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Old 12-27-2007, 08:06 AM   #1
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Testing FuelMaster 21 Module? Heat Pump no Run.


I have a Lenox natural gas furnace with a heat pump system. My old lenox thermostat was mercury based and not very precise. I wanted to upgrade my thermostat so bought a digital one for heat pumps. After hooking it up, my heat pump would not run, so I put my lennox thermo back and it still will not run. Only the nat. gas furnace runs.

When I tilt the thermo so the only the first mercury switch (for the heat pump) is tripped, the nat gas furnace automatically engages. I checked the voltages. At thermo, there was power to R (power), Y1 (1st stage heat), and W1 (2nd Stage heat), no power at O (reversing valve) or E (emergency heat). The same voltages were found on the input side of the FM21 module. But there was no voltage found on the Heatpump/output side of the FM21. Only voltage going to furnace.

So I pulled the W1 wire from the FM21 to see if the thermostat or thermostat wire had a short. I moved thermostat to engage only the 1st stage heating. Checked voltage at Thermostat and power only at R, and Y1. Voltage at input side of FM21 was still R, Y1, and W1.

I flipped the emergency heat switch and it appeared to operate correctly.

So it appears something in the FM21 is causing or forcing the secondary heat routine to run no matter what. What can I check now? I couldn't find any fuses that might have blown. It's been in the 50s around here, so I'm fairly confident the heat pump should be running. I hope someone can offer some good troubleshooting advice for me. Thanks in advance.

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Old 12-29-2007, 01:03 AM   #2
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Testing FuelMaster 21 Module? Heat Pump no Run.


Any luck? Why the dual fuel system? I think you might've landed one of your wires on a wrong terminal.

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Old 12-29-2007, 08:24 AM   #3
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Testing FuelMaster 21 Module? Heat Pump no Run.


No. No luck. Everything is hooked up correctly. But for some reason, when the FM21 board receives a signal for the 1st stage heat (heat pump) it automatically engages the 2nd stage heat?

Heat pumps are supposed(??) to be more efficient. But I seriously doubt my cheapo heat pump really saves me energy. It just saves my Nat. gas bill by using more electricity.
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Old 12-29-2007, 03:11 PM   #4
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Testing FuelMaster 21 Module? Heat Pump no Run.


Hmm? Switch the lead where whatever you want to come on first will...?
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