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Energy in buildings
Energy in buildings

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2.3.2 Ventilation heat recovery

Many office buildings use mechanical ventilation driven by electric fans. One way of reducing ventilation heat loss is to use mechanical ventilation with heat recovery (MVHR), which involves allowing warm outgoing air to preheat cold incoming air. This can be done by passing both air streams through a heat exchanger (see Figure 19), consisting of multiple layers of thin, flat, metal or plastic plates with incoming and outgoing air passing through alternate layers. This gives a large area through which heat can flow. Obviously such a system can be used only if the inlet and outlet ducts are adjacent to each other.

Described image
Figure 19 Flat plate heat exchanger
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This diagram shows a layer cake of ductwork. The two sets of alternate layers carry fresh air into the building and stale air out from it. Heat can pass through the walls of the ducts into the adjacent air stream, although the streams themselves are separate. At the top left of the diagram three blue arrows labelled ‘cool fresh air’ show air entering the first air path through the heat exchanger. At the bottom right three yellow arrows show this air stream emerging and are labelled ‘warm fresh air’. At the top right three red arrows labelled ‘warm stale air’ show air entering the second air path through the heat exchanger. At the bottom left of the diagram three orange arrows labelled ‘cool stale air’ show this air leaving.

Figure 19 Flat plate heat exchanger

MVHR is a mixed blessing. On the one hand it gives controllable ventilation adjustable to almost every room. On the other it requires complex ductwork and air pumping, which can consume considerable amounts of electricity.

MVHR systems are available for domestic applications (see Figure 20) but it is essential that they are installed in buildings that are airtight to start with, otherwise any attempt to pump air around the system may just increase the flow of air through unwanted air infiltration paths. However, once the fabric heat losses of a building have been tackled with thick insulation and high-performance windows, this may be the only satisfactory way to deal with the remaining major heat loss, that from ventilation.

Described image
Figure 20 Whole-house mechanical ventilation with heat recovery (Source: Nicholls, 2002)
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This is a diagramatic representation of a single storey house shown looking obliquely down on it. At the left of the diagram is a labelled living room containing a sofa. In the middle of the diagram is a labelled bathroom and at the right of the diagram is a labelled kitchen. At the top of the diagram above the ceiling is a grey box labelled ‘heat exchanger’. Coloured lines show the path for air ducts between the heat exchanger and the rooms below. A blue duct labelled ‘intake’ is shown with an arrow indicating an air flow into the heat exchanger. On the opposite side three yellow ducts emerge flowing into each of the rooms with arrows indicating the direction of air flow. Another set of ducts coloured red are shown rising from each of the rooms with small arrows showing that the direction of the flow of air is into the heat exchanger. An orange duct labelled ‘exhaust’ emerges from the opposite side of the exchanger with an arrow indicating that the air flow is outwards.

Figure 20 Whole-house mechanical ventilation with heat recovery (Source: Nicholls, 2002)

Activity 8

What is the difference between ventilation and infiltration?

Answer

Ventilation is air movement that is considered essential and may be deliberately encouraged by opening windows or using mechanical ventilation. Infiltration is uncontrolled air movement through various cracks in the building fabric or through air paths such as chimneys.

Activity 9

A small 19th century terraced house has a volume of 240 m3, an air change rate of 2 ACH and an inside–outside temperature difference, ΔT of 20 K. What is its ventilation heat loss rate in kilowatts?

Answer

Using the equation given above:

  • Qv = 0.33 × n × V × ΔT watts

and taking n = 2 ACH, V = 240 m3 and ΔT = 20 K

  • Qv = 0.33 × 2 × 240 × 20 watts = 3.17 kW

    (i.e. rather a lot).