Heat Pump Lockout Temperature

 
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Old 12-14-2010, 06:24 PM   #1
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Heat Pump Lockout Temperature


I am using 16 SEER, 9.8 HSPF Amana heat pumps. I want to know what the optimum temperature is to lockout the heat pump and switch to the 95.5 AFUE backup furnace.

Our electricity is $0.09/kWh and natural gas is $0.95/therm. To compare the two fuels, I convert the electricity to therms. It turns out that one therm of electricity is $2.64. This makes the gas furnace as economical to run as the heat pump when the heat pump's COP is .955x2.64=2.5.

The chart for the heat pump shows that its COP is 2.5 at about 20 degrees F. Do these charts list the steady state COP, ignoring losses due to defrosting, or are they reasonable to use for this analysis?
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Old 12-14-2010, 07:51 PM   #2
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Re: Heat Pump Lockout Temperature


COP does NOT include defrost losses.

HSPF includes defrost losses.

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Old 12-14-2010, 08:03 PM   #3
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Re: Heat Pump Lockout Temperature


For many, it's more of a comfort issue. When as the outdoor temps drop, so does the supply temp. I lock them out at 30-35. All of our past customers have been quite pleased with the savings already. Here is an energy calculator that is easy to navigate. http://partner.goodmanmfg.com/goodma...calGoodman.jsp

P.S. , you probably would get the best usage / efficiency out of the system by simply installing the coil in the return. ...
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Old 12-14-2010, 08:36 PM   #4
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Re: Heat Pump Lockout Temperature


Thanks a lot guys. I did not realize that the supply temp would drop with the outside temp, but it can't be much. I just had my HVAC guy install one of these systems in my own home to replace my 20 year old heat pump. I just now switched it from 20 to 30 degrees and I will see how it works.

Relative to our old system, we have been quite comfortable, though the temperatures have been dipping in the low teens at night and barely hitting the thirties during the day. My main concern is efficiency, and if I am understanding you both, I am probably splitting hairs either way unless the defrost cycle losses are excessive.

Thanks again.
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Old 12-14-2010, 08:41 PM   #5
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Re: Heat Pump Lockout Temperature


Your heat pump has a time and temp defrost control. it defrost every X minutes(of accumulated compressor run time) weather its needed or not once the outdoor temp is below I believe 28 on your unit. The X minutes is what ever it been set to.
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Old 12-14-2010, 08:52 PM   #6
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Re: Heat Pump Lockout Temperature


I have another problem with the new system. The outdoor temperature sensor right now is reading 39 degrees, but it is actually 25 outside. It sounds like the outside sensor is not placed properly. When the wind is really blowing, it seems to be more accurate.

So do you guys think these are good systems for my climate (Zone 4)?
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Old 12-14-2010, 08:56 PM   #7
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Re: Heat Pump Lockout Temperature


Sounds like a good set up. Your gas rate is relatively low. So you have a high economic balance point. But, if your gas rate ever goes up un-proportionally to electric. You got the heat pump to keep your bill as low as possible.
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Old 12-14-2010, 09:06 PM   #8
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Re: Heat Pump Lockout Temperature


Good advice.

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