Thermal comfort in buildings10 July 2009

Kostas Pavlou

Definition according to ISO 7730:

“That condition of mind which expresses satisfaction with the thermal environment”

Building is a construction that protects people from the environmental conditions and provides a healthy and comfort indoor conditions with the possble use of systems

Users expectansions

Low running cost

Low initial cost

Environmental friendly

Why thermal comfort is important?

• having the “right temperature” is one of the things people considered most important in a building

• `air freshness' is an important requirement • The subjective feeling of the freshness of

the air has been found to be closely related to temperature of the air

Achievement of thermal comfort

• It is not always possible due to the indoor environmental conditions – technical reasons

• The thermal sensantion usully changes over the time

• It is subjective

Thermal Environment

The thermal environment consist of:

• The building environment (building skin, systems)

• The subjective (the human body)

The human body (1)

• The normal body core temperature is 37ºC.

• We have separate heat and cold sensors.

• Heat sensors are located in the skin. Signals when temperature is higher than 37ºC.

• Cold sensors are located in the skin. They send signals when skin temperature is below 34ºC.

• There are more cold sensors that warm sensors.

The human body (2)

• Heating mechanism:– Reduced blood flow.– Shivering.

• Cooling mechanism:– Increased blood flow.–Sweating (Evaporation).

The body surface area

ADU = 0.202 * (Wb^0.425) * (Hb^0.725) (m²)

Wb the body weightHbthe body height

Body surface area

1.00m²

1.25m²

1.50m²

1.75m²

2.00m²

2.25m²

2.50m²

150 160 170 180 190 200Body height (cm)

FemaleMale

The body surface area

The energy balance

HeatProdu-ced

HeatLost

Thermal Comfort can only be maintained when heat produced by metabolism equals the heat lost from body

Heat produced

Food

Heat

Mechanical work

The efficiency of “man machine” is in the range of 0 – 0. 25 and it is the ratio of mechanical work to chemical energy

Heat losses

C : Heat loss by convection from outer surface of clothed body to air

R : Heat loss by radiation from outer surface of the clothed body to its environment

E : Heat loss by evaporation from the skin

Heat losses by radiation

Thermal comfort in air – conditioned building

ISO 7730 describes the:

Predicted Mean Vote Scale (PMV)

Predicted Percentage of thermally Dissatisfied persons (PPD)

Predicted Mean Vote scale (PMV)

PMV-index (Predicted Mean Vote) predicts the subjective ratings of the environment in a group of people

- +3 Hot

- +2 Warm

- +1 Slightly warm

- +0 Neutral

- - 1 Slightly cool

- -2 Cool

- -3 Cold

- +3 Hot

- +2 Warm

- +1 Slightly warm

- +0 Neutral

- - 1 Slightly cool

- -2 Cool

- -3 Cold

Calculation of PMV and PPD

The PMV and PPD values are function of the Metabolic Rate and the Clothing value.

Metabolic rate is given in Met1Met = 58.15W/m²

Clothing value is given in Clo1Clo = 0.155m²ºK/W

Met value tableActivity Metabolic Rates [M]

