Moisture loads in apartment buildings and their impact on central AHU functions and energy use

In energy calculations, high temperature efficiency of air handling units (AHUs) is often sought to achieve lower theoretical energy consumption for multi-family buildings. However, for those AHUs that are monitored, it appears that the moisture in the extract air sometimes leads to more condensation, freezing and defrosting than expected, resulting in increased heating energy and heat output requirements. The moisture content of extract air in multi-family buildings is a challenge to maintaining the function and performance of the ventilation system and its components in cold weather. The higher moisture content of the extract air compared to the supply/outdoor air is due to moisture generated by people, showers, cooking, washing and drying clothes, potted plants, etcetera. In Swedish multi-family dwellings, the moisture generation is about 2.0 g/kg air (varying 1 - 3 g/kg air during the day), and depending on the behaviour of the occupants, the moisture generation can be even higher or lower. It is difficult to derive theoretically the actual energy and power demand due to defrosting needs, so this must be based on analysis during winter of detailed measured data from the AHUs in actual operation in the multi-family buildings. With calculated average efficiency as function of outdoor temperature and extract air humidity derived from measured data better energy calculation can be performed.