Heatpump at 26 Degrees ?

sdilleyDecember 11, 2009

Last night the temperature went down to 26 degrees and I noticed the heat pump was running instead of the gas furnace. My house was up to temperature and the outside coils were ice free, but I thought that the gas furnace would provide the heat when it got this cold.

My heat pump is an American Standard Heritage 12 Model 2A6H2036A1000AA, will running the heat pump when it is this cold hurt the unit? How efficient is the heat pump at this temperature?

Note that the thermostat only turns off the heat pump and switch on the furnace if the indoor temperature is more than one degree below the thermostat setting and the thermostat does not have a outside temperature sensor.

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1. Will running the HP damage the unit? no.
2. How efficient is the HP at 26F? depends on the specs for the unit, but probably 'not very'.

Without an external temp sensor, the thermostat makes the decision whether the gas has to turn on based on whether the HP can meet the heating demands or not. Basically, it looks for the temperature to rise X degrees in Y time. If the rise doesn't occur, it switches to the auxiliary heat (gas in your case). The fact that it did not means you HP was still doing the job at 26F which is pretty good.

    Bookmark   December 11, 2009 at 10:50AM
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is HP running continuously at 26 degrees?

if so, it really would be better to switchover to the backup gas furnace.

keep in mind the gas furnace may be more efficient at 26 degrees. that would depend on your electric rate and nat gas rate.

for DF systems, I prefer a thermostat with outdoor system where you the homeowner is making the decision.


    Bookmark   December 11, 2009 at 11:25AM
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It got down to 28F or so in ATL last night. I noticed the HP running while it was still 30F and mostly just moving air (lukewarm air). Our switchover is set to 30F, so once it dropped a degree, the gas kicked in and kept the place warm overnight.

Still hate getting the dual bills in the winter, but with the outdoor sensor and Honeywell running the setup, I'm at least confident we're somewhat optimized for our setup.

    Bookmark   December 11, 2009 at 2:42PM
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If the house IS being held to temp at 26°F outdoor, even if the heat pump is running continuously, there's no reason to manually switch the gas on. That's why you installed a heat pump, yes? ... to have it heat your house? Let the thermostat do its job in deciding if the switchover is required.

My heat pump can hold 70°F inside and even cycle off occasionally at 26°F. It'll ice-up and need defrosting regularly if the weather is wet/rainy/snowy ... but that's how it's supposed to work.

    Bookmark   December 11, 2009 at 3:20PM
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Don't confuse heat pump efficiency with capacity.

At 25F ambient, my 9 HSPF Goodman has a C.O.P. of 2.82, which means that it is still 282% cheaper than straight electric resistance.

However, the heating capacity is reduced from 36,000 btu at 47F (the high heat rating temp) to 25,000 btu at 25F. The ouput air temp also drops from 100 at 47F to 91.6 at 25F.

To determine if the gas furnace or heat pump is cheaper to run at 25F, you need to do a cost comparison per btu based on your heat pump's COP at 25F, electric rate, furnace efficiency, and cost of natural gas.

Take care.

    Bookmark   December 11, 2009 at 9:47PM
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"keep in mind the gas furnace may be more efficient at 26 degrees...."

The gas furnace efficiency doesn't change with ambient temps like a heat pump does. The gas furnace is 80% (or whatever its rating) at all ambient temps. It may be cheaper or more expensive to run than a heat pump, but its efficiency is basically constant.

Take care.

    Bookmark   December 11, 2009 at 9:53PM
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gary is correct. a slip of the pen. my bad.

"keep in mind the gas furnace may be more efficient at 26 degrees...."

I meant to say "keep in mind the gas furnace may be less expensive to operate at 26 degrees than the heat pump".

efficiency of gas furnace remains the same regardless of outside temp.


    Bookmark   December 12, 2009 at 7:16AM
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We've had a bit of a cold snap here the last 12 days wth temps dipping to the single digits. My heat pump runs until it gets 20 degrees outside. I have a 3,000 sq. ft. story and a half heated by a 3.5 ton Trane XL 14i with 90% single stage furnace backup.

My house stays comfortable at 70 degrees even at 20 degrees outside with the HP providing the heat. Under that, I switch over to gas heat via compressor lockouts where the furnace then becomes the better choice both for heating and the pocketbook. This is even after the 22 cents a therm drop in natural gas pricing.

    Bookmark   December 14, 2009 at 9:26AM
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