I have a York combination system with an oil fired furnace and a heat
pump. Everything is working fine but I have a question about how the
heat pump and furnace work together.
The thermostat is a 2 stage, when the first stage calls for heat, the
heat pump alone comes one. If the room temperature drops another
degree or so, the second stage calls for heat also and the furnace
then comes on. Here is where my question comes in. When the second
stage calls for heat and the furnace comes on, the heat pump
compressor still remains on. As the furnace heats up, the heat pump
condenser input air is getting hotter and hotter. The heat pump is
trying to heat air that has already been heated by the furnace. At
some point when the furnace warms up enough, the heat pump does
eventually shut off , but it seems that it is shutting off due to a
high pressure safety or something and that the line pressure is very
high at that point. I imagine I can hear the compressor straining to
pump heat into the already heated air and I'm thinking this can't be
good for the compressor.
Is it normal for a dual system to be designed so that the heat pump
and furnace both are operating at the same time or should the controls
be wired to turn the heat pump off immediately when the furnace comes
on for second stage?
It's wired wrong , they should NOT both run at the same time. Get the
installer back ASAP to correct the situation and shut off the HP until it's
fixed.Switch the T-stat to the Emerg.Heat position and leave the main power
on to the compressor
well I've live din the house for 14 years and it has been fine..
I usually use the EM heat setting which is oil only, but with the
price of oil such as it is, I am starting to use the heat pump more
when it is above 30 deg F outside and I noticed this situation happens
I have traced the problem to the bonnet sensor. The compressor will
shut off when the bonnet sensor voltage gets below 3.75 Volts so the
controller works and is wired correctly. But the air temperature out
of the furnace and into the condensor gets well above 90 deg F and
the bonnet sensor voltage decreases only a little and is about 4.71
Volts when the air is 90 deg F. So I suspect the bonnet sensor is
bad. Does anybody know what the temperature vs resistance curve or
the voltage vs temperature for the bonnet sensor is supposed ot be so
that I can check it. It measures about 34 kOhms at room temperature.
Conclusion,,, it's fixed.
I replaceed the bonnet sensor with a 10 kOhm NTC thermistor. It works
perfectly now, when the second stage calls for heat, the furnace comes
on, when the bonnet temp reaches 91 deg F (and the senor voltage drpos
below 3.78Volts) the heat pump shuts off and the furnace continues to
run. When the second stage is satisfied, the furnace shuts off. When
the bonnet cools down to 89 deg F, (and the sensor voltge raises to
3.83 Volts) the heat pump comes back on. So with the correct bonnet
sensor in place, the heat pump never runs when the supply air is above
90. What I don't know is...if the sensor that was in there was the
wrong one from day one or if the sensor went bad. Either way...it's
fixed now. Thanks for the advice.
Now....if you actually had a clue......
The furnace heat and heat pump are not supposed to run at the same time.
when the heat pump runs, the furnace blower runs only to condition the air.
The *normal* air discharge temp for a heat pump is 95F - 105F.
The sensor that tells the system when to switch between the heat pump and
the furnace heat is the *OUTDOOR TEMP* sensor.
Here is some more food for thought.....
If the furnace heat runs at the same time as the heat pump, it will run the
refrigerant pressures through the roof and will eventually do terminal and
catastrophic damage to the compressor.
Still think you got it "fixed"??
the bonnet temp sensor is measuring the air coming out of the furnace
BEFORE it gets to the indoor coil.. it turns off the heat pump as soon
as the air out of the furnace and into the coil starts to get warm,
i.e. reaches about 90 deg F and yes it keeps the pressures from going
read this about how it is supposed to work:
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