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To improve energy eﬃciency, mechanical ventilation systems are often

equipped with heat recovery for recovering the heat contained in exhaust air.

This heat is in most cases given back to suppl y air. Such heat recovery exchan-

gers are eﬃcient during both cold and hot seasons, saving heating and cooling

energy. Some of these heat exchangers also transfer humidity, thus decreasing

the energy used to humidify or dehumidify the air.

As shown in Figures 0.3, 0.4 and 0.5, air handling units may have parasitic

shortcuts and leakages. Such leakages have often been observed in buildings

(Presser and Becker, 1988; Hanlo, 1991; Fischer and Heidt, 1997; Roulet

et al., 1999). They can dramatically decrease the eﬃciency of ventilation and

heat recovery (Roulet et al., 2001). Moreover, leakage in a building’s envelope

allows indoor air to escape outdoors without passing through the heat recovery

system. In addition, these units use electrical energy for fans, which may, in

some cases, exceed the saved heat. The inﬂuence of these various phenomena

on the real energy saving is addressed in this chapter.

Types of heat exchangers

Water-to-air heat exchangers are in most cases made out of ﬁnned tubes in

which the water circulates. The ﬁns increase the exchange area between the

exchanger surface and the air.

The heat exchangers most commonly used for heat recovery are plate heat

exchangers, rotating heat exchangers and heat pipes. Most common air-to-air

exchangers are plate heat exchangers, in which the exhaust air is blown in

several channels limited by plates made of glass, metal or plastic (see Figure

5.4). The other side of these plates is in contact with inlet air, so that heat

Figure 5.4 Close view of a ﬂat plate heat exchanger

84 Ventilation and Airﬂow in Buildings

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Sharp JX-9400 Technical Information Seite 105