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Old 02-11-2010, 12:14 PM   #1
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What temp does my Heat Pump stop producing good heat or what is the threshold?


I have an 850 sq foot house, single story with crawlspace, good insulation and decent windows, carpet. I have a one year old Lenox 2 ton 13HPD Merit series Heat Pump. It is 13 seer for Air conditioning and 7.7 for heating seasonal performance factor. I live in southern TN.

What is the approx. temperature where my heat pump starts to not heat the house well or what is the temperature where it has a hard time maintaining a 68-70 deg. F indoor temp? I am strictly talking about the heat pump without the use of heat strips. Can it perform at 25 degrees without too much trouble or ?. Thanks.

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Old 02-11-2010, 01:13 PM   #2
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What temp does my Heat Pump stop producing good heat or what is the threshold?


It depends how much heat your house loses.

A HP can generate heat at 0 degrees, just not as much as it can at 50.

Without having a load calculation done, it is nearly impossible for one to answer your question. There are too many variables.

Insulation;
Number of windows;
Doors, type of construction;
Vapor barrier;
Etc.

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Old 02-11-2010, 06:43 PM   #3
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What temp does my Heat Pump stop producing good heat or what is the threshold?


My 2 ton in a 1000 square foot slab on grade home maintains 72 degrees indoors down to 9 degrees outside. Windows are new but need more insulation. She runs non stop down that low so I have the HP locked out at 15 degrees.
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Old 02-11-2010, 07:09 PM   #4
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What temp does my Heat Pump stop producing good heat or what is the threshold?


I wouldn't lock it out at that temp. It would still be cheaper then the strip heaters.

OP: There is no one set temp. Depends on how much hat your house needs at what temps.
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Old 02-11-2010, 07:36 PM   #5
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What temp does my Heat Pump stop producing good heat or what is the threshold?


I have a 94.4% AFUE gas furnace for backup and a 18.5 seer HP with a 8.55 HSPF beenthere. I know electric is 4 cents a kw after the first 900 @ 8 cents a kw. Gas is 80 some cents a therm. Next time the bills come I'll ask you to figure what the real economic balance point is.

Electric back up I'd run it down to zero.
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Old 02-11-2010, 07:42 PM   #6
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What temp does my Heat Pump stop producing good heat or what is the threshold?


Don't for get to post the COP at various temps.

Just as an example. At the rates you posted. Heat pump is cheaper at COP of 1.5 and higher.
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Old 02-12-2010, 07:44 AM   #7
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What temp does my Heat Pump stop producing good heat or what is the threshold?


The heat pump's computer should make the switch automatically at the most cost-effective point. Seal and insulate your house as much as possible, especially the attic and basement.
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Old 02-12-2010, 08:59 AM   #8
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What temp does my Heat Pump stop producing good heat or what is the threshold?


Quote:
Originally Posted by sweaty View Post
The heat pump's computer should make the switch automatically at the most cost-effective point.

What computer would that be?
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Old 02-12-2010, 11:25 AM   #9
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What temp does my Heat Pump stop producing good heat or what is the threshold?


Doesn't everything have a computer?
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Old 02-12-2010, 12:03 PM   #10
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What temp does my Heat Pump stop producing good heat or what is the threshold?


Quote:
Originally Posted by sweaty View Post
Doesn't everything have a computer?
Nope.

Many defrost boards are just elaborate timers. With a temp sensor, and a couple relays. No logic built into them.

Does your electric company and gas company provide online up to date daily rates for computers to automatically download?
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Old 02-12-2010, 01:08 PM   #11
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What temp does my Heat Pump stop producing good heat or what is the threshold?


Quote:
It runs non stop down that low so I have the HP locked out at 15 degrees.
Why do that? Let it run, it's cheaper! Heat pumps will never be below a 1.0 COP and will almost always be at least a little bit above it. If it is dropping to much in temp for your taste, bump the thermostat up, or use a smaller differantial, or get one that will turn on the strip heat after a long run time. But running Aux heat 100% below any preset temp is expensive!
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Old 02-12-2010, 03:25 PM   #12
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What temp does my Heat Pump stop producing good heat or what is the threshold?


Yes, my plan was to leave my heat pump with no lockout and I was going to do what beenthere suggested and put my outside sensor in my return air and lockout my heat strips at 60F since my Th8000 Honeywell won't allow me to lockout the heat strips any lower than 40F.
My plan is to let my heat pump run and only use the heat strips when I really need them because most of the time our lows don't go much past 27 and that is only for a few weeks and the days are usually at the lowest 32-35F where I live.
The problem for me is finding the threshold where my heat pump is having a hard time keeping up and needs that little boost from the heat strips.

I hope that my heat pump can handle it without bringing my heat strips on or at least if I do need them just for a very short time period.
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Old 02-12-2010, 03:28 PM   #13
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What temp does my Heat Pump stop producing good heat or what is the threshold?


That temp will vary between daylight and night time at the same outdoor temp. Day time solar gain lessens the heat needed to heat the house.

So at 25 outdoor temp, and the sun shining the heat pump may be able to heat the house. But at 25 outdoor temp and night time, it may not.
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Old 02-12-2010, 06:29 PM   #14
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What temp does my Heat Pump stop producing good heat or what is the threshold?


Quote:
Originally Posted by Marty S. View Post
My 2 ton in a 1000 square foot slab on grade home maintains 72 degrees indoors down to 9 degrees outside. Windows are new but need more insulation. She runs non stop down that low so I have the HP locked out at 15 degrees.
im in nebraska too,but mine kicks off around 25 degrees
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Old 02-12-2010, 08:15 PM   #15
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What temp does my Heat Pump stop producing good heat or what is the threshold?


What about running hours and wear and tear on the compressor? It might make more sense to shut it off sooner and extend the life of the compressor a year or two. I use some electric heaters when it is mild and extend the life of my furnace. Costs a bit more but in the long run not really. Disclaimer: the Pros get parts/compressors at wholesale cost and supply their own free labor. The homeowners has to pay top dollar for both.

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