Combined airtoair heat exchanger and a heat pump

Some air-to-air HR units incorporate an additional HP for further exhaust air HR. These systems consist of a conventional balanced air-to-air HR unit combined with a balanced ventilation system. Inserted in the exhaust duct is the evaporator unit of an HP This extracts further heat, which is transferred to the supply air stream via a condenser unit located in the supply duct. COP HP values for domestic systems of 3.0 and larger can be achieved. Output air temperatures range typically from 30°C to 50°C. Obviously, the air-to-air heat exchanger and the heat pump are competing for the same heat source. The heat pump typically comes after the heat exchanger, so its performance is reduced accordingly because it has a cooler heat source. The better the thermal efficiency of the air-to-air HR system, the worse the opportunities for HR by HP are.

Nevertheless, when used in an air tight, well-insulated building, this approach can offer additional useful heat gain and reduce the period in which auxiliary space heating becomes necessary. Efficient use depends on a good control strategy that prevents the operation of conventional space heating, while sufficient heat is being supplied by the HR unit. Air distribution must also be controlled for optimum performance. This means satisfying the ventilation needs of each room and providing sufficient air flow to meet thermal requirements. The advantage of air-to-air HR with HP is that:

• additional, enhanced HR is possible from the exhaust air stream. The disadvantages of air-to-air HR with HP are:

• as with air-to-air HR, air-tight construction is essential and costs in terms of operation and maintenance have to be considered; and

• extra capital costs and maintenance fees will be necessary.

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