A heater includes a heater housing; a fuel cell stack assembly within the heater housing and having a plurality of fuel cells; a combustor within the heater housing for combusting a mixture of fuel and air to form a heated combustor exhaust that is discharged into the heater housing; a combustor fuel supply conduit for supplying the fuel to the combustor; an anode exhaust conduit for communicating anode exhaust from the fuel cell stack assembly out of the heater housing; and a combustor bypass conduit in fluid communication with the combustor fuel supply conduit and the anode exhaust conduit for allowing a portion of the fuel to bypass the combustor.