigned with new names to dif-ferentiate them from the original OTTV equations.Similar to Singapore, Malaysia also adopted in 1987 OTTV as a building envelope thermal

performance index in its energy standard for new commercial buildings. Having found that thesolar absorptivity of external wall surfaces would affect the chiller load by 8–9%, wall surfaceabsorptivity was included as a multiplicative factor in the term for heat conduction throughopaque walls and roofs in the OTTV equation [6,7]. In 1980s and early 1990s, some other Asiancountries, including the Philippines [8], Thailand [9,10] and Indonesia [11], have also imple-mented OTTV control over the energy performance of new buildings.The Hong Kong Government started to consider building energy conservation in the late

1980s and commissioned a consultant to develop a method of control over the OTTV of com-mercial and hotel buildings. The consultant’s report [37] emphasised that the OTTV developedwas not meant to be a measure of the maximum rate of heat transfer across the building envel-ope nor an indication of the size required of the air-conditioning plant. Despite the resemblancebetween the OTTV equations and those in ASHRAE Standard 90-75, they were meant to relatecooling energy use to envelope characteristics (with all other influential factors fixed).The study was based on assumed cooling periods in the year, which embraced the daylight

hours for 5.5 days per week from April to October. Finding that the conduction heat gainthrough fenestration areas had insignificant effects, the proposed OTTV equations comprisedonly two terms that account for the building cooling energy use contributed by conduction heatgain through opaque parts of walls and roofs, and by solar gain through fenestrations. Therecommended maximum OTTV for commercial buildings was: for walls, 16 W/m2 for no

Table1

Summary

ofassumptionsmadebyresearchersin

thedevelopmentofOTTVequationforcommercialbuildingsin

HongKong

Shillinglaw&

Chen

[50]

JRP[24]

Lam

etal.[53-55]

Chan[59]

Chow&Yu[61,62]

Basisforquantifi-

cationofOTTV

Mean

energy-gain

Coolingenergyuse

Instantaneous

envelopeheatgain

Netpositive

envelopeheatgain

Chilledwater

load(BLAST)

Space

coolingload

Annualenergy

consumption(or

chilledwaterload)

(TRACE600)

Annualenvelopheat

gain

(TRACE600)

Durationofanalysis

periodin

theyear

March–Novem

ber

basedon12hdays

(0700–1800)

Totaldaylight

hoursfrom

April

toOctober

Heatgain:8760,

3650,5880,2450,

4416&1840h

Forallhourswitha

netpositiveenvelope

heatgain,which

varied

from

the

buildingto

another

Noinform

ation

1920h

Commercial

buildings:2190h

Space

coolingload:

5880,2450,4416

&1840h

Hotel:2675h

Buildinglayout

Basedonseveral

existingbuildings

Amodelbuilding

withsquare

floor

layout,1400m2per

floor,121m2wall

areawithwindow

towallratio0.3on

each

ofthefour

principalorientations

(N,E,S&W)

A40-storeybuilding

withsquare

floor

layout,1225m2

per

floor

A40-storeybuilding

withsquare

floor

layout,1296m2

per

floor

Asinglecompartment

ofavolumeranging

from

4,000to

40,000

m3

Indoor/

Outdoor

temperature

Indoor:25.2

vC

Indoor:Commercial

building:25.5

vCfor

summer,20.5

vCfor

winter;Hotel:22.5

vC

forsummer,20.5

vC

forwinter

Indoor:25.5

vC

forcooling,21

vC

forheating

Indoor:25.5

vCfor

cooling,22

vCfor

heating

Indoor:25

vC,

23

vC,21

vC

(continuedonnextpage)


Table1(continued)

Shillinglaw&

Chen

[50]

JRP[24]

Lam

etal.[53-55]

Chan[59]

Chow&Yu[61,62]

Outdoor:28.1

vC,

beingthemeanvalue

for1500heach

year

from

1968to

1977

Outdoor:33

vC

forsummer,7

vC

forwinter

Operatinghours

Commercialbuildings:

Weekday

08:00to

19:00

08:00to

18:00

08:00to

18:00

08:00to

18:00

Saturday

08:00to

13:00

08:00to

13:00

08:00to

13:00

09:00to

13:00

SundayorPublic

Holiday

Off

Off

Off

Off

TotalAChours:2800

TotalAChours:2780

TotalAChours:2780

Hotel:24hours

Internalloads

Lighting,35W/m

2Lighting,20W/m2

Lighting,20W/m2

Nointernalload

Equipment,35W/m

2Equipment,15W/m

2Equipment,5W/m2

Noventilationload

Occupancy

density,

14.28m2/person

Occupancy

density,

3.7m2/person

Occupancy

density,

7m2/person

Outsideair,

9,4l/s/person

Outsideair,

3.3l/s/person

Outsideair,

7l/s/person

Infiltration,0.6ach

Air-sideACsystem

VAVreheatsystem

VAVsystem

with

40%

minimum

turn

downratio

Fancoilsystem

Methodofheatgain

orcoolingload

predication

DOE-2.1D

Transfer

Function

Method

DOE-2

BLAST

DOE-2

TRACE600



provisions for daylight and 23 W/m2 for including provisions for daylight; and for roofs, 11 W/m2. For hotel buildings, the maximum OTTV was 30 W/m2 for walls and 17 W/m2 for roofs.The regulatory control over the OTTV of commercial and hotel buildings that has been

enforced in Hong Kong since July 1995 is based largely on the consultancy study. The methodand data for OTTV calculation, and the compliance requirements, are stipulated in a Code ofPractice [14], but the control figures (35 W/m2 for the tower block and 80 W/m2 for thepodium portion of a building) were less stringent. These figures have been tightened in 2000, to30 and 70 W/m2, respectively [38].Similar to ASHRAE Standard 90-75 and 90A-80, the building energy efficiency regulation of

Hong Kong has been criticised to be limited in scope and restrictive, as it only controls thebuilding envelope design and does not consider other aspects of building design, such as opti-misation of and trade-offs between the performance of the building and the services systemdesigns [39,40]. Local building professionals commented that the use of the OTTV method islimiting freedom in architectural design and restricting innovations [41].

