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Posted by Bubba on November 25, 2007, 8:44 am
wrote:
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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 -
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>> > - 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
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>> Mark- Hide quoted text -
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>> - 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
You need some fresh air. You just "aint right".
Bubba
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