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The Design and Delivery of Low Carbon Buildings
Ventilation 1
Ventilation
Introduction ................................................................................................................................... 2 Historical Approaches to Ventilation ............................................................................................ 3 Selecting the Ventilation Strategy ................................................................................................. 5 General Principles ......................................................................................................................... 8 Natural Ventilation: Details ...................................................................................................... 9 Mechanical Systems with Heat Recovery (MVHR): Details ................................................. 12 Earth Tubes ............................................................................................................................. 15
The Design and Delivery of Low Carbon Buildings
Ventilation 2
Introduction
Ventilation is required for several reasons: 1. To provide oxygen to enable the occupants to breathe;
2. To remove waste carbon dioxide from the occupants;
3. To remove other trace gases and particulates from the outgasing of surface coverings (for example carpets), or processes;
4. To keep odours within reasonable bounds;
5. To provide cooling; 6. To allow a psychological connection between the indoor and outdoor realm.
For every design the importance of each these needs to be assessed and any complicating
factors, such as the potential for noise ingress from adjacent roads considered. Reason 1, the
need for oxygen, requires relatively little fresh air: approximately 0.03 litres of air per second
per person (l/s/p). For 1 person in a house of volume 500 m3, this equates to 0.0002 air changes
per hour (ach/hr). This is very little air and is more than likely to be provided by ingress around
doors and windows. The other reasons given above typically imply much higher rates: the
removal of carbon dioxide requires around 3 to 5 litres per second per person and summertime cooling possibly 20 litres per second per person (l/s/p) (depending on room size and design). In
general it is meeting the need for cooling that is the most difficult. Equations 1 and 2 shows the
relationship between air changes per hour and litres per second; Table 1 gives explicit values for three room sizes. (The constants used in Equation 1 and 2 arise from there being 3600 seconds
in an hour and 1000 litres in a cubic metre.) Some of the values given in Table 1 imply very
high velocities if the air enters through small openings, or large (or numerous) openings. For
example a classroom of 30 occupants being cooled by 20 l/s/p of outside air provided by a 1
metre wide window open to 100 mm, implies an air velocity of 13.5 miles per hour (or force 4
on the Beaufort scale). This is easily fast enough to cause annoyance or to move paperwork.
1000).(
3600)./()/(
3mV
slqhrachQ = Equation 1
3600
1000).()./()/(
3mVhrachQslq = Equation 2
Table 1 . Relationship between air changes per hour and litres per second. This table should be used to
convert between requirements set in different units, or to get an idea of the quantity of air required.
ach/hr l/s
l/s 10 m3 100 m
3 1000 m
3 ac/hr 10 m
3 100 m
3 1000 m
3
1 0.36 0.036 0.0036 0.1 0.278 2.78 27.8
3 1.08 0.108 0.0108 1 2.78 27.8 278
8 2.88 0.288 0.0288 2 5.56 55.6 556
15 5.4 0.54 0.054 4 11.1 111 1110
20 7.2 0.72 0.072 8 22.2 222 2220
30 10.8 1.08 0.108 16 44.4 444 4440
In winter, ventilation can also be a source of heat loss. gives the heat loss from a ventilation
rate q (litres per second), given the specific heat capacity (spht, 1000 J.kg-1.K
-1) and density (1.1
kg/m3) of air, for each 1 degree centigrade difference been the internal and external air, i.e. the
loss in watts is the same as the ventilation rate in litres per second (per degree centigrade):
The Design and Delivery of Low Carbon Buildings
Ventilation 3
)/(1000
)/(..)( slq
slqsphtwattsloss air ≈=
ρ Equation 3
For a building of 500 occupants being ventilated at 5 litres per second per person and a temperature difference of 20°C, Equation 3 gives a loss of 2500 watts per degree centigrade, or
50 kW in total. Depending on the external climate and the hours of operation, this could imply
over 70,000 kWh over the heating season, or £3,600 worth of natural gas just to keep the concentration of carbon dioxide within the building within acceptable limits. It is illustrative to
compare this with the fabric losses from such a building. If the mean U-value of the fabric is 0.3
and the surface area 3000 m2, the loss will be 18 kW, i.e. the ventilation loss exceeds the fabric
loss during occupation. This is a little appreciated result and implies that as much care should be
given to the design of the ventilation system and air-tightness as given to minimising fabric
losses. It also suggests that if the heat from ventilation air leaving the building could be
captured and passed to the incoming air it would be more than enough to heat the building during occupancy. Given a mechanical ventilation system, this is relatively easy to do and is
one of core technologies behind the Passivhaus approach.