4. Research studies into OTTV in Hong Kong

Since the Hong Kong Government started considering regulating energy performance ofbuilding envelope designs, many local researchers studied into the OTTV of commercial build-ings in Hong Kong. Most studies made reference to the ASHRAE and the Singaporean Stand-ards but re-evaluated the coefficients in the OTTV equations on the basis of local weatherconditions [42–44]. Table 1 summaries the key assumptions made by various local researchers intheir studies, including the basis upon which OTTV was quantified and the characteristics of themodel buildings they used. The latter includes the assumed building dimensions, the operatinghours of the air-conditioning system, the indoor temperature set point and the internal loads.The simulation tools used are also shown in the table.Shillinglaw and Chen [42] developed an OTTV calculation method and a grading system for

OTTV assessment. Their equation embraced the same three major envelope heat gain terms asin ASHRAE’s. The derivation, however, was based on mean energy gain through the envelope,under the average weather condition over a 12-hour daily time frame (07:00–18:00) from Marchto November. The choice of the diurnal and seasonal time frames is important as they have sig-nificant impacts on the values of the coefficients in the OTTV equation [44].Lam et al. [45–47] studied the use of two approaches to develop OTTV equations for Hong

Kong. In the first, OTTV was evaluated based on heat gains and the results were used to deter-mine TDEQ for opaque walls and roofs. In the second, the ‘OTTV’ actually represented theannual total cooling load due to the three heat gain components. The simulation program DoE-2 [48] was used to predict the heat gains and the resultant cooling loads. The cooling periodsstudied include a 10-hour day and a 24-hour day, each for a 5.5-day week over a 9-monthperiod (March to November). They recommended that OTTV calculation should be based onheat gains while TDEQ, DT and SF should be evaluated based on fixed air-conditioning sched-ules for avoiding the need for different sets of TDEQ, DT and SF data [47].Chow and Chan [49–52] also used DoE-2 to predict heat gains but took a different approach

to establish OTTV equations. They argued that due to the weather changes among the four


seasons in Hong Kong, it would be inappropriate to base the calculation of OTTV on the totalheat gain throughout the year. Instead of from outdoor into indoor, heat transmission throughthe envelope may reverse in direction during certain air-conditioned hours in the year. Theyproposed to determine the total envelope heat gain by summing algebraically the hourly heatgains from all envelope elements over just those hours where the total envelope heat gain in thehour remained positive. OTTV was then determined by averaging the total envelope heat gainover such hours, and the OTTVs determined for a range of building models were used in aregression analysis to yield TDEQ, SF, and DT as coefficients in the OTTV equations.Chow and Yu [53,54] also developed OTTV equations based on the heat gain, chilled water

load and annual energy consumption, and considered the method proposed by Chow and Chan[50] the most appropriate. Their study was based on a rectangular shape atrium hall and cov-ered a range of set point indoor temperatures, including 25, 23 and 21

vC. They opined that the

use of OTTV alone would not ensure energy efficient and cost effective building designs; the airleakage, the selection of heating, ventilating and air-conditioning (HVAC) systems and equip-ment, building energy management and other energy saving options, such as daylighting andsolar heating, should also be considered.Hui [40] also made similar comments and proposed to apply prescriptive requirements to

building design and components, and system performance requirements to building services sys-tems in assessing their energy performance. He also developed a general methodology for devel-oping OTTV equation that incorporates building energy simulation and multiple regressiontechniques, but suggested to evaluate the solar factor separately by means of ASHRAE’s orother methods.Recently, the Hong Kong Government has developed a performance-based energy code [55]

intended to provide an alternative compliance route to the five existing prescriptive codes; oneon OTTV and four on building services installations. However, compliance with the OTTVrequirement remained a basic requirement for which no trade-offs of performance with servicessystems would be allowed.

5. Adequacy of OTTV as an envelope energy performance index

Several modifications to the original definition of OTTV (which was based on peak heatgains) have been proposed, such as basing it on annual heat gains, annual cooling loads orannual air-conditioning energy use, all with the objective to obtain a parameter that can reflectthe impact of envelope characteristics on the energy use for air-conditioning. For buildings inHong Kong or places with similar climate, heat transmission through walls and windows cantake place in opposite directions at different times, which makes it difficult to derive a consistentenvelope energy performance index. In parts of buildings where there are high internal loads orsolar gains, envelope conduction loss can help reduce cooling load on the air-conditioning sys-tem, and thus lower air-conditioning energy use. For intermittently air-conditioned buildings,envelope heat loss may also help reduce the pull-down load during the start-up period,particularly after a prolonged shut-down period (e.g. a weekend). Therefore, a well insulatedenvelope with low OTTV may not necessarily mean reduced energy use. To circumvent this


problem, proposals have been made to focus only on several hotter months in the year or toignore periods with net heat losses.Besides the above problem, all the methods have more fundamental drawbacks due to the

assumptions made implicitly, which include:

1. The heat gains from each wall, roof, window or skylight could be determined independentlyfrom pre-calculated values of equivalent temperature difference (TDEQ) and solar factor (SF).

2. The same value of TDEQ would apply to walls or roofs of the same construction and facingthe same direction, and likewise the same value of SF would apply to windows or skylightsat the same orientation, irrespective of the room dimensions and configurations.

3. The OTTV of the entire envelope could be determined from the OTTVs of different walls,windows, roofs and skylights in the envelope, as their area-weighted average.

4. The OTTV would reflect the impact of the envelope on the energy use for air-conditioningthe building.

Assumption 4 above is obviously linked to the goal of OTTV controls over buildings.Assumptions 1 to 3 serve to simplify the calculation method such that OTTV for buildings canbe evaluated based on a limited set of pre-calculated TDEQ and SF data. These assumptionsresemble those made for simplifying the design cooling load calculation methods for buildings.In such methods, the heat gains from different envelope components and the resultant coolingload are determined using pre-calculated values for the influential parameters, such as the walland roof conduction transfer function coefficients and the room weighting factors for use withthe transfer function method [16,56,57]. Similar applies to the periodic wall and roof conductiontime series and the periodic room weighting factors in the newly introduced radiant time series(RTS) design cooling load calculation method [56], and the cooling load temperature difference(CLTD) and solar cooling load (SCL) in the simplified method [58] that the RTS methodreplaced.However, it has been well recognised that significant discrepancies between predictions and

