referring initially to fig1 , in a representatively illustrated embodiment thereof the present invention provides a specially designed reversible circuit heat pump - based system 10 comprising an indoor heat pump coil unit 12 ( representatively , as denoted in fig1 , having no electric resistance type of secondary heating structure ), an outdoor heat pump unit 14 , and a fuel - fired supplemental heating source which is representatively a non - condensing type gas - fired modular blower 16 . illustratively , the indoor heat pump coil unit 12 rests atop the modular blower 16 which has the schematically depicted blower and heating sections 18 and 20 , the heating section 20 being supplied with gas ( or another type of fuel ) via a fuel supply line 22 . the indoor heat pump coil unit 12 has an indoor refrigerant coil 24 which , via refrigerant lines 26 and 28 , is operatively coupled to an outdoor coil and associated circuit components ( not shown ) within the outdoor unit 14 . three control components are associated with the system 10 — a heat pump thermostat 30 representatively mounted external to the indoor heat pump unit 12 on a wall 32 , a specially designed modified modular blower control 34 representatively associated with the blower section 18 of the modular blower 16 , and a heat pump controller 36 representatively mounted on the outdoor heat pump unit 14 . the heat pump thermostat 30 is electrically coupled to the modular blower controller 34 , as schematically depicted by the numeral 38 , and to the heat pump controller 36 , as schematically depicted by the numeral 40 . during cooling operation of the heat pump system 10 , the blower section 18 of the non - condensing fuel - fired modular blower 16 sequentially flows system return air 42 upwardly through the modular blower heating section 20 ( which is unfired during heat pump cooling cycles ), and then upwardly across the indoor refrigerant coil 24 which cools the air 42 so that it exits the heat pump 12 as conditioned ( i . e ., cooled ) air 42 a . alternatively , the heat pump coil unit 12 may be a downflow or horizontal unit if desired . during normal heating operation of the heat pump system 10 , the blower section 18 of the non - condensing fuel - fired modular blower 16 sequentially flows system return air 42 upwardly through the unfired modular blower heating section 20 , and then upwardly across the indoor refrigerant coil 24 which heats the air 42 so that it exits the indoor heat pump coil unit 12 as conditioned ( i . e ., heated ) air 42 b . using a specially designed overall control technique for the system 10 , as subsequently described herein , in a heating cycle thereof the system 10 normally produces the heated discharge air 42 b using only the refrigerant heat from the indoor coil 24 but if its heating output is detected as being insufficient to meet a particular heat demand , the overall system control automatically terminates heat pump operation and initiates firing of the modular blower heating section 20 to replace the refrigerant - based heating of the indoor coil 24 with combustion heat and thereby raise the temperature of the heated supply air 42 b being discharged from the heat pump 12 . when the overall system control detects that the replacement combustion heat from the modular blower heating section 20 is no longer required , such combustion heat is terminated and the heat pump coil unit 12 is re - activated until the heat demand is met by the indoor refrigerant coil 24 . in the depicted representative embodiment of the present invention , the use of a non - condensing gas - fired modular blower 16 as an alternative to electric resistive elements for back - up heat is uniquely coupled with the specially designed modified modular blower control 34 in a manner assuring that the modular blower 16 is only utilized for secondary heat when the heat pump portion 12 , 14 of the system 10 cannot provide adequate heat ( e . g ., at extremely low outdoor ambient temperature conditions or if the heat pump portion 12 , 14 has failed ). a simplified wiring diagram for the system 10 is shown in fig2 and indicates the arrangement of the connection terminals , and the electrical wires interconnecting them , for the heat pump ( hp ) thermostat 30 , the modified non - condensing gas - fired modular blower ( mb ) electronic control 36 , and the outdoor heat pump ( hp ) control 36 . with the following exceptions relating to the modular blower control 34 , the thermostat 30 and the controls 34 and 36 may be of conventional construction and configurations . first , the modified modular blower control 34 is provided with a new connection terminal “ b ” that indicates when a call for heat pump heating operation is requested from