Historical Approaches to Ventilation The Roman architect Vitruvius Pollio (approx 80-15 BC) said towns should be located “without marshes in the neighbourhood, for when the morning breezes blow toward the town at sunrise,
if they bring with them mists from marshes and, mingled with the mist, the poisonous breath of
the creatures of the marshes to be wafted into the bodies of the inhabitants, they will make the site unhealthy.”. During the Industrial Revolution, most physicians believed polluted external air
was responsible for various chronic conditions. By 1866, technology had progressed such that
B.F. Sturtevant Co. was equipping the U.S. with ventilating fans. In 1884, Dr. John S. Billings, U.S. deputy surgeon general, published “The Principles of Ventilation and Heating and Their
Practical Application1” a comprehensive text on standards and specifications.
Vitruvius was also the author of De architectura, known today as The Ten Books on
Architecture,2 Vitruvius is famous for asserting in De architectura that a structure must exhibit
the three qualities of firmitas, utilitas, venustas — that is, it must be solid, useful, beautiful.
According to Vitruvius, architecture is an imitation of nature. As birds and bees built their nests,
so humans constructed housing from natural materials, that gave them shelter against the
elements. Our material pallet is now far wider, but an equally great change has arisen because of access to cheap energy such we need no imitate nature but rely on mechanical systems to cool
and heat our shelters and ignore the elements.
Buildings such as Frank Lloyd Wright’s Guggenheim museum (1943–1959) (Figure 1) were
design to completely isolate the occupant from the external environment by the use full
mechanical ventilation. Such an approach to ventilation is relatively modern and there are
plentiful examples of naturally ventilated buildings of all scales and in all climates that work
well. Figure 3 and Figure 4 show two examples, one old, one new, of the use of natural
ventilation in extreme climates. Given the success of these, it is sensible to conclude that for
most climates it should be possible to design a naturally ventilated solution. However the need
to minimise heat losses as we move toward zero-carbon design may paradoxically move us
away from a natural solution in many climates and towards a mechanical solution in winter and a natural one in summer.
1 Billings, J.S. (1889). The principles of ventilation and heating and their practical
application (2nd ed.). New York: The Sanitary Engineer. 2 Vitruvius, Pollio (transl. Morris Hicky Morgan, 1960), The Ten Books on Architecture. Courier Dover Publications. ISBN 0-486-20645-9.
The Design and Delivery of Low Carbon Buildings
Ventilation 4
Figure 1. Guggenheim museum. http://en.wikipedia.org/wiki/File:Guggenheim_museum_exterior.jpg
Figure 2 . Zion National Park Visitors’ Centre. This uses downdraft cooling towers (shown) with
evaporative (water) at the top, and exhaust through high clerestory windows.
Figure 3. Mansion in Jaisalmer, India. The image clearly shows the open nature of the facade needed to
give the large ventilation needed to cool the high mass architecture through the use of night time cooling.
http://en.wikipedia.org/wiki/File:Jaisalmer-4.jpg
The Design and Delivery of Low Carbon Buildings
Ventilation 5
Figure 4. Photo of visitor centre at Zion National Park showing downdraft cooling tower with
evaporative media at the top, and exhaust through high clerestory windows.
