This is usually an electrical heater. When the outside air will get to all around 6*c (42.8F) the heater will occur on and heat the air up to guard the elements within from frost. Or else This may freeze the heating and cooling coils within and burst them. AHUs casing https://air-handling-units-ahus05937.pointblog.net/fascination-about-ahu-s-74439619