Abstract: An external gas-fired heater for heating a water/glycol fluid is comprised of a housing having a combustion chamber therein. The chamber has a coil through which the water/glycol fluid is circulated and heated by a cylindrical gas burner disposed inside the coil with the coil spaced thereabout. The housing is adapted for mounting outside a building to which the heated fluid is convected for heat exchange with heating devices contained within the building. A cover is removably secured to the housing for access to the internal heater devices and is equipped to convect air within the housing for the air-gas mixing blower. Quick connect couplings permit the housing to be easily removed and replaced, if necessary. The cover also provides ease of access to the heater devices for servicing or replacement.