(Courtesy of Robb Williamson)
Natural ventilation in many climates may not move interior conditions into the comfort zone
100% of the time. There is therefore the need to make sure the building occupants understand
that some of the time thermal comfort may not be achieved. There is also a need to consider the
form of the building at an early stage. A naturally ventilated structure is often of articulated plan with large window and door openings, while an artificially conditioned building might well be
more compact in plan with sealed windows.
Selecting the Ventilation Strategy Table 2 lists the issues influencing the choice of ventilation strategy for a low-energy building
and Table 3 gives suggested ventilation rates for differing environments (these should be
checked against current national regulations before commencing detailed design work). Table 2
and Table 3 provide a first step to what will need to be considered but should only been send as a starting point, not for setting out the only possibility: for example double opening windows
with much higher and lower than typical openings may well be able to provide adequate
ventilation to a much greater room depth (work by the UK Building Research Establishment
indicates that ….)
Table 2. Issues influencing the choice of ventilation strategy (adapted from CIBSE-B p2-6)
Issue Comments
Location Large adjacent buildings can adversely affect wind patterns and imply greater
opening areas are required. The proximity of external sources of pollution can
influence the feasibility of natural ventilation. The proximity of external sources of
noise can impact on the feasibility of natural ventilation.
Pollution Local levels of air pollution may limit the opportunity for natural ventilation. It may
not be possible to provide air inlets at positions suitable for natural ventilation given
the inability to filter the incoming air successfully.
Orientation Buildings with their main facades facing north and south are much easier to protect
from excessive solar gain in summer as the north side will be in shade and shading
can easily be provided on the southside, as the sun will be high during the hottest
part of the day.
Form At building depths greater than 15 m the ventilation strategy becomes more
complex; the limit for daylighting and single sided natural ventilation is often taken
as 6 m. (But is probably higher.) Adequate floor to ceiling heights are required for
displacement ventilation and buoyancy driven natural ventilation; a minimum floor
to ceiling height of 2.7 m is recommended.
Infiltration Ventilation strategies and the whole low-energy approach, whether natural or
mechanically driven, depend on the building fabric being appropriately airtight.
Shading The appropriate use of external planting or other features can reduce solar gain.
The Design and Delivery of Low Carbon Buildings
Ventilation 6
These need to be external, not internal and it is important to consider making the
windows smaller rather than relying on shading as this will also reduce heat losses.
Window choice Openable areas must be controllable in both summer and winter, e.g. large openings
for still summer days and trickle ventilation for the winter time. Window shape can
affect ventilation performance: Single sided ventilation provided by top or bottom
hung windows is rarely effective except in domestic situations where gains and
occupancy levels are low. In high gain situations, maximise the height difference
between the top and bottom of the window, or better have a high and a low opening
(if at all possible use double sided ventilation).Windows need to be easy to use—
remember large triple glazed units are heavy and can be difficult to open if sited too
high.
Glazing Total solar heat transmission through window glazing can vary over a six fold
range, depending on the combination of glass and shading mechanisms selected.
Figure 5 shows the relative effectiveness of eight glazing and shading systems. Thermal mass Thermal mass is used to reduce peak cooling demands and stabilise internal air
temperatures. In winter it can be used to store excess heat for the next day—
however for this to be effective in energy terms insulation and infiltration levels
need to be improved to ensure the heat is retained.
Table 3. Summary of recommendations (adapted from CIBSE-B, p2-13)
Building sector Recommendation (ac/hr, unless otherwise stated)
Assembly halls 3-4 air changes per hour (but pay particular attention for the
potential to overheat).
Music studios 6–10 (but heat gain should be assessed)
Call centres 4–6 (but heat gain should be assessed)
Catering (inc. commercial
kitchens)
30–40
Communal residential buildings 0.5–1
Computer rooms Positively pressurised to 1 ac/hr to prevent local build-up of heat and
contamination for external air. However unless active cooling is
used much higher rates are typical.
