Alt Heat not turning on during heat pump defrost cycle

[Ahren]

I have a duel fuel (2 stage heat pump plus 2 stage natural gas furnace) forced air system.  Everything works fine except that alt heat (i.e. gas furnace) does not turn on during heat pump defrost cycles.

The defrost cycle signal wire from the heat pump defrost board (X2) is wired to the W1 terminal on the furnace controller and from there also to the W1 terminal on the Nest Learning Thermostat 3rd Generation.  When the heat pump goes into defrost mode, a 24 V signal can be measured at the furnace panel on the W1 terminal, but the gas burner does not fire.  The air handler fan keeps running during defrost and blows cold air into the house.  I thought in this scenario it was supposed to fire the gas burner to help provide heat to the indoor HP coil to help defrost the outdoor unit, and to keep the house temp comfortable during defrost (nothing like a blast of cold air when you step out of the shower on a cold morning when the HP is working hard).  The furnace fires correctly when Alt heat stage 1 is called for (when outdoor temp drops below Duel Fuel - Alt Below: setting temp), as does Alt heat stage 2 when the furnace is running for longer.  Could there be something coming from the Nest Thermostat controls that is stopping the furnace from firing?

[Markjosephp]

Hello Ahren,

Thanks for reaching out. It appears that you have thoroughly diagnosed the problem with your dual-fuel system. The fact that the furnace does not fire during the heat pump defrost cycle despite getting a signal from the defrost board is baffling. 

Given your setup, it's always a good idea to double-check the thermostat settings to make sure there are no competing schedules or settings that could interfere with the furnace's operation during defrost cycles. Also, the furnace controller itself is one possible place to look at more. The controller may be receiving the signal from the heat pump, but it may not be able to activate the gas burner during defrost cycles due to a setup problem or other malfunction. 

If everything seems to be hooked and adjusted correctly but you're still having problems, it might also be helpful to speak with a qualified HVAC technician. In order to identify any underlying problems that could be preventing the furnace from firing during defrost cycles, they can conduct a thorough evaluation of your system.

Best regards,

Mark

[Ahren]

Thanks Mark,

the recommendation I have so far from my HVAC technician is to replace the furnace control board, but I wanted to check if there were potential other issues before spending the $$.  Reading through the OEM thermostat installer manual (American Standard) with Control mode wiring there is a setpoint you can change to turn Alt heat on/off during defrost ISU 0364… but I can’t find such a setting in the Nest… so I’m wondering if it’s just a software setting I need to turn on somehow on the control board somehow…