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Old 11-21-2012, 09:53 AM   #1
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Heat pump to gas fired.


Assuming a heat pump is in place in a residence and there is a desire to change it over to a gas fired furnace. Any simply way to size the gas furnace by using the heat pump specs? Tonnage vs btu?
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Old 11-21-2012, 04:26 PM   #2
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Heat pump to gas fired.


Not really. You can get an idea from the amount of strip heat installed. But I've seen some that had 1.5 to twice as much strip as the house needed.
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Old 11-22-2012, 12:34 PM   #3
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Heat pump to gas fired.


it would be better to have a contractor calculate the proper size furnace needed
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Old 11-22-2012, 07:22 PM   #4
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Heat pump to gas fired.


Perhaps if I re-phrase the question it may be more clear. Let's say the heat pump is a 3 ton unit with 10kw electric resistance. What size gas furnace would be adequate to replace that? And let's assume that a manual j and d was used to determine the sizing of the heat pump. What is the correlation between the two? And about the strip heat....in a heat pump it's function is limited to only a few scenarios, not normal usage, correct?
From what I have seen and learned from my own heat pump, the strip heaters will only become energized when the hp is in defrost mode or if the set-back of the thermostat is too much for the heat pump alone to recover in a reasonable amount of time. I have seen the heat pump alone sustain the temperature when it has been 0 deg., with the heat strips only coming on in defrost mode. It is that observation that makes me question why the heat strips need to be in the overall sizing calculation.
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Old 11-22-2012, 07:48 PM   #5
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Heat pump to gas fired.


Well that heat pump would have roughly 10K BTU at zero and the 10KW elements would add another 32K. If that's what the manual J called for then a 50K BTU furnace should be good.
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Old 11-22-2012, 08:51 PM   #6
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Heat pump to gas fired.


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Perhaps if I re-phrase the question it may be more clear. Let's say the heat pump is a 3 ton unit with 10kw electric resistance. What size gas furnace would be adequate to replace that? And let's assume that a manual j and d was used to determine the sizing of the heat pump. What is the correlation between the two? And about the strip heat....in a heat pump it's function is limited to only a few scenarios, not normal usage, correct?
From what I have seen and learned from my own heat pump, the strip heaters will only become energized when the hp is in defrost mode or if the set-back of the thermostat is too much for the heat pump alone to recover in a reasonable amount of time. I have seen the heat pump alone sustain the temperature when it has been 0 deg., with the heat strips only coming on in defrost mode. It is that observation that makes me question why the heat strips need to be in the overall sizing calculation.
Is your heat pump an air to air, or a geo.
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Old 11-22-2012, 08:55 PM   #7
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Heat pump to gas fired.


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Is your heat pump an air to air, or a geo.
Air to air.
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Old 11-22-2012, 09:02 PM   #8
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Heat pump to gas fired.


If your area is a high heat area, then your heat pump is larger for your home then it would be if you lived in my area. So it may be sized large enough to handle the heat loss 95% of the time.
The aux is probably sized to be able to handle the load 125% of the time. Based on what you posted.

A gas furnace in the 40,000 BTU input range would be what you would use.
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