Court rooms As for typical naturally ventilated buildings
Dwellings 0.5–1
Factories and warehouses highly dependent on use
High-rise (non-domestic)
buildings 4–6 ACH for office
areas; up to 10 ACH for meeting
spaces
Hospitals and health care
buildings
6-10 toilets and bathrooms, 10 (minimum) isolation rooms, 15
recovery rooms, 6 (minimum) treatment rooms. There are usually
filtration requirements for hospitals and hence most of these will be
supplied via a mechanical systems.
Hotels 10–15 minimum for guest rooms with en-suite bathrooms
Industrial ventilation Sufficient to minimise airborne contamination
Laboratories 6-15, likely to be mechanical (allowance must be made for fume
cupboards)
Museums, libraries and art
galleries
Depends on nature of exhibits
Offices 1.8 l/s/p if seated quietly; 5.6 l/s/p if light work
Schools and educational buildings teaching areas: 3 l/s/p minimum
The Design and Delivery of Low Carbon Buildings
Ventilation 7
. Figure 5. Summary of recommendations (adapted from CIBSE-B p2-13)
The key decision to be made is whether the building will use natural or mechanical ventilation, as this will define much of the energy philosophy and layout of the building. Table 1 shows the
advantages and disadvantages to each. In summer ventilation is likely to be provided by
openable windows much of the time as this reduces the electrical demand from fans, so it is unlikely that a mechanically ventilated building will be able to do without opening windows. In
situations such as sites exposed to high levels of external noise opening windows may not be
possible. This suggests either using mechanical ventilation all year, which implies much larger systems to give the substantial ventilation rates needed in summer for cooling, or relying on
acoustically damped passive vents. The later should be viewed with caution. There is not the
same pressure on occupants to close these when not required as they do not present a security
issue, it can be difficult to see if they have been left open, any motor driven unit may fail open
or the control system may become incorrectly programmed, and there is little evidence on
whether they will be airtight for the whole life of the building.
It is worth remembering that mechanical ventilation with heat recovery (MVHR) is becoming
increasing common within continental Europe and is well worth considering even for domestic
properties, however they is the need to ensure the occupants will be able to successfully operate such a system and maintain it.
It is critical if the building is to be a low energy one that air conditioning is avoided at all cost.
In the UK’s climate for example, the need for air conditioning does not arise from high external
temperatures usually, but from too high solar, electrical or metabolic gains. Little can be done to
tackle the latter, but the others are amenable to adaption—as are expectations and clothing
levels. The engineering out of air conditioning is a particularly good use of a thermal model.
There is the need to identify early in the design process how much of any overheating is due to
these, and how much it could be reduced by reducing the solar gains or reducing the electrical
Shops and retail premises 5–8 l/s/p
Sports centre halls 8-12 l/s/p
Swimming pools 4-6 or 8-10 if extensive water features
Toilets Regulations usually apply; opening windows of area 1/20th. of floor
area or mechanical ventilation at 6 litres/s per WC or
3 minimum for non-domestic buildings; opening
windows of area 1/20th. of floor area (1/30th. in Scotland) or
mechanical extract at 6 litres/s (3 ACH in Scotland) minimum
for dwellings
Transportation buildings (inc. car
parks)
6 for car parks (normal operation) 10 (fire conditions)
The Design and Delivery of Low Carbon Buildings
Ventilation 8
load. The thermal model should be run with a series of values for these gains and the results presented to the whole design team. Imagination and a joined up design team are needed here.
