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:
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.