measurements could arise due to the use of pre-calculated room weighting factors for coolingload estimation. Efforts had been made to provide weighting factors [59–61] for a range of zonetype groups, and conduction transfer function coefficients for wall and roof groups [62], suchthat cooling load estimation may still be based on pre-calculated parameters without significantloss in accuracy, provided the right set of parameters is selected for the calculation. The resultsof the ASHRAE Research Project 472 [61] showed that when the zone geometry (ratio of roomwidth to depth), zone height, type of internal partitions or the number of external walls enclos-ing an air-conditioned zone was changed, the weighting factors for a different zone group mayneed to be selected, which highlights the significance of these parameters on the thermalresponse of an air-conditioned zone. For improving prediction accuracy, procedures for calcu-lating custom weighting factors have been incorporated into the building energy simulation pro-gram DoE-2 [63].The more recently conducted study for verification of the prediction accuracy of the newly

introduced RTS method [64,65] included parametric studies that embraced variations in thezone geometry and glazed portions of external walls. However, consistent with the aim of thestudy, the periodic room weighting factors used in the simulation were generated specifically for


each modelled zone. The results, therefore, did not unveil the impact of these factors on the pre-dicted cooling load had just one single set of weighting factors been used. Nonetheless, themessage, as re-iterated by McQuiston et al. [57], is clear: the zone geometry and constructioncharacteristics, including those of the walls, roofs or ceilings, floors and internal masses areinfluential, whilst use of data categorised by zone types could lead to less than satisfactoryresults. Therefore, McQuiston et al. advised users of the RTS method to generate custom radi-ant time factors for the specific zone in question using a computer program that implements thedetailed heat balance model.It is worth noting that the ASHRAE Research Project 472 [59–61] for enhancing the design

cooling load calculation methods in the ASHRAE Handbook [59–62] were conducted in the late1980s, which coincided with the time of upgrading Standard 90A-80 to Standard 90.1-1989.Therefore, it is reasonable to believe that these research studies had influenced, to certain extent,the decision of abandoning the use of OTTV in Standard 90.1.

6. Evaluation of the influence of zone arrangement on the heat gain through the building envelopeof an office building

The need to take detailed account of the zone geometry and characteristics of envelope com-ponents, partitions and internal heat sources applies to both building cooling load estimationand OTTV calculation. Assumptions 1 to 3 above are, strictly speaking, invalid. The simulationstudies described below were meant to unveil the significance of the impacts of envelope con-struction and room configuration on envelope heat gains, which are the basis of OTTV calcu-lation.

6.1. Basis of the case studies

A 40-storey office building model was devised to provide a basis for the study. The base-caseused as the reference for comparison with other cases had the area on each floor divided intofour perimeter zones of equal areas (Zones 1–4) and an interior zone (Zone 5), as shown inFig. 1. The glazed area accounted for 40% of the exterior surface of the external wall in eachperimeter zone (i.e. the window-to-wall area ratio (WWR) was 0.4). Except the window size andglazing property, the building models for all the cases studied had the same characteristics,including the building size and shape; wall and roof construction; the lighting, equipment andoccupant loads and their patterns of variations; and the air-conditioning schedule, as summar-ised in Tables 2–4. The internal load characteristics are same as the reference conditions forassessing new office buildings under the Hong Kong Building Environmental AssessmentMethod (HK-BEAM), which is a voluntary scheme that has been implemented since 1996 [66].For all the cases studied, the indoor temperature set point for all air-conditioned areas wastaken as 24

vC, and no provision of reheat was assumed.

Calculation of heat gains for OTTV evaluation followed the method due to Chou and Lee[33,34], i.e. by summing algebraically over all hours in the year the heat gains from and the heatlosses to individual envelope elements during both the air-conditioned and unconditioned hours,and dividing the sum by the total number of air-conditioned hours in the year and the surface


Table 2Key construction characteristics of the building model

Building type O
ffice building
No. of storeys 4
0 storeys (above ground) Floor to floor height 3.2 m
Floor area 3
6 m � 36 m, square in shape Air-conditioned area 1058 m2
Non-air-conditioned area
225 m2
Orientation N
orth (N), East (E),South (S) and West (W)
Opaque wall construction M
aterials Thickness(mm)
Conductivity(W/m K)

Sc
pecific heatapacity (J/kg K)
Granite panel
25 2.90 9 00.0 Normal cavity 50 6.54 Concrete 100 2.16 6 53.0 Plaster 20 0.38 1 000.0
Fenestration G
lass 6 1.05 7 50.0 Shading coefficient 0.4 Window-to-wall area ratio 0.4
Roof construction S
and/screed 25 0.72 8 40.0 Insulation 37 0.034 8 37.0 Asphalt 30 1.15 8 37.0 Screed 25 0.72 8 40.0 Concrete 100 2.16 6 53.0
Fig. 1. Floor layout of the building model (Base-case).


Table 3Occupation characteristics of the building model

Indoor design conditions
Cooling indoor dry-bulb temperature 24vC
Indoor relative humidity
54% Normal occupied andair-conditioned hours
Weekdays
8:00–19:00 Saturdays 8:00–13:00 Sundays and public holidays Not occupied
Occupancy density
Maximum density 9 m2 per person Daily patterns Table 4
Ventilation rates
When ventilation system is ON 10l/s per person When ventilation system is OFF 0l/s Daily patterns Table 4
Infiltration rates
When ventilation system is ON 0.1 air change/h When ventilation system is OFF 0.5 air change/h
Lighting load
Maximum intensity 25 W/m2
Daily patterns
Table 4 Appliances load Maximum intensity 20 W/m2 (assumed
constant load)

Table 4Occupancy density and lighting load profiles (in fractions of maximum density), and ventilation system operatingschedule

Day in theWeek

Hours
Occupancy Lighting(perimeter)
Lighting(interior)

