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Posted by Bumpy on November 25, 2007, 9:48 pm
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>> >> > > > 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?
>>
>> >> > > > thanks
>>
>> >> > > > Mark
>>
>> >> > > re: Everything is working fine...
>>
>> >> > > Based on the responses so far, perhaps that statement was a bit
>> >> > > premature. ;-)- Hide quoted text -
>>
>> >> > > - Show quoted text -
>>
>> >> > 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
>> >> > occasionaly.
>>
>> >> > Mark- Hide quoted text -
>>
>> >> > - Show quoted text -
>>
>> >> OK
>>
>> >> 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.
>>
>> >> thanks
>>
>> >> Mark- Hide quoted text -
>>
>> >> - Show quoted text -
>>
>> > 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.
>> > Mark
>>
>> 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"??- Hide quoted text -
>>
>> - Show quoted text -
>
> 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
> too high,,,
>
> read this about how it is supposed to work:
>
> http://johnsonsupply.com/york2001/PUB/54501/OUT/N10y.pdf
>
Depending on the climate conditions, my Trane system, calls for the oil
burner
to run during the Defrost cycle to keep the air warm to the output plenum
because it is actually in the cooling mode.
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