This is frequently an electrical heater. When the outside air receives to close to 6*c (forty two.8F) the heater will occur on and warmth the air up to safeguard the elements inside from frost. In any other case this could freeze the heating and cooling coils within and burst them. https://aircooledchillers71592.mybjjblog.com/the-fact-about-ahu-s-that-no-one-is-suggesting-47034365