At what outdoor cold temperature are heat pumps pretty worthless?

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  #1  
Old 06-10-09, 06:16 PM
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At what outdoor cold temperature are heat pumps pretty worthless?

I was doing all this work at this duplex yesterday, when the lady from the unit next door asks me if I heard that they are getting a new furnace. No. She said her winter electric bill was outrageous and she sat in the house shivering on cold days! What?!

I said the landlord never asked me to come over and see if there was a problem! Sort of makes me mad, actually.

And then to learn my boss is springing for 2 new gas furnaces, and getting rid of the heat pumps at the advice of his furnace guy. The lady said when it was 23 below zero out this winter (we actually even had colder), the heat pump was worthless. I told her that she should have had electric auxillary heat that would have come on.

But maybe it also is sized inadequately for super cold weather?

But at least I could have come over to see if they were fully staged. But I also have the hunch they were, because she said the elctric bill was outrageous. But she would not tell me how outrageous.
 
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Old 06-10-09, 08:19 PM
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In my experience newer heat pumps can provide enough heat down to about 10 deg before the aux heat has to kick in. There is one heat pump mfg that has technology that allows the heat pump to work down to 30 below though the name eludes me right now.
 
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Old 06-11-09, 07:04 AM
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There is a theoretical limit, but in short, once an HP's COP reaches 1 you might as well use electric resistive heat. So if you have the COP charts for the particular unit and know it is operating at peak efficiency, the temp at which you reach COP of 1 is where it become worthless, as you put it, or pointless.

However, since that's somewhat theoretical, and doesn't take into account the dwelling heat loss, it is likely most dwellings heat loss is greater than the HP's BTU output above that COP 1, necessitating a supplement heat source.
 
  #4  
Old 06-11-09, 07:30 AM
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I'm in an area of about 7500 degree days, which is rather cold, northern NE. In my field I have had several opportunities to discuss poor performance from air source HP's. Ground source are of course much better. The old mark was generally 40 degrees with newer units, as zoneout mentioned, remaining somewhat effective down to 10 degrees. The one exception he was trying to remember is the Acadia from Hallowell International, it is rated down to minus 30. Long story. But the problems I have encountered are less the HP and more the house. Extracting any amount of heat from 10, 20, or 30 degree air is a daunting task and should not be expected to be very efficient unless one has taken extreme measures to reduce the total amount of heat they need. Not that it improves the efficiency, but it reduces the demand. In a southern climate yes, but when you want primarily heat, you have to help the process out significantly.

My recommendation, since a HP relies on electricity, is a back-up source of heat. One, it will be there when the power is out, which always seems to occur when you need heat the most. And two, an alternate source of heat can be kicked on to prevent the HP from dipping deep into your pockets when it has to switch to the electric heaters.

To be fair, my opinion is biased a bit due to our electric rates, 17.5 cents per KW, last I looked. Paying half that amount is just like doubling the efficiency.

Bud
 
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Old 06-11-09, 08:36 AM
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Okay guys. Is ti recommended the furnace guy tear out the heat pumps? Or leave them and somehow join the new gas furnace in tandem with the heat pump? As I have heard one tenant actually say that the unit was very economical in the summer. I could look into if the HP's were new to the house or later retrofitted, as this might influence the decision also. The place was built in the 70's or 80's.
 
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Old 06-11-09, 08:52 AM
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Originally Posted by ecman51` View Post
Okay guys. Is ti recommended the furnace guy tear out the heat pumps? Or leave them and somehow join the new gas furnace in tandem with the heat pump? As I have heard one tenant actually say that the unit was very economical in the summer. I could look into if the HP's were new to the house or later retrofitted, as this might influence the decision also. The place was built in the 70's or 80's.
I couldn't make that recommendation without knowing more about the existing system. What is it? How old or what is its SEER or EER or HSPF rating? Do you want or need A/C? What's the added cost for ductwork? Will the new furnace fit or is it a shoe-horn job if you keep the heat pump? Maybe the pro knows more.

I hate throwing out perfectly good equipment, but if its a hassle or costly to integrate, or on its last leg, or extremely inefficient, then its time to say good-bye.
 
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Old 06-15-09, 07:14 AM
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Originally Posted by ecman51` View Post
Okay guys. Is ti recommended the furnace guy tear out the heat pumps? Or leave them and somehow join the new gas furnace in tandem with the heat pump? As I have heard one tenant actually say that the unit was very economical in the summer. I could look into if the HP's were new to the house or later retrofitted, as this might influence the decision also. The place was built in the 70's or 80's.
Going dual fuel is a great setup. It allows people to hedge against high electrical or gas/propane/oil costs. It is highly recommended, IMO, if you can get AHRI matched furnaces for those heat pumps.
 
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Old 06-15-09, 08:58 AM
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I will ask the landlord today if that possible plan was even brought up. I have a funny feeling it will be a destroy/replace project. The landlord I work for often throws big money at big jobs, but then questions throwing small money at small jobs.
 
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