Ventilation

Weekday
1–7 0.0 0.05 0.05 Off 7–8 0.05 0.1 0.1 Off 8–9 0.4 0.5 0.5 On 9–13 0.95 0.9 1.0 On 13–14 0.45 0.8 0.9 On 14–17 0.95 0.9 1.0 On 17–18 0.5 0.8 0.8 On 18–19 0.25 0.5 0.5 On 19–20 0.1 0.3 0.3 Off 20–21 0.05 0.2 0.2 Off 21–24 0.0 0.05 0.05 Off
Saturdays
1–7 0.0 0.05 0.05 Off 7–8 0.05 0.1 0.1 Off 8–9 0.3 0.5 0.5 On 9–13 0.6 0.75 0.8 On 13–17 0.1 0.2 0.2 Off 17–18 0.05 0.1 0.1 Off 18–24 0.0 0.05 0.05 Off
Sundays andpublic holidays

1–9
0.0 0.05 0.05 Off 9–17 0.05 0.1 0.1 Off 17–24 0.0 0.05 0.05 Off


area of the corresponding element, as depicted by Eq. (3). The heat gain from a fenestration

was the algebraic sum of the transmitted solar radiation, the part of the absorbed solar energy

that flowed toward the indoor space and the conduction heat flow due to outdoor/indoor

temperature difference.The heat gains were predicted by using a detailed building heat transfer simulation program

HTB2 [67] under the assumption that the air-conditioning system would be operating during the

occupied periods (Table 3), keeping the indoor temperatures steadily at the set point level

(24vC). It was assumed that no air-conditioning would be provided during the unoccupied peri-

ods. In such periods, the indoor temperatures would float according to heat balance in the

zones. The annual heat gains from the opaque and fenestration parts of individual external

walls, normalised by the air-conditioned hour and the respective component surface areas, are

referred to here as the heat gain intensities (W/m2). Such heat gain intensities would have been

the basis for evaluation of the OTTV equation coefficients TDEQ, SF and DT in deriving OTTV

equations for buildings in a specific region.For ease of reference, the heat gain intensities from the surface of the opaque part of a wall

and a fenestration in the same wall in a particular zone I that faced a specific direction D in the

base-case are denoted respectively by IW,I,D,B and IG,I,D,B. The same under a specific case stud-

ied are denoted respectively by IW,I,D,C and IG,I,D,C. The percentage difference between the

corresponding heat gain intensities (DIW,I,D and DIG,I,D) was calculated as shown below, to

show the impacts of changing the building characteristics of the base-case to those of the

specific case being considered.

DIW;I;D ¼ IW;I;D;C IW;I;D;B

IW;I;D;B� 100% (4)

DIG;I;D ¼ IG;I;D;C IG;I;D;B

IG;I;D;B� 100% (5)

The OTTV’s of the entire building under the base-case (OTTVB) and under the specific case

(OTTVC) were determined according to Eqs. (6) and (7).

OTTVB ¼P

ðAW;I;D;BIW;I;D;B þ AG;I;D;BIG;I;D;BÞPðAW;I;D;B þ AG;I;D;BÞ

(6)

OTTVC ¼P

ðAW;I;D;CIW;I;D;C þ AG;I;D;CIG;I;D;CÞPðAW;I;D;C þ AG;I;D;CÞ

(7)

where AW,I,D,B is area of the opaque part of the external wall in zone I facing direction D in the

base-case, m2; AG,I,D,B is area of the fenestration part of the external wall in zone I facing direc-

tion D in the base-case, m2; AW,I,D,C is area of the opaque part of the external wall in zone I

facing direction D in a specific case, m2; AG,I,D,C is area of the fenestration part of the external

wall in zone I facing direction D in a specific case, m2.However, if IW,I,D,B and IG,I,D,B were taken as the heat gain intensities that would apply to

the specific case, similar to applying an OTTV equation with pre-determined coefficients to


different buildings, the OTTV for the specific case (OTTVC,B) would have been calculated as:

OTTVC;B ¼P

ðAW;I;D;CIW;I;D;B þ AG;I;D;CIG;I;D;BÞPðAW;I;D;C þ AG;I;D;CÞ

(8)

The percentage error in the OTTV so calculated (DOTTV) would, therefore, be:

DOTTV ¼ OTTVC;B OTTVC

OTTVC� 100% (9)

For the base-case, the OTTV of the entire building (OTTVB) was found to be 6.65 W/m2. Ifthe calculation method and data given in the Code of Practice [14] was used instead, the OTTVwould be 27.4 W/m2. The large difference between these two figures was due to the very differ-ent approaches adopted for evaluating OTTV.The calculation results for the heat gain intensities IW,I,D,B and IG,I,D,B for the four external

walls of the building model are summarised in Table 5. The results show that for the topmostfloor, the opaque part of the walls facing north, east and south would incur a heat loss fromindoor to outdoor over the year (balance of gains and losses), but that facing west would leadto a small heat gain. On a typical floor, the opaque part of all the walls would incur a heat lossover the year. Due to the large positive solar gain, which dominated the fenestration heat gain,all windows would lead to a positive heat gain.

6.2. Effects of varying area of fenestrations in external walls

For examining the impacts of varying the proportion of fenestration areas in external wallson the heat gain from the opaque part of the walls, the window-to-wall area ratio (WWR) ofthe building model was reduced from 0.4 (base-case) down to 0.3 and 0.2, and enlarged up to0.5 and 0.6, but the construction of the wall and properties of the glazing remained unchanged.The simulation results for the two extreme cases (WWR ¼ 0:2 and 0.6) are summarised inTable 6, which show that increasing the fenestration area will lead to significantly increasedtotal envelope heat gain, and vice versa, as reflected by the calculated OTTVs (2.88–10.3W/m2). For those walls that would incur a heat loss through the opaque part over the year, theintensity (per unit area value) of this heat loss would increase with the fenestration area (thus

Table 5Heat gain intensities of envelope elements in the base-case building model and the OTTV of the building

Zone (I)
Exposure (D) Topmost floor Typical floor
Opaque wallIW,I,D,B (W/m2)

FenestrationIG,I,D,B (W/m2)

Opaque wallIW,I,D,B (W/m2)

FenestrationIG,I,D,B (W/m2)

1
N 4.01 12.80 4.63 11.22 2 E 4.05 15.16 4.69 13.50 3 S 1.89 28.82 3.35 25.07 4 W 1.43 40.96 0.53 36.04 OTTVB (W/m2) 6.65


DIW,I,D is positive) but, at the same time the heat gain intensity from the fenestration woulddrop (thus DIG,I,D is negative). This is because the transmitted solar intensity remained the samebut the increased total room solar gain raised the temperatures of the surfaces enclosing theroom, leading to a rise in heat loss through the glazing. The results also show that using theheat gain intensities for the base-case to estimate the OTTVs for the other cases would lead tovery significant errors, with DOTTV ranging from 41% to +16%, as shown in Fig. 2.

