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I am running a central heat system that uses a heat pump as stage 1 and a gas furnace as stage 2. The thermostat is not directly connected to the furnace and heat pump motor, instead to goes through a control system (part 2AC02700701) that takes care of switching to gas if it gets too cold outside independently of the thermostat request. The control system schematic if shown below:

Heat pump, add-on control system

My furnace will operate full speed when Y and W are powered but will use a low fan quiet mode only when G alone is powered. Therefore the heat pump control system will result in the following strange behaviour:

  • Thermostat requesting cool (G,Y,O are powered) => G,Y powered on furnace and Y,O powered on heat pump. Running full speed, all is ok
  • Thermostat requesting S1 heat (G,Y are powered) => G only powered on furnace and Y powered on heat pump. Heat pump running heat but furnace only operating at low speed, I don't think this is ok
  • Thermostat requesting S2 heat (W is powered) => W is powered on furnace and none on heat pump. Running full speed gas furnace, all is ok

I could not find any comment anywhere proposing to run a heat pump with low fan speed. It seems the fan speed used for cooling should be the same than the one used for heating. Then why would the heat pump controller apply power to the G/Y terminals for heat pump cooling but not for heat pump heating, in which case it applies power to G terminal only?

My concern is that with this setup, it takes forever to heat the house using the heat pump and I feel like I am wasting lots of energy. As a test I tried to hard wire the furnace fan to high speed and it started heating much more rapidly with the heat pump, however on the long run, I don't want to let the furnace run at high speed continuously.

  • It seems odd that you have a gas furnace that requires G to be energized to run the blower. What make/model is it? Also, have you considered getting a new thermostat that has an outdoor temperature sensor, so it can perform the lockout itself without the need for an extra module in the system making things confusing? – ThreePhaseEel Oct 20 '18 at 3:52
  • The furnace will run the blower properly if the stage 2 heat mode is triggered with W. The issue I believe is my controller which powers G only from the furnace when running in heat pump mode. G activates the blower in very low speed only which seems insufficient for the heat pump. If the Y was also powered in heat pump mode it would work properly. – jmbouffard Oct 20 '18 at 4:09
  • Are you comfortable modding the control module depicted in your wiring diagram? Because it's either that, or rip out the control module and the 'stat and replace it with something that's going to be less quirky... – ThreePhaseEel Oct 20 '18 at 4:27
  • I'm starting to think that removing this control module and using a better thermostat unit would be the best solution. Thanks for your comment. – jmbouffard Oct 20 '18 at 4:30

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