For example, could the IT equipment be spread throughout the building therefore reducing the
need for a cooled server room, or could more heat-tolerant processors be considered? Could recessed light fittings be replaced by simple more efficient batten ones, or the artificial lighting
load be reduced by adapting the relevant lighting codes to the aims and objectives of the
client—namely a low carbon building. Could higher thermal mass and passive night time
cooling of the building be used rather than active cooling? Such multidisciplinary thinking is
unlike to occur unless the whole design team is involved from the start. Except in very rare
situations, the use of active cooling should be considered a failure of architecture and of the design team in general, as it will either greatly increase the carbon footprint of the building in
use, or require far larger renewable electricity generation from the site—the cost of which is
likely to be considerable.
Table 4. Advantages and disadvantages of mechanical and natural ventilation.
Natural Mechanical with heat
recovery
Advantages Disadvantages Advantages Disadvantages
Easy to operate Hard to use night time
cooling
Much more energy
efficient in winter.
Higher maintenance
cost.
Reduce size of plant
room.
Ingress of external noise
in some environments
Easy to use for night
time cooling
Higher electrical load
(because of fans)
User control Can not recover heat
from ventilated air.
Predicable performance:
will still work in
summer if needed
Larger plant room
Low maintenance costs
(unless automatic
openers used)
Risk of draughts Better control of
external noise
Need to leave room for
ductwork
No fan energy Difficult to achieve
night time cooling
without the use of
louvered systems and
these may prove to no
be airtight, or be left
open in winter.
Ability to deal with
highly polluted
environments
Potential for noise and
higher room-to-room
sound transmission
A greater physical and
psychological
connection to the
outdoor realm.
Ventilation rate is likely
to be at its lowest in
summer, just when it
need to be at its greatest
Risk of draughts with
some systems, although
these should be easy to
engineer out
Can not deal with
highly polluted
environments
User control: normally
little and adds cost
Potential for fan noise
as moving elements
age. Again, good
engineering can reduce
this
General Principles There are four possible combinations of natural and mechanical systems:
1. Natural supply and extract. Essentially openable windows, but possibly with the use of
louvres. Heat recovery is not possible, so all energy in the ventilation air will be lost. However
no energy is needed to provide the ventilation air. 2.Natural inlet, mechanical outlet. Typically fans in roof areas to extract air provided by
opening windows and louvres. The fans cause a negative pressure in the building which sucks
air in through the windows and other openings. Although a heat recovery unit could be used to
The Design and Delivery of Low Carbon Buildings
Ventilation 9
recover the energy in the outgoing air, there is no ductwork to reinject it back into the building. If ductwork is created to allow for this, it would seem most sensible to use this to provide the
incoming air in the first place—which is option 4, below.
3. Mechanical inlet, natural outlet. Air is blown into the building using fans and is allowed to exit from windows and other openings. No opportunity to recover energy from exhaust air.
4.Mecanical inlet and outlet. Supply and extract fans inject and remove air from the building.
Easy to include heat recovery and hence the method adopted by Passivhaus. In summer opening
windows can be used to remove the energy requirement of the fans. Easily to include night time
cooling without compromising surety.
Natural Ventilation: Details
Natural ventilation can be defined as ventilation that occurs due to air moving through the
building under the forces of buoyancy and wind. Natural ventilation can be used in most
building types, however care will be need if the building is great than 15m in depth [A2-8]. If
gains are greater than 40 W/m2 CIBSE Guide-A (p2-8) concludes that some form of mechanical
ventilation maybe required [A2-8,ref27]. However, a classroom of 70 m2 with 30 occupants
implies a metabolic gain of 43 W/m2, in addition there might be a lighting gain of 10 W/m
2 and
a few computers, yet most classrooms in the UK, USA and Europe do not rely on mechanical
ventilation. This is a typical example of conservatism within the building services industry and
a point where clients and architects need to question all assumptions and preferably do their own back-of-the-envelope calculations.
Table 5 and Table 6 show standard recommendations for options for various room sizes and levels of gains. These should be seen only as a starting point as experience has shown that good
architecture and engineering will be able to provide a successful naturally ventilated solution for
larger room and great gains.