6.3. Effects of redefining the zoning

A further case study was performed to show the impacts of zoning on the envelope heat gainsof the air-conditioned spaces. For this case study, the floor layout was modified from thatshown in Fig. 1 to as shown in Fig. 3, where there are 10 perimeter zones and one interior zone.All the envelope and internal load characteristics were same as those in the base-case. Table 7summarises the predicted heat gains from the envelope elements and the OTTV for the entirebuilding. As the sizes of Zones 1, 3, 6 and 9 were not significantly smaller than the respective N,E, S and W zones before zoning was applied, the effects were just marginal. The large percent-age change in the heat gain intensity from the south wall in Zone 9 was due to the small valueof the intensity in the base-case (Table 5).

Table 6Percentage changes in heat gain intensities of envelope elements in the building model and the OTTV of the building

Zone (I)
Exposure (D) T opmost floor Typical floor
Opaque wallDIW,I,D (%)

FenestrationDIG,I,D (%)



(a) Window-to-wall area ratio (WWR) enlarged to 0.6
1 N 1 0.1 10.9 8.1 11.7 2 E 1 1.3 10.5 9.3 11.4 3 S 3 7.2 8.1 24.7 10.7 4 W 59.0 6.5 201.7 9.0 OTTVC
(W/m2)
1 0.03
OTTVC,B

(W/m2)
1 1.61
DOTTV
1 5.8%
(b) Window-to-wall area ratio (WWR) reduced to 0.2
1 N 12.3 12.8 10.4 14.2 2 E 13.7 12.2 11.8 13.7 3 S2 44.3 9.4 30.6 12.8 4 W 6 8.8 7.4 244.1 10.6 OTTVC (W/m2) 2 .88 OTTVC,B (W/m2) 1 .69 DOTTV 41.4%


For small perimeter rooms, significant changes in the heat gain intensities can be observed

even if those rooms were still enclosed by only one external wall (e.g. Zone 5). This is due to the

largely increased partition wall surface area that shares the transmitted diffuse solar radiation

and the long wave radiation re-emitted by the floor surface after it absorbed the transmitted

direct solar radiation (a method commonly used to treat transmitted solar energy in room heat

transfer modelling [57]). Consequently, the temperatures of the enclosing surfaces are lower,

which reduces the conduction heat loss through both the opaque and glazed parts of the exter-

nal wall. The latter increased the heat gain intensity from the window.For the corner zones (Zones 2, 4, 7 and 10, Fig. 3), the layout modification incurred very sig-

nificant changes to the heat gain intensities (Table 7), especially the intensities of conduction

gain from the opaque parts of the external walls. The effect can be an increase (e.g. the north

wall in Zone 10) or a reduction (e.g. the west wall in Zone 10 and the north and east walls in

Fig. 2. Percentage errors in the calculated OTTV.

Fig. 3. Modified floor layout of the building model.


Zone 2) in heat loss intensity, depending on whether there is a general rise or drop in theenclosure surface temperatures.If the heat gain intensities were unaffected by the layout change, the OTTV for this building

should equal that of the base-case building, as the envelope characteristics of the two wereidentical. However, the calculation results show that using the heat gain intensities predicted forthe base-case for estimating the OTTV of this building led to an error (DOTTV) of 9%.

6.4. Effects of varying WWR and SC on OTTV and annual cooling load

Table 8 shows the calculated OTTVs for the base-case building model and five other buildingmodels that differ from the base-case model only in the window-to-wall area ratio (WWR)and/or the shading coefficient of glazing (SC) (denoted as Case I to V). The tabulated datainclude the OTTVs of these building models, determined according to the method and datagiven in the Code of Practice [14], and the predicted annual space and total cooling loads forthese building models. The total cooling load exceeds the space cooling load because it includesthe cooling load for treating ventilation air and those loads incurred by fan and pump heatgains. Fig. 4 shows the correlation between the OTTV and the annual cooling loads of thesebuilding models.

Table 7Percentage changes in heat gain intensities of envelope elements in the building model and the OTTV of the buildingwith the modified floor layout (Fig. 3)

Zone (I)
Exposure (D) Topmost floor Typical floor




1
N 11.85 9.37 4.76 5.30 2 N 38.99 30.64 31.94 33.01 2 E 43.97 30.57 36.59 33.10 3 E 15.62 11.45 1.32 0.56 4 E 18.74 12.34 3.51 2.40 4 S 105.50 17.86 66.32 22.91 5 S 92.71 17.87 47.66 19.04 6 S 46.64 8.58 9.51 4.19 7 S 40.58 6.56 14.13 4.51 7 W 116.67 10.07 354.96 13.06 8 W 63.29 5.99 39.17 2.01 9 W 41.99 3.73 42.90 1.49 10 W 190.49 16.57 625.15 23.12 10 N 10.14 6.74 7.13 9.16 OTTVC (W/m2) 7.25 OTTVC,B (W/m2) 6.65 DOTTV 9.02%


This set of results shows that OTTV is highly sensitive to changes in WWR and SC. Simply

reducing or enlarging the window area will only lead to moderate changes in the annual space

cooling load and the annual total cooling load, which are much less significant than changing

the glass property (SC). This is because enlarging the window area will increase the solar gain

but this will be compensated by the increase in conduction heat loss through the glazing. On the

other hand, increasing the shading coefficient without changing the glazed area will increase the

solar gain but the heat loss through the glazing will be virtually unaffected. Fig. 4 shows that

the annual cooling loads are linearly, but just moderately, related to OTTV, primarily because

the cooling load due to envelope heat gains is relatively small compared to other more domi-

nant loads, such as lighting, equipment and ventilation loads.

