Energy efficiency of exhaust air heat recovery while controlling building air humidity: a case study

One modern method of lowering the energy use by buildings is to exchange heat between ventilation air leaving a building and incoming fresh, typically colder air. Moisture in the exhaust air condenses in the heat exchanger, giving liquid water that must be removed. With a low absolute humidity of inlet fresh air especially at low ambient temperatures, a significant overall drying effect results for the air inside the ventilated building. Increasing the air humidity by, for example, spraying water or increasing the watering of plants may give an increased energy penalty. This paper presents the analyses of the energy efficiency of a modern house equipped with external heating supplied by a geothermal heat pump and an exhaust air heat recovery system (EAHR). Indoor temperature is controlled at 22°C and the relative humidity at between 30 and 70%. Temperature and humidity of inlet air and its flow rate into the building are varied and the effect on the power use by the heat supply is assessed. The results show that without moisture recirculation in the EAHR a net water loss up to 1 kg/h from the building may be possible. Also several kWh electricity may be consumed daily by a (geothermal) heat pump that supplies heat to a 99 m2 modern house depending on outdoor temperature and indoor and outdoor air humidity. Besides water consumption, this may imply 10-15 % of total electricity use.