The type of boiler used in an HVAC system depends on the application and the size of the system. Generally, boilers used in HVAC systems are either hot water boilers or steam boilers. Hot water boilers are typically used for smaller residential and commercial applications, while steam boilers are used for larger industrial applications. Hot water boilers are typically fueled by natural gas, propane, or electricity, while steam boilers are typically fueled by natural gas or oil. Depending on the application, the boiler may also be equipped with a heat exchanger to provide additional heating or cooling.