Table 8Predicted OTTV and annual cooling load from the original to the modified construction

(a) Shading coefficient offenestration ðSCÞ ¼ 0:4

(b) Shading coefficient offenestration ðSCÞ ¼ 0:7

Case I
Base-case Case II Case III C ase IV Case V
Window-to-wallarea ratio (WWR)

0.2
0.4 0.6 0.2 0 .4 0.6
OTTV based on Codeof Practice [10]

(%
W/m2) 16.36 27.41 38.47 25.41 4 5.51 65.62 Diff.a 40.3% 40.4% 7.3% 6 6.0% 139.4%
OTTV based on heatgain predictions

(%
W/m2) 2.88 6.65 10.03 10.75 2 1.06 29.96 Diff.a 56.7% 50.8% 61.7% 2 16.7% 350.5%
Predicted spacecooling load

(%
GW h) 5.37 5.59 5.78 5.77 6 .31 6.78 Diff.a 4.0% 3.4% 3.2% 1 2.9% 21.3%
Predicted annualtotal cooling loadb

(%
GW h) 10.02 10.18 10.33 10.25 1 0.62 10.95 Diff.a 1.6% 1.4% 0.7% 4 .3% 7.6%
a With reference to the corresponding value in the base-case.b Including the space load, fresh air load and equipment heat gains.

nnual space cooling load (ASCL) and annual total cooling load
Fig. 4. Correlation of a (ATCL) with OTTV.


7. Heat gain through the building envelope of a high-rise air-conditioned residential building

So far, the regulatory OTTV control in Hong Kong applies only to commercial and hotel

buildings. As shown in Fig. 5, among various energy end-uses of the domestic sector, air-

conditioning was the lowest before 1986 but has surpassed all other energy end-uses since 1995

[68]. This trend shows that besides commercial and hotel buildings, good envelope energy

performance is also needed for reducing energy use for air-conditioning in residential buildings

in Hong Kong. Similar trend is expected to arise in many southern China cities along with the

rapid economic growth.Compared to commercial buildings, heat gains from internal sources in residential flats are

generally less intensive. Consequently, the envelope heat gain dominates the cooling load of a

residential flat [69] and thence OTTV would be a highly appropriate energy performance index

if it could adequately reflect the thermal performance of the envelope. However, a new set of

coefficients in the OTTV equation would need to be evaluated specifically for residential build-

ings to account for the different air-conditioning patterns.A study similar to that on office buildings as reported above has been conducted to examine

if OTTV is applicable to high-rise air-conditioned residential buildings. Since air-conditioners

are used primarily in living and dinning rooms and bedrooms in residential buildings, the study

focused only on the envelope elements enclosing these types of rooms. Other types of rooms,

such as kitchens and bathrooms, were excluded as they would normally be either naturally or

mechanically ventilated. The parametric studies performed were based on a residential flat

model and a set of occupancy profiles and internal load patterns that are representative of high-

rise residential buildings in Hong Kong.

Fig. 5. Various energy end-uses of the domestic sector in Hong Kong.


7.1. The flat model and the internal load profiles and air-conditioning patterns

The characteristics of the flat model used in the study were assigned with values that rep-resent the majority type of flats in residential buildings in Hong Kong, in respect of the numberof various types of rooms in a flat, the dimensions of the rooms, and areas and thermal charac-teristics of external walls and windows, as found in a recently conducted building characteristicssurvey [70]. Fig. 6 shows the configuration of the living and dining room and the bedroom inthe flat model. Table 9 summarises the building characteristics of the living and dining roomand the bedroom. In addition to the living and dining room (denoted as Zone 1) and the bed-room (denoted as Zone 5), the flat model includes other zones (Zones 2–4 and 6–8) that areconnected to these rooms (Fig. 6). These room models were the basic components of the flat

oom layout model for: (a) a living and dining room; and (b) a bed
Fig. 6. R room.


Table 9Characteristics of the rooms (Zones 1 and 5) in the flat model of residential building

Building characteristics
Zones 1 and 5
Wall thickness
150 mm Wall construction Heavy concrete w/cement plastering at both sides Window-to-wall ratio (WWR)a 0.45 Glass thickness 6 mm Shading coefficient of fenestration 0.95 Wall solar absorptivity (a) 0.58 External shading Nil
Table 10Daily occupancy, lighting (in fractions of the installed lighting power intensity) and appliances power profiles of resi-dential building

Hours
Occupancy L ighting H ousehold appliancespower (W)
(a) Living and dining room
24–6 0 0 .0 2 7.1 6–7 0 0 .3 5 2.0 7–8 0.5 0 .5 7 7.0 8–9 1 0 .0 7 7.0 9–12 1 0 .0 7 7.0 12–13 0.9 0 .0 7 7.0 13–14 1 0 .5 8 8.6 14–18 1 0 .0 6 0.6 18–19 1 0 .5 6 0.6 19–20 1.5 1 .0 1 42.3 20–23 2 1 .0 1 42.3 23–24 0 0 .5 1 42.3 (b) Bedroom 24–1 2 0 .3 3 5.8 1–6 2 0 .0 0 .0 6–7 1.9 0 .5 0 .0 7–8 0.5 0 .2 0 .0 8–9 0 0 .3 0 .0 9–13 0 0 .0 0 .0 13–14 0.5 1 .0 0 .0 14–17 0.5 1 .0 1 5.0 17–18 0.5 0 .0 1 5.0 18–19 0.5 1 .0 1 5.0 19–20 0.5 1 .0 3 5.8 20–22 1 1 .0 3 5.8 22–23 1 1 .0 4 4.8 23–24 2 0 .6 4 4.8


model, and the model can be modified conveniently to represent flats with different combina-tions of external walls and windows and exposure directions.The occupancy, lighting and appliances load profiles and the patterns of utilisation of

air-conditioners that are representative of typical households in Hong Kong were applied to theliving and dining room and the bedrooms in the flat model to account for the impacts of inter-nal heat sources and the air-conditioning pattern on the heat gain from the envelope. Theseload profiles and air-conditioning patterns were determined from the data gathered in an energyend-use survey of households in Hong Kong [70]. Table 10 shows the daily internal load profilesfor the living and dining room and the bedroom. The installed lighting power intensity for a liv-ing and dining room and a bedroom were 14 and 17 W/m2, respectively, which were the corre-sponding mean values found in the survey. The air-conditioned period for the living and diningroom was between 13:00 and 22:00 and that for the bedroom between 13:00 and 07:00 (on thenext day), which would remain the same every day from April to October inclusive, but no air-conditioners would be operated outside these months in the year, as confirmed in the survey tobe the case for the vast majority of the households in the sample.In the simulation study, the indoor temperature of an air-conditioned room was set steadily at

22vC, and the infiltration rate at 0.5 air change/h (ach), during air-conditioned periods. For

rooms without air-conditioners and for those with air-conditioners but during unconditionedperiods, it was assumed that while the room was occupied, the occupants would turn on a venti-lation fan and/or open windows or doors to increase the ventilation rates in the rooms. Thelower ventilation rate, at 3 ach, was used when the indoor temperature stayed below 22

vC, and

the higher rate, at 12 ach, was used whenever the indoor temperature equalled or exceeded 22vC.

