Thermostat setup for Dual Fuel Heating with Heat Pump and Gas Furnace

Our dual fuel system consists of a Honeywell thermostat, a single stage split heat pump, a traditional gas furnace, a Bill Porter (BPC) dual fuel relay board and an Outdoor Sensor (ODS) thermostat. I want the system to use the gas furnace when below the ODS setpoint and use the heat pump above the setpoint. I want to manually call for the gas furnace if the heat pump has failed. I do not want the gas furnace to augment the heat pump as an auxiliary heat source. I do not want the gas furnace to operate when the heat pump defrosts. The heat pump should never defrost or need auxiliary heat if the ODS is set properly. I want the emergency heat option in case the heat pump loses refrigerant (as it too often does).

After much Internet research, I concluded I had to figure this out myself. I needed to know what system type to use on the Honeywell thermostat and also which terminals to use on the thermostat. I also needed to know whether to use the jumper on the BPC.

Equipment Details

• Honeywell TH8320ZW Z-Wave thermostat
• Bill Porter BPC-1 rev 3 dual fuel relay board
• Amana single stage gas furnace and single stage heat pump

Conclusion

The thermostat does not know about the gas furnace. It is configured as a “code 0170 Type 7 - 2 heat/1 cool heat pump (with aux. heat)”. This system type is required to enable the Emergency Heat system setting. If I didn’t want Emergency Heat, I would have used the “Type 2 - 1 heat/1 cool heat pump (no aux. heat)”.

The system cycle rate appears to be a personal preference. I have chosen code 0220, 1st state compressor cycle rate of 2. This cycles the compressor at most 2 times per hour. The thermostat does not know about the gas furnace so this is the cycle rate for the furnace when the outside temperature is below the ODS setting. There is a setting 0250, 2nd state heat cycle rate but it is not used because I did not use the Aux terminal. Setting 0270, Emergency heat cycle rate is set to 2 also. If it was electric emergency heat, I would probably set it higher, probably 9 as the manual suggests.

Use the “2H/1C Heat Pump (with auxiliary heat)” wiring guide. Do not jumper the E and Aux terminals. Connect the white (W) wire to the E terminal. Do not connect anything to the Aux terminal. This enables the thermostat to call for Emergency Heat but not Auxiliary heat.

The thermostat will show “Aux Heat On” if the thermostat heat setpoint gets too far above (about 4F) the room temperature. Ignore the display. Since there is no wire on the Aux terminal it is not doing anything. The heat pump will continue to run. When the thermostat system setting is “Emer”, the heat pump will stop and the furnace will start. In this mode the display will show a red LED in the upper right corner and will show "Aux Heat On" but its really enabling the E terminal which we connect to the gas furnace. The thermostat can be used in the Emergency mode if the heat pump has failed.

Remove the 1-2 jumper from the BPC board. This jumper permits the heat pump to start the furnace when the heat pump is in defrost mode. If the heat pumps defrosts, it probably means the refrigerant is low or the ODS is set too low.

Bill Porter Schematic BPC-1 Rev 4 This a description of how I think the BPC works, taken from a rev 4 schematic.

R2 is made by the O terminal getting voltage from the thermostat (cooling mode) which will not allow furnace (W) to come on except during defrost when W receives 24v from the defrost board in the outdoor unit if jumper 1-2 is in place. All R2 N/C circuits open when in cooling mode, so the furnace cannot run. Remove jumper 1-2 to prevent furnace operation during defrost.

R1 is energized by W from the indoor thermostat OR when the outdoor sensor (ODS) drops below set point AND Y is energized. The energized R1 disables the heat pump compressor while the furnace is running. R1 energizes and closes the circuit to W on furnace side. The furnace starts with ODS low (cold) and Y (heating) or W (emergency or aux). R1 seals in with the outdoor sensor so even if the temperature rises, it can’t go back to heat pump until Y and W drop.

The furnace starts either by W or ODS and Y.

Other revisions of the BPC are different. I have a rev 3 but a schematic for rev 4. The differences are unknown but the principles are probably the same.