Table 5. Natural ventilation options and their effective depth (adapted from CIBSE-B, p2-9)
Strategy Effective depth relative to room height
Single sided, single opening 2 x floor-to-ceiling height
Single sided, double opening 2.5 x floor-to-ceiling height
Cross flow 5 x floor-to-ceiling height
Stack ventilation 5 x floor-to-ceiling height
Atria 10 x floor-to-ceiling if centrally located
Table 6. Relationship between design features and heat gains (adapted from CIBSE-B, p2-9)
Design features Total heat gains* (W·m–2) floor area
10 20 30 40
Minimum room height (m)
2.5 2.7 2.9 3.1
Controllable
window opening
(to 10 mm)
Essential Essential Essential Essential
Trickle vents for
winter
Essential Essential Essential Essential
Control of indoor
air quality
May be required May be required Essential Essential
Design for
daylight to reduce
gains
May be required Essential Essential Essential
Daylight control
of electric lighting
May be required May be required Essential Essential
100% shading
from direct sun
May be required Essential Essential Essential
The Design and Delivery of Low Carbon Buildings
Ventilation 10
Cooling by
daytime
ventilation only
Essential Essential Problem Problem
Cooling by day
and night
ventilation
Not necessary May be required Essential Essential
Exposed thermal
mass
Not necessary Not necessary Essential Essential
* i.e. people + lights + office equipment + solar gain
The following schematics show the six most common ways natural ventilation can be used in a
building. The equations associated with each of these might seem slightly complex for early
stage design work as several of the parameter they contain might well not be known, for
example the separation between the top and bottom of the windows. However the key is to
realise in each case what the sensitivities are and how the ventilation rate can be improved in
each case of by changing the strategy, e.g. from single to double sided.
In a naturally ventilated building, the flow of air will arise either from the difference in air
pressure on across the building due to wind, or from the lower density of warm over cold air. The latter will cause the warm air to rise and exit through the top of windows or the windows or
other openings at high level. In general wind driven cross ventilation is far more effective than
single sided ventilation that relies on buoyancy. Because buoyancy drive ventilation relies on
there being a substantial difference between the internal and external air temperature it is
particularly ineffective in summer as the outside air may well be of a similar temperature as the
inside. Hence although such a strategy is likely to provide enough ventilation to keep carbon
dioxide concentrations at reasonable levels, it is unlikely to be able to help cool the building.
(Note: because the internal/external temperature is lower in summer the mass of fresh air
needed to remove each unit of heat from the building is also larger.) The effectiveness of each of the solutions is examined in Error! Reference source not found. for various conditions.
In all cases the effective area of a number of opening across which the same pressure difference is applied—e.g. single sided ventilation with two low level inlets and two high level outlets, or
wind driven double side ventilation—can be obtained by simple addition. When buoyancy and
wind effects are possible, then it is likely the situation will be dominated by whichever gives the
greatest flow rate using the equations given.
Strategy: wind driven single sided, openings all at same
height
Sensitivity
Flow rate is proportional to the opening
area. So doubling the area of opening will
double the flow. Flow is also proportional to
the wind speed. Insensitive to
internal/external temperature difference. At
low wind speeds little flow will occur and
Buoyancy driven flow will dominate.
The Design and Delivery of Low Carbon Buildings
Ventilation 11
Strategy: buoyancy driven single sided, openings all at same height Sensitivities
Flow rate proportional
to the square root of
the height between the
mid points of the top
and bottom window.
Also proportional to
the root of the
temperature
difference.
Proportional to root of
the opening area.
Single sided, openings all at same height, buoyancy driven
Note ha is the distance
between the top and
bottom of the opening.
Double sided, wind driven
Double sided, temperature driven
Note Za is the distance
between the midpoints of
the two openings.