7.2. Influences of room envelope configuration on heat gains from individual envelope elements

Two series of case studies had been conducted. The study targeted at analysing the heat gainfrom an external wall with fenestration, denoted as Surface 1 in the room model (Fig. 6). As inthe study on office buildings, the year-round total heat gain from the external wall (Surface 1) isthe algebraic sum of the heat gains throughout the air-conditioned and unconditioned hoursover the year, evaluated for each of the cases from hourly predictions obtained using the simu-lation program HTB2 [67].

7.3. External wall with fenestration

In the first series of cases, Zones 1 and 5 (Fig. 6) each had only one external wall (Surface 1)with fenestration in the base-case. The window-to-wall area ratios in the two zones were both0.45 (based only on the overall area of Surface 1). The room models were then modified suchthat the external walls in Zones 1 and 5 were both an opaque wall without fenestration. Table 11shows the original heat gain intensities (the annual total heat gain per unit area per air-conditioned hour) from the opaque and glazed parts of the external wall, and the percentagechange in the heat gain intensity from the opaque part after the window in the wall wasreplaced by the opaque component. Four cases were studied, each with the wall facing a differ-ent direction. Compared with the heat gain intensities from envelope elements in the officemodel, the heat gain intensities in the residential flat model are much greater because of the


much higher glazing shading coefficient and wall U-value, the lower indoor temperature duringair-conditioned periods and the fewer air-conditioned hours in the year.The results show that the heat gain intensity from the opaque part of Surface 1 in Zone 1

(a living and dining room) increased by more than 35% and that in zone 5 (a bedroom) by morethan 22%. There are only small variations in these percentage changes when Surface 1 was posi-tioned to the four different directions. The heat gain intensity from the opaque part of a wallwith or without a window in it would have been identical if it was determined following thestandard procedures for OTTV calculation using the same pre-calculated TDEQ. However, whenthere was no window and thus no solar gain, the internal surfaces of fabric elements enclosingthe room will have lower temperatures, which caused the increase in heat gain from the externalwall.

7.4. Numbers of external wall

In the second series of case studies, the walls denoted as Surface 2 in the room models forboth Zone 1 (a living and dining room) and Zone 5 (a bedroom) (Fig. 6) were each modifiedinto an external wall without fenestration. Thus, Zones 1 and 5 each had two external walls(Surface 1 and 2). Table 12 shows the consequential percentage changes in the intensities of heat

Table 11Heat gain intensities from the external wall (Surface 1) of the building model and the percentage changes in the heatgain intensity from the opaque part when the fenestration is removed

Exposure(D)

L
iving and dining room Bedroom
Base-case (W/m2)
Opaque wall (%) Base-case (W/m2) Opaque wall (%)
IW,I,D,B
IG,I,D,B DIW,I,D IW,I,D,B IG,I,D,B DIW,I,D
N 2
2.27 140.94 35.06 18.22 79.97 22.48 E 2 1.82 150.49 38.96 18.00 84.77 24.76 S 2 2.67 157.97 39.10 18.49 88.59 25.01 W 3 0.58 220.80 38.04 22.53 119.97 26.21
Table 12Percentage changes in the heat gain intensities of envelope elements in the original external wall when an additionalexternal wall was included

Exposure (D) L
iving and dining room Bedroom




N
18.64 4.19 17.55 5.68 E 19.56 4.05 18.10 5.49 S 26.06 5.36 21.63 6.47 W 12.07 2.38 13.24 3.53


gains from the envelope elements in Surface 1 of Zones 1 and 5. The presence of another exter-nal wall affected the radiant energy exchanges among the surfaces enclosing the room and thiscaused changes in the heat gain intensity from the opaque part of the original external wall(Surface 1) by 12% to 18%, and from its fenestration part by 2% to 4%, for the livingand dining room (Zone 1), and by 13% to 18% and 4% to 6%, respectively, for the opa-que and fenestration parts of the external wall (Surface 1) in the bedroom (Zone 5).

7.5. Further case studies

Further simulation studies had been performed to provide a more comprehensive pictureabout whether OTTV would be an appropriate envelope energy performance index for residen-tial buildings. This series of simulation study was based on six room models, referred to asCases 1–6, which had characteristics as summarised in Table 13. The study focused on examin-ing the heat gain intensity from one external wall, denoted as the 1st external wall. In Case 1,there was only one external wall that faced south. In Case 2, there was another external wall,the 2nd external wall, which faced west. The room model in Case 3 had also two external walls,but the 2nd external wall included a west facing window of the same characteristics as that inthe 1st wall. The room model in Case 4 included 3 external walls, with the 1st and the 2nd wallexactly the same as those in Case 3, but the 3rd external wall was an east facing opaque wallwithout window. Simulation was performed for a series of cases, with the same adjustmentsmade in each case to the constructions of the 1st and the other external wall(s), including thewall thickness for the opaque part and, where applicable, the window-to-wall ratio (WWR).Details of the adjustments made are shown in Table 13.Fig. 7 shows a scattered plot of the annual total heat gain intensity from the 1st wall (includ-

ing the window and the opaque part of the wall) predicted in the simulations for Cases 1–4,denoted as the OTTV (of the 1st wall only), against the parameter ð1WWRÞ �Uwall, whereWWR is the window-to-wall ratio and Uwall the U-value of the walls. A higher value ofð1WWRÞ �Uwall would mean a more significant contribution from the opaque part of theexternal wall. Here, WWR refers only to the area ratio of the fenestration in a wall to the totalarea of the wall, but Uwall is the U-value for all the external walls in the flat model, since theconstruction of the external walls were assumed to be identical for all the cases.According to the conventional OTTV calculation method, cases of the same value of

