2.3 Cutting ventilation losses
Buildings also lose heat by ventilation, i.e., the passage of air through them. In houses this normally means the controllable air movement through openable windows, extractor fans, or, in the case of larger buildings, a mechanical ventilation system. However there is also an uncontrolled component called infiltration. This is the air flow through gaps in the fabric of the building – cracks around windows, doors and electrical or plumbing outlets, or between skirting boards and floors. In common use, the term infiltration is used as a component of ventilation rather than something completely different.
Some form of ventilation in a building is essential. For example in a house it is needed in living spaces:
- to provide combustion air in winter for boilers, fires and gas cookers, although it is not necessary for heating systems with balanced flues (see Section 3.1) or for electric fires
- to remove moisture from kitchens, toilets and bathrooms, which should be equipped with controllable ventilation openings and/or their own extractor fans
- to provide fresh air for occupants and to keep them cool in summer.
Ventilation is also needed in other areas of the house, to remove moisture in the roof space or loft above the insulation, or under suspended ground floors (which are usually of wood, but in more recent construction can be made of concrete). Figure 17 illustrates the ventilation and infiltration air paths through a normal house and also where it is important to maintain essential ventilation. Note that an air flow must be maintained through the loft space and not be blocked by insulation pushed into the eaves of the roof.
The main driving forces for this air movement are the buoyancy (or stack) effect of warm air and the wind pressure on a building. Warm air inside a building in winter is less dense than cold air outside and, like a hot air balloon, will tend to rise. This has the effect of sucking in cold air from outside into the rooms on the ground floor. Wind pressure will attempt to force air through gaps in the walls on the windward side of the building and out again on the leeward side. Wind speeds increase with height above the ground, so wind-driven infiltration in high-rise buildings can be a major problem.
Houses are normally naturally ventilated, i.e. they are dependent mainly on the stack effect to provide adequate air movement.
In larger buildings mechanical ventilation is often used. This is often also the means of space heating, with air being centrally preheated (or cooled in summer) before being distributed throughout the building and extracted again through more ductwork. The term ‘air conditioning’ normally implies the use of mechanical ventilation with central air cooling.
The key factor in determining the ventilation heat loss in a building is the ventilation rate, i.e. the average rate at which air flows through it. Any warm air that escapes through the windows, doors and various gaps in the outer fabric is immediately replaced by a new supply of fresh cold air from outside. We may be unaware of how substantial this ‘invisible’ air really is – an average house contains about a quarter of a tonne of it!
The ventilation rate is normally specified as the number of complete air changes that take place per hour (ACH). Actually measuring this scientifically is a fairly complex process. Typically, in a new, well-built, naturally ventilated house where windows are closed, and with few gaps in the building fabric, it might take two hours for the air to be completely replaced by new, incoming air. We would say that the ventilation rate of this house was 0.5 ACH.
If the volume of a house is V m3, and the air change rate is n ACH, then the total amount of air passing through it per hour will be n × V m3. This air needs to be heated up through the temperature difference ΔT between the external temperature and the internal temperature. The energy required to raise one cubic metre of air through one kelvin is 0.33 watt-hours, i.e. its heat capacity per cubic metre is 0.33 Wh m–3 K−1. Thus the total ventilation heat loss, Qv , will be:
- Qv = 0.33 × n × V × ΔT watts
For any given building, the actual ventilation rate will depend on its age and location. Many buildings built before 1918 had an open coal fire and chimney for almost every room. They are also likely to have been designed for gas lighting, with high ceilings and air bricks in the walls to remove the combustion fumes. Draughty wooden ground floors are also common. Since the pressure of the wind on a house has a great influence, buildings in sheltered locations are likely to have a lower air change rate than those in exposed positions. For example, a house built before 1918 might have an average ventilation rate of over 2 ACH in an exposed location.
After 1920, houses and offices were designed for electric lighting and had lower ceilings. It was only in the 1970s, with the advent of cheaper electricity and gas central heating, that houses began to be built without open fireplaces. They could then (theoretically at least) be designed to be reasonably airtight. Section 2.3.1 looks at how to reduce heat loss by improving the airtightness of buildings.
Heat loss can also be reduced by recovering some of the heat from ventilation air before it is released. This is the topic of Section 2.3.2.