The heat exchanger's output (burner side) in directly heated fresh air units with bypass control increases considerably, thus reducing the formation of condensate water to negligible amounts.

In order to attain low blow out temperatures, only some of the warming air is directed via the heat exchanger and subsequently distributed amongst the remaining body of air. The ratio of mixture of the two bodies of airflow controls the blow out temperature of the unit.
Bypass units are always equipped with dual control blower burners.