the heat pump thermostat 30 ( via its connection terminal “ b ”), for example , when the heat pump reversing valve has been switched to its heating orientation . second , the modified modular blower control 34 is provided with a new input connection terminal “ e ” indicating when a call for secondary heat operation is requested from the heat pump thermostat 30 ( via its connection terminal “ e ”), such request being indicative of a call for emergency heat during a heat pump heating cycle . the new terminal “ e ” of the modular blower control 34 may be the former “ w ” input terminal from a conventional control used in a non - condensing gas furnace . third , the software algorithms in the modular blower control 34 are modified to prevent utilization of the non - condensing gas - fired modular blower 16 except under secondary conditions ( i . e ., when the “ e ” input to the modular blower control 34 is active ). an alternate embodiment of the fig2 simplified control wiring diagram is depicted in fig3 . the fig3 wiring diagram is identical to the fig2 wiring diagram with the exception that in the fig3 electrical wiring circuit an outside air temperature lockout portion 44 is incorporated therein . the lockout portion 44 has an outside air temperature ( oat ) sensor portion 46 and functions to limit when the modular blower ( mb ) 16 can be used for back - up heat . a logic flow diagram 48 is shown in fig4 and illustrates the operation of the modified modular blower control 34 in controlling the use of combustion heat from the modular blower 16 to replace the refrigerant - based heat provided by the heat pump portions 12 , 14 ( see fig1 ) during a space heating demand cycle . subsequent to a suitable starting step 50 , a query is made at step 52 as to whether a heat pump thermostat output signal “ b ” is being input to the modular blower control 34 . if it is not , the system loops back to step 52 . if the step 52 query answer is “ yes ” a transfer is made from step 52 to step 54 at which a query is made as to whether the heat pump thermostat output signal “ e ” is being input to the modular blower control 34 . if it is not , the system loops back to step 52 . if the step 54 query answer is “ yes ”, a transfer is made from step 54 to step 56 . at step 56 the heat pump is turned off and the modular blower heating section 20 ( see fig1 ) is energized to replace the refrigerant - based heat previously being generated by the now idle heat pump indoor refrigerant coil 24 ( see fig1 ). a transfer is then made from step 56 to step 58 at which a query is made as to whether the signal “ e ” transmitted to the modular blower control 34 has terminated . if it has not , the system loops through steps 56 and 58 , continuing to provide replacement combustion heat from the modular blower until the answer to the step 58 query becomes “ yes ”. when this occurs , a transfer is made from step 58 to step 60 at which the heating of the modular blower section 20 is terminated , heat pump operation is re - enabled , and a transfer is made from 60 back to step 52 . when the previously described outside air temperature lockout circuit portion 44 ( see fig3 ) is utilized , the fig4 logic flow diagram is modified , as shown in fig5 , by interposing an additional step 62 between the previously described steps 50 and 52 . at the additional step 62 shown in fig5 , a query is made as to whether the outside ambient temperature is less than a predetermined magnitude . if it is not , the system pauses at step 62 until the query answer becomes “ yes ” at which point a transfer is made from the additional step 62 to the previously described step 52 in the logic flow diagram of fig4 . as can be seen from the foregoing , in an illustrated representative embodiment of the present invention , combustion heat from a fuel - fired modular blower , preferably a non - condensing type fuel - fired modular blower , is used to replace the refrigerant - based heating capacity of a reversible circuit heat pump only when the heat pump refrigerant heat is insufficient to meet a heat pump heating demand as evidenced by the receipt by a modular blower control from a heat pump thermostat of both a first signal indicative of a call for heat pump heat ( by , for example , the heat pump reversing valve having been set to its heating orientation ), and a second signal indicative of a call for emergency heat during a heat pump heating cycle . additionally , this is done in a manner desirably restricting the user from overriding the heat pump . the foregoing detailed description is to be clearly understood as being given by way of illustration and example only , the spirit and scope of the present invention being limited solely by the appended claims .