This is often an electrical heater. When the outside air will get to around six*c (42.8F) the heater will occur on and heat the air up to protect the parts inside of from frost. In any other case This may freeze the heating and cooling coils inside and burst them. https://sap-ewm63431.anchor-blog.com/14652033/the-best-side-of-air-handling-units-ahus