Double sided, wind and temperature driven
The Design and Delivery of Low Carbon Buildings
Ventilation 12
Table 7. Relative effectiveness of natural ventilation strategies. Heat removed, watts (at a wind speed of 3
m/s (where relevant), an internal air temperature of 20°C, for total area of openings of 1 m2, a height
difference (where relevant of 2 m), and two external temperatures represent summer and winter
conditions.
Heat removed (watts)
Strategy No. of openings Method External Temp =
5°C
External Temp =
18°C
Single sided One Wind 1395 186
Single sided One Buoyancy 3347 560
Single sided Two Buoyancy 5664 273
Double sided Two Wind 3969 529
Double sided Two Buoyancy 2003 96
Double sided Two Both 3969 529
From Table 7 we can conclude that:
• None of the strategies provide much cooling in summer—just when it might be needed,
and hence larger (or a great number of) openings might be needed if the gains are
substantial (a classroom, for example, might have 3 kW of gains).
• A large single hole can provide a reasonable amount of air if it is 1m high—a top hung
window would not be this.
• In the single sided case, the air flow, and hence the cooling, is improved if the single
1m2 opening is replaced by two openings, each of area 0.5m2, separated vertically by
1m.
• The greatest flow rates and hence cooling will come from a double sided solution where
there is a difference in height between the openings on either side of the space.
Mechanical Systems with Heat Recovery (MVHR): Details
Mechanical ventilation may be defined as the movement of air around a building under the
assistance of fans. The incoming air is either:
1. via displacement (laminar flow), i.e. at low level and modest speeds and at a temperature
close to the room temperature. Warm air is then extracted at a high level. Or
2. by mixing (turbulent flow) at higher speeds with complete mixing with the room air typically
via ceiling supply.
If the system is primarily design to supply winter air to ensure reasonable levels of air quality
the system can be modest in scale. If in addition there is the need to provide cooling in summer
than the much larger supply rates will imply a considerably larger system and corresponding
energy costs. Hence the common approach in low energy buildings of using a small mechanical
system with heat recovery in winter and providing the much larger quantities of air needed for
cooling in winter using opening windows. Figure 1 shows an example MVHR system. The first
thing to note is the complexity compared to a window. It is worth noting that the lifetime of
parts with moving elements is likely to be far less than the lifetime of the building.
Another common approach in buildings with extensive corridors is to supply the fresh air to the
main rooms and extract it from the corridors with little return ductwork. This however requires
a “hole” of some form between each room and the corridor, this can be a source of unwanted noise transmission.
The Design and Delivery of Low Carbon Buildings
Ventilation 13
Figure 6. Basic domestic scale MVHR system.
The Design and Delivery of Low Carbon Buildings
Ventilation 14
Figure 7. An MVHR unit being used to supply warm air in winter (top) and cool air in summer (bottom).
http://www.sunwarm.com/MVHRbrochure.pdf
The Design and Delivery of Low Carbon Buildings
Ventilation 15
Figure 8. Section through a domestic scale MVHR. http://www.sunwarm.com/MVHRbrochure.pdf
Earth Tubes
One further approach to the provision of fresh air to a building, and that can be used with either
a mechanical or natural system, is the earth tube. The temperature of the ground a few metres
below the surface is typically similar to the mean annual air temperature (approximately 12ºC in
the UK, depending on location). This means that if the supply air is brought to the building via a long tube buried in the ground its will adjust is temperature closer to the ground temperature.
Thus in winter cold air will be slightly warmed and in summer hot incoming air will be slightly
cooled. Thus free heat of cooling is provided. The approach has been used in the UK, but is far more popular in locations where there is a much greater swing in annual temperature, for
example Sweden and the USA. Figure 9 illustrates the basic principle. It is worth noting that is
possible to model the effectiveness of the approach in standard thermal models such as IES.
Figure 9. Installation of an earth tube system in Wisconsin, USA. The inlet is the upright pipe just in front
of the earth mover, the building is on the left.
http://www.zigersnead.com/current/blog/post/earth-tubes/04-06-2008/1045/