An air handler is used to condition and circulate air as part of a heating, cooling and ventilating system. It contains a blower, heating and/or cooling elements, a filter rack or compartment, sound attenuators, and dampers.
The air handler can be an up flow air handler, down flow air handler, or horizontal flow air handler. Air handlers are connected to ductwork that distributes the conditioned air through the building, and returns it back to the unit.
It can work in combination with a boiler as a hydronic heating application and can replace your forced air furnace as a heating source.