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Passive Heating

The main objectives in passive heating are to minimise all areas of heat loss from the building whilst maximising heat gains within each space.

Minimising Heat Losses

One of the main sources of heat loss from many buildings is air infiltration: warm air leaking out of high level vents and cracks, drawing cold air in at a lower level. In a manner similar to preventing infiltration gains in summer, infiltration losses can be minimised by:

  • Airtight Construction
    This requires using windows and doors with good quality airtight seals as well as caulking and sealing cracks/gaps around them. It also means carefully considering the details of all junctions and building section to ensure a complete seal.
  • Avoiding Thermal Bridges
    A thermal bridge is any part of a construction through which heat can travel faster and with less resistance than other parts. A poorly designed aluminium window frame, for example, can often pass more heat than the entire area of the double glazing it holds. Similarly, metal studs in a wall provide a means by which heat flow can easily bypass the insulation bats packed between them. Avoiding thermal bridges simply requires some forethought and attention to detail in both the design and construction phases.
  • Non-Porous Materials
    Using less porous materials in the building fabric can prevent some of the air infiltration that passes through the materials themselves. However, careful consideration must be given to the potential for interstitial condensation when using materials with high vapour resistances. This is especially true when internal temperatures are up around 18-20°C and external are below zero.

    Another significant source of heat loss is by conduction through the fabric of the building. Conduction losses can be minimised by:
  • Insulation
    In very cold climates, all surface in a building that are exposed to the outside should be highly insulated. The best location for insulation is always on the outside surface (preventing losses or gains before they reach the main structure), however this may not always be practical. In climates where heat loss is only an issue at night (such as in a desert), it may be preferable to store daytime heat in the building fabric for release later at night when the temperature falls. In these cases, some uninsulated heavyweight materials with a high time lag could be strategically placed adjacent to sensitive areas of the building.
  • Using Double Glazed Windows and Heavy Drapes
    Glass is a very good thermal conductor so the more glass in a building, the greater the overall heat loss. However glazing allows in daylight and can often provide direct solar gains during the day, so it is often a desirable element of the equator-facing facade. Therefore, it is important to minimise any losses by using double-glazing (or even triple glazing in some areas) to increase the thermal resistance of the window. Also, large areas of cold glass can generate a convective loop within a room, cooling the air immediately near it so that it becomes heavier and sinks to the floor, continually drawing warm air from above to replace it.

    Such a process can quickly and effectively cool the entire room. The convective effect can be prevented by the use of full-height curtains or thick drapes with sealed pelmets at the top.
  • Avoiding Thermal Bridges
    A thermal bridge is any part of a construction through which heat can travel faster and with less resistance than other parts. A poorly designed aluminium window frame, for example, can often pass more heat than the entire area of the double glazing it holds. Similarly, metal studs in a wall provide a means by which heat flow can easily bypass the insulation bats packed between them. Avoiding thermal bridges simply requires some forethought and attention to detail in both the design and construction phases.

Maximising Heat Gains

Passive heating techniques are usually based on the collection and distribution of solar energy as that tends to be the highest grade thermal source available on most sites. Another source is geothermal energy, however this is often very low grade heat requiring large air flows to have any significant effect. However, in areas with high summer temperatures and very cold winters it has been used quite successfully.

Passive Solar Heating

Solar radiation is very important for winter heating. This assumes, however, that there are some solar gains available throughout the day, either direct or diffuse. At very high latitudes, where the Sun is at a very low altitude in winter and may only be visible for 4-6 hours, solar heating may not be an effective a solution. However, in most parts of the world it is and should definitely be considered in any passive design solution.

Effectively collecting solar heat usually involves using glass covering a dark material with a high thermal mass, thus invoking the well-known greenhouse effect. There are many ways to harness this energy, however the following generic systems illustrate the major principles involved.

Geothermal Heating

The deep ground temperature usually tends towards the annual average air temperature. This depends also on the nature of local ground cover and the ground water table, however it is usually very close. Thus, in areas which are relatively hot in summer and cold in winter, the ground temperature will usually be colder then the air temperature in summer and warmer in winter

It is possible to make use of this differential by drawing any air that enters the building first through an underground plenum or a series of buried pipes. As the cold air comes into contact with the sides of the plenum or pipe, it will absorb some of the heat from these surfaces. The longer the air is in contact with the surfaces, the closer to the ground temperature the air will come. This can be a source of heating in winter as well as cooling in summer.

The effect of pipes and plenums is usually relatively small as the volumes of air required to enter a building mean that either the air does not spend much time in them, or they have to be very long and extensive. As a result, geothermal systems tend to be used in combination with heat pumps. These compress or expand the medium to significantly change its temperature before it enters the underground system. The higher temperature differential leads to a much greater heat exchange which, when returned to its original pressure, means that the temperature of the medium is significantly changed by the process.

Related Links

Passive Solar Heating - Sustainable Building Sourcebook
http://www.greenbuilder.com/sourcebook/PassSolGuide1-2.html
Thermal Bridges in Buildings - CBD-44
http://www.nrc.ca/irc/cbd/cbd044e.html
A Catalog of Thermal Bridges in Commercial Construction (PDF)
http://www.commerce.state.mn.us/pages/Energy/Builders/pdfs/ThermalBridgesCat.pdf
Passive Design: Natural Ventilation
Passive Design: Direct Gain

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