Reclining 46 W/m2 0.8 Met

Seated relaxed 58 W/m2 1.0 Met

Clock and watch repairer 65 W/m2 1.1 Met

Standing relaxed 70 W/m2 1.2 Met

Car driving 80 W/m2 1.4 Met

Standing, light activity (shopping) 93 W/m2 1.6 Met

Walking on the level, 2 km/h 110 W/m2 1.9 Met

Standing, medium activity (domestic work) 116 W/m2 2.0 Met

Washing dishes standing 145 W/m2 2.5 Met

Walking on the level, 5 km/h 200 W/m2 3.4 Met

Building industry 275 W/m2 4.7 Met

Sports - running at 15 km/h 550 W/m2 9.5 Met

Heat gains of occupants, ISO 7730

Clothing tableGarment description Iclu Clo Iclu m2 °C/W

Underwear PantyhoseBriefsPants long legs

0.020.040.10

0.0030.0060.016

Underwear,shirts

BraT-shirtHalf-slip, nylon

0.010.090.14

0.0020.0140.022

Shirts Tube topShort sleevesNormal, long sleeves

0.060.090.25

0.0090.0290.039

Trousers ShortsNormal trousersOveralls

0.060.250.28

0.0090.0390.043

Insulatedcoveralls

Multi-component fillingFibre-pelt

1.031.13

0.1600.175

Sweaters Thin sweaterNormal sweaterThick sweater

0.200.280.35

0.0310.0430.054

Clothing tableGarment description Iclu Clo Iclu m2 °C/W

Jackets VestJacket

0.130.35

0.0200.054

Coats over-trousers

CoatParkaOveralls

0.600.700.52

0.0930.1090.081

Sundries SocksShoes (thin soled)BootsGloves

0.020.020.100.05

0.0030.0030.0160.008

Skirt,dresses

Light skirt, 15cm above kneeHeavy skirt, knee-lengthWinter dress, long sleeves

0.100.250.40

0.0160.0390.062

Sleepwear ShortsLong pyjamasBody sleep with feet

0.100.500.72

0.0160.0780.112

Chairs Wooden or metalFabric-covered, cushionedArmchair

0.000.100.20

0.0000.0160.032

0

10

20

30

40

50

60

70

80

90

100

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

PMV

PP

D

Acceptable thermal comfort conditions

PMV – PPD calculations

Thermal comfort in non air – conditioned buildings

The adaptive approachThe adaptive approach to thermal comfort starts, not from a consideration of the heat exchange between man and the environment, but from the observation that there are a range of actions that man can and does take in order to achieve thermal comfort. The seat of temperature regulation in man is the temperature of the brain, from where he controls the equilibrium between himself and the environment by means of actions taken which tend to maintain this temperature within close limits. If a change occurs, in the environment or elsewhere, causing the brain temperature to deviate from these close limits, then an action is taken which will tend to restore it to these limits.

Constraints

• change too fast for adaptation to take place

• are outside normally accepted limits

• are unexpected

• are outside individual control

Discomfort may arise where ambient or indoor conditions:

Some actions in response to cold• Vasoconstriction (reduces blood flow to the surface tissues)

• Increasing muscle tension and shivering (generates more heat in the muscles)

• Curling up or cuddling up (reducing the surface area available for heat loss)

• Increasing the level of activity (generates body heat)

• Adding clothing (reduces the rate of heat loss per unit area)

• Turning up the thermostat or lighting a fire (usually raises the room temperature)

• Finding a warmer spot in the house or going to bed (select a warmer environment)

• Insulating the loft or the wall cavities (hoping to raise the indoor temperatures)

• Improving the windows and doors (to raise temperatures/reduce draughts)

• Building a new house (planning to have a warmer room temperature)

• Acclimatising (letting body and mind become more resistant to cold stress)

Some actions in response to heat • Vasodilation (increases blood flow to surface tissues)

• Sweating (evaporative cooling)

• Adopting an open posture (increases the area available for heat loss)

• Taking off some clothing (increases heat loss)

• Reducing the level of activity (reduces bodily heat production)

• Having a beer (induces sweating, and increases heat loss)

• Drinking a cup of tea (induces sweating, more than compensating for its heat)

• Adopting the siesta routine (matches the activity to the thermal environment)

• Turning on the air-conditioner (lowers the air-temperature)

• Switching on a fan (increases air movement, increasing heat loss)

• Opening a window (reduces indoor temperature and increases breeze)

• Finding a cool spot

• Going for a swim (selects a cooler environment)

• Acclimatising (letting body and mind adjust so that heat is less stressful)

Local thermal discomfort

Assuming that the heat balance is neutral, discomfort may occur by local conditions.

The followings should be avoided:

• draughts

• radiation asymmetry

• temperature differences

Draught

At lower air temperatures a higher number of occupants will be dissatisfied

Radiation asymmetry

The following may result to radiation asymmetry:

•Building elements with different thermal resistance

•Different materials

•Heat sources

•Direct solar gains

Even the design target is the achievement of thermal comfort the behavior of the people under extreme thermal conditions has been studied very intensive.

The WBGT index is used to determine the time that a healthy man can do a specific work without any heat illness.

The WBGT is calculated by the following equations:

Indoor

WBGT = 0.7*Twb + 0.3*Tgl

Outdoor

WBGT = 0.7*Twb + 0.2*Tgl + 0.1*Tdb

Where, Twb: Wet bulb temperature

Tgl Globe temperatureTdb Dry bulb temperature

Thermal stress

Energy and comfortAs the design target is the achievement of thermal comfort the design of the building and the systems are focused on it.

An important issue is that the initial and running cost of the building and the systems should be kept as low as possible.

School

OfficeBedroom

Train station

Source: Derek Clements-Croome (1997). "Naturally ventilated buildings". E&NN SPON, Chapman & Hall

House in Oslo

External shading

Software tools

Software tools may be used in order to predict the internal condition and / or the comfort sensation.

In order to predict the internal conditions, such as the air temperature, the humidity, the building elements surface temperatures, the use of dynamic building thermal simulation software is necessary. The use of such a software is rather complicated and time – consuming.

Given the internal conditions the calculation of the possible comfort sensation is quite easy.

It has to be mentioned that results of all software depends on the required input.

Questions