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

Abstract One modern method of lowering building energy use is heat exchange between outgoing and incoming air. Exhaust air moisture (partly) condenses in the heat exchanger, giving liquid water to be removed. With low absolute humidity of inlet air especially at low outdoor temperatures, a significant drying effect results for the air inside the building. Increasing air humidity by, for example, spraying water or increasing watering of plants will give an energy penalty. This paper presents an analysis of the energy efficiency (recovered versus input exergy) of a modern house equipped an exhaust air heat recovery system and external heating supplied by a geothermal heat pump. Indoor temperature is 22 °C and relative humidity is 50 or 30%. Temperature and humidity of inlet air and ventilation flow rates are variable and the effects on power use by the heat pump are assessed. Results show that without moisture recirculation a water loss up to 1 kg/h from the building may occur. Several kWh of electricity may be consumed daily by a (geothermal) heat pump supplying heat depending on outdoor temperature, and indoor and outdoor air humidity. Besides water consumption, this may imply 10–15% of total electricity use.