an air heating and cooling system includes a heat pump , illustratively devoid of auxiliary electric resistance type air heating structure , operative to provide refrigerant - based heating or cooling of air being delivered to a conditioned space , a fuel - fired modular blower selectively operable to generate combustion heat , and a control system associated with the heat pump and the modular blower . the control system has a heat pump thermostat electrically connected to a modular blower control and operative to transmit to the modular blower a first signal indicative of heating operation of the heat pump , and a second signal indicative of a need for alternative heat during a heating demand cycle . the modular blower is operative , in response to receiving both signals , to provide combustion - based air heating in place of refrigerant - based heat pump air heating .