Why does the truck heater only blow hot air?

A truck heater only blows hot air because it relies on the engine's coolant system to generate heat. The engine heats the coolant as it runs, and this hot coolant circulates through the heater core, warming the air blown into the cabin by the blower fan. If the heater is only blowing hot air, it may indicate that the coolant is not circulating properly or that the temperature control is set to maximum heat. Additionally, if the system lacks a functioning thermostat, it may lead to constant hot air output.