Ventilation

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


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):


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.


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


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.


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


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)


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


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


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.


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


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.


Ventilation 13

Figure 6. Basic domestic scale MVHR system.


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


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/

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