Fossil fuel application with a heat pump

What is going on

This furnace can pair with a heat pump as a fossil fuel system, where an outdoor temperature sensor decides whether the heat pump or the gas furnace does the work. The heat pump gets priority unless a compressor or backup heat balance point lockout is active. The control adjusts stage times after the first call. If the heat pump cannot satisfy the thermostat in the target runtime, it is locked out until the furnace can do so on low stage. It is a hybrid or dual fuel setup, and it saves on fuel cost when gas is cheaper than electricity at low outdoor temperatures.

Likely causes

• Dual fuel or hybrid install with a heat pump and gas furnace
• Heat pump not keeping up at low outdoor temperatures
• Non-communicating single stage heat pump needing extra control hardware
• Wiring mixed between the furnace control and the fossil fuel board

Check in this order

1. On non-communicating single stage heat pump systems, use a fossil fuel kit and follow its wiring guide.
2. Connect all furnace wiring to the furnace integrated control module and to the FURNACE terminal strip on the fossil fuel control board.
3. Turn power off before wiring, and confirm 24 V low voltage connections are made to the correct terminals.
4. Confirm the outdoor temperature sensor reads correctly.
5. Test heat pump and furnace operation separately and check changeover.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)