ð1WWRÞ �Uwall (which are of the same wall construction, glazing properties and exposuredirection) should lead to the same OTTV, because the same solar factor (SF) and equivalenttemperature difference (TDEQ) values would have been used in the calculation. Therefore, thefour points for Cases 1–4 that correspond to the same ð1WWRÞ �Uwall value should appearin Fig. 7 as just one point if indeed single-valued SF and TDEQ could be used for predictingOTTV of envelope elements. However, as Fig. 7 shows, there are substantial vertical spreads inthe OTTV of the 1st wall among the four Cases. It can be observed that the OTTV of the wallwould drop with increase in the number of external walls that enclosed the same room and withthe inclusion of a window at the other external wall. This shows that the interactions of the heattransfers at the opaque part and the fenestration part of an external wall with those of otherexternal wall(s) are highly significant. Therefore, the OTTV calculated based on pre-evaluatedparameters may not be a true reflection of the heat gain.

Table13

InputvariablesforCases1–6

Room

type

Livinganddiningroom

Case

1Case

2Case

3Case

4Case

5Case

6

No.ofexternalwall(s)

12

23

12

Configurationofthe

1stexternalwall

Orientation

South

Window-to-wall

ratio(W

WR)

0.05–0.65in

step

increm

entsof0.10,0.45,0.45

Shadingcoeffi

cient0.95

Wallthickness

100–300mm

instep

increm

entsof50mm

Absorptivity

0.98

Shadingdevice

No

LSF¼0:0,

LSF¼0:1,0.3&

0.5

LSF¼0:0,

LSF¼0:2,0.4

Configurationofthe

2ndexternalwall

Orientation

–West

West

West

–West

Window-to-wall

ratio(W

WR)

–0.0

Sameas1stwall

–Opaquewallonly

Shadingcoeffi

cient–

–Sameas1stwall

––

Wallthickness

–Sameas1stwall

–Sameas1stwall

Absorptivity

–Sameas1stwall

–Sameas1stwall

Shadingdevice

–No

No

No

––

Configurationofthe

3rd

externalwall

Orientation

––

–East

––

Window-to-wall

ratio(W

WR)

––

–0.0

––

Shadingcoeffi

cient–

––

––

–Wallthickness

––

–Sameas1stwall

––

Absorptivity

––

–No

––

Shadingdevice

––

––

––



Studies have also been made into the effects of including an external shading device, a side-finat the left side of the window in a south facing external wall, on the OTTV of the external wall.The effects of the co-existence of another external wall in the same room were also investigated.These cases are denoted as Case 5 and 6, and details about the flat models used are summarisedin Table 13. The predicted surface heat gain intensities for the 1st wall are shown in Fig. 8. Inaddition to the significant effects of the shading device, Fig. 8 also shows the interesting resultthat the heat gain intensity in Case 5 (with the 1st wall being the only external wall) with leftside-fin with a projection factor (LSF, as defined in Fig. 8) of 0.5 was even higher than the heatgain intensity in Case 6 without any external shading devices but with the presence of anotherexternal wall with no window.

sity from the 1st external wall in the air-conditioned living and dining r
Fig. 7. Heat gain inten oom model for Cases1–4 (see Table 13 for details).
sity from the 1st external wall in the air-conditioned bedroom for Cases
Fig. 8. Heat gain inten 5 and 6 (see Table 13for details).


The above results are considered sufficient evidence to show that the use of OTTV that isbased on the assumption that heat gain from each wall or window could be determined inde-pendent of other walls and windows and could be summed to yield to the overall heat gain,would not be a good indicator of the thermal performance of the envelope of an air-conditionedresidential building. Accurate determination of heat gains of indoor spaces from the buildingenvelope should take into account the interactive heat transfer among the walls and fenestra-tions, which would require the use of a detail building energy simulation program.

8. Conclusion

Overall thermal transfer value (OTTV) is meant to be a measure of envelope thermal per-formance for air-conditioned buildings with which to set an energy efficiency standard. Whenfirst introduced in ASHARE Standard 90-75 in 1975, it was based on the heat transfer throughthe envelope that contributed to the peak cooling load. Its use in ASHRAE Standard 90 lastedfor 14 years, and has been abandoned since Standard 90.1 was launched in 1989. However,OTTV continues to be used in the building energy codes of a number of countries, especiallyAsian countries. The concept and definition of OTTV have also evolved along with the continu-ous efforts made to improve its application, with the key objective to make OTTV an indicatorof the impact of the envelope on the energy use for air-conditioning in buildings.For buildings situated in a sub-tropical climate region like Hong Kong, research studies

showed that acceptable correlation between OTTV and energy use for air-conditioning (with allother things being equal) could be achieved only if the heat transfer in buildings during the coolmonths was ignored. Even though OTTV calculated in such a way may be a good reflection ofthe impact of envelope performance on energy use for air-conditioning, it remains an inad-equate measure of the envelope performance. As the case study results for office and residentialbuildings presented in this paper show, the use of pre-calculated coefficients for OTTV calcu-lation has inherent deficiencies, as the interacting effects among heat gains from different envel-ope elements and internal sources, and the impacts of room configuration cannot be properlyaccounted for. The OTTV determined from such methods, therefore, is subject to uncertaintiesand may be inconsistent with the envelope performance.OTTV is simple to use and thus the cost of implementing the regulatory control can be kept

low, which may be a valid reason for basing the control on OTTV. To be effective, regulatorycontrol over building energy performance needs to include requirements on the energy perform-ance of building services installations. In order to provide designers with flexibility in meetingthe control requirements in the most economical manner, compliance through an alternativeroute that is based on the total energy budget approach, as in ASHRAE Standard 90.1, shouldbe provided. In this case, detailed computer simulation becomes an integral part of compliancedemonstration. When detailed simulation is used, minimum performance required of individualtypes of envelope components can be specified on the basis of more basic characteristics, e.g. thecharacteristics of a particular wall construction and glazing and a window-to-wall area ratiolimit, instead of using the simplistic OTTV method, which is prone to errors. Therefore, forregimes that are still using OTTV as a means for controlling building energy performance, a


second thought should be given to whether or not to continue with its use as a regulatory

instrument.

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