An after-heating element is often installed if you want to be able to control the supply air temperature. A low supply air temperature may, at times, feel like a draught. By ensuring that the supply air temperature does not fall below a certain level, you achieve a good, comfortable indoor climate.
Energy neutral houses, such as houses built in line with the German Passive House concept, have very low heating requirements. The houses therefore have no traditional heat supply, such as underfloor heating. The ventilation unit heats the dwelling, but during cold spells a small amount of supplementary heating may be required. During such periods the after-heating element will heat the supply air so the dwelling remains comfortable.