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Old 10-08-2014, 06:15 PM   #1
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Home heat pumps

Something does not add up here, a compressor refrigerator is like a heat pump, it cools by pumping the hot air out of the box. As anyone knows, heat pumps are ideal in temperate zones, but poor performers in cold and extreme heat. So explain why a compressor refrigerator will work more efficiently/effectively in -0- weather as well as 100 degree weather than the absorption unit?
The heat exchange does suffer in both scenarios in both units does it not? Basic physics here. So why would one out perform the other?
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Old 10-08-2014, 06:44 PM   #2
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All I know is I lived in a house with a heat pump for 20 years and it never got the house warm when it was below 20 degrees nor cooled when it got over 95 degrees out side, it ran continuously in those extremes and when natural gas became available, I jumped on it and added a gas furnace backup, used the heat pump down to freezing and then turned it off and the gas furnace on when it got colder, I was warm then!
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Old 10-08-2014, 07:24 PM   #3
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Originally Posted by cpaharley2008 View Post
All I know is I lived in a house with a heat pump for 20 years and it never got the house warm when it was below 20 degrees nor cooled when it got over 95 degrees out side, it ran continuously in those extremes and when natural gas became available, I jumped on it and added a gas furnace backup, used the heat pump down to freezing and then turned it off and the gas furnace on when it got colder, I was warm then!
Jim

A heat pump is a different animal. Your heat pump is also your air conditioner I assume. I'm sure it cools your house in the summer just fine below 95 degrees. When it goes into heat mode it reverses the refrigerant flow and absorbs heat from the outside and transfers it into the house by way of the refrigerant flowing through the system. At 0 degrees there is no heat for it to absorb so your auxiliary heat (electric or gas) kicks in to help out. Heat pumps are not designed for northern climates and will not ever work good in really cold temperatures. Heck they don't work good in Tennessee. Just like the A/C side of it will poop out around 110 degrees. Luckily we don't have many 110 degrees days and your A/C will work fine up to around 100 degrees. It just a matter of design of the system. It's designed to heat fine down to around 25 degrees then after that the heating capicaty falls way off. Above 95 degrees the cooling capicaty falls way off. Because the capicaty falls way off under extreme temperatures it will run continuously by design.

Same as our fridges are doing above their design conditions. Above 85 degrees its capicaty is way down. Add to that a lack of a well insulated box, leaking doors, under size cooling system and a few other designs issues and here we are 5000 post later still talking about it.
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Old 10-08-2014, 08:17 PM   #4
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Agree with KKamper - heat pumps can't handle extremes well. Have to wonder how well they do in Coleman ACs that tout "heat pump" in their ads.
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Old 10-08-2014, 08:59 PM   #5
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Heat pumps are not designed for northern climates and will not ever work good in really cold temperatures.
I agree - if the heat source is the outside air, they're pointless at freezing temperatures. In addition to the problem of forming frost on the (outside) evaporator, their coefficient of performance (the ratio of heat moved to energy taken to operate the unit) drops so low that it makes more sense (economically and environmentally) to burn fuel in any modern furnace to get the required heat.

I think in an RV with heat pump (which includes our large non-Escape trailer), the intent is to cover cool nights in the camping season, not real winter.
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Old 10-20-2014, 01:58 PM   #6
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I agree - if the heat source is the outside air, they're pointless at freezing temperatures. In addition to the problem of forming frost on the (outside) evaporator, their coefficient of performance (the ratio of heat moved to energy taken to operate the unit) drops so low that it makes more sense (economically and environmentally) to burn fuel in any modern furnace to get the required heat.

I think in an RV with heat pump (which includes our large non-Escape trailer), the intent is to cover cool nights in the camping season, not real winter.
The latest generation of heat pumps actually work very efficiently even in very cold climates. Ours keeps our house warm down to about -15 to -20F and even at -10F it is far cheaper to run than our LP gas furnace. Partly that is due to the fact that we have a dual fuel - demand control agreement with our electric coop. They can cut power to our heat pump when they are experiencing high demand, and with that they give us 1/2 price per kwh during the winter. But even at regular prices it is still usually cheaper than our super efficient LP furnace, just not as drastically cheaper.

This heating fuel comparison calculator is a very useful tool: www.eia.gov/tools/faqs/heatcalc.xls

You do have to spend some time finding the exact ratings of your heating devices. At the bottom of that spreadsheet it has adjustments to make on your efficiency based on the state - sorry, it is US only no adjustment data for Canada is provided.
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Old 10-20-2014, 04:27 PM   #7
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The latest generation of heat pumps actually work very efficiently even in very cold climates. Ours keeps our house warm down to about -15 to -20F and even at -10F it is far cheaper to run than our LP gas furnace. Partly that is due to the fact that we have a dual fuel - demand control agreement with our electric coop. They can cut power to our heat pump when they are experiencing high demand, and with that they give us 1/2 price per kwh during the winter. But even at regular prices it is still usually cheaper than our super efficient LP furnace, just not as drastically cheaper.
I think you're seeing dissimilar prices for two different sources of energy. My guess is that at the lower temperatures your electrical generation utility is burning more fuel (coal, oil, natural gas) of more energy value to run the heat pump than the energy value of LPG which would go into the furnace. LPG (or "propane") is not cheap; for many people, the choice would be electricity for the heat pump or natural gas, and the economics would be different. In the linked calculator, propane is basically tied with electricity and kerosene as the most expensive energy source - I wouldn't use any of those three if I had a choice (I do have a choice, so I use natural gas).

In the linked calculator, the heat pump is assumed to put out 2.4 times as much heat energy as it takes in as electricity. Unfortunately, power generating stations are less than 50% efficient, so one unit of coal/oil/gas energy going into the station produces perhaps 0.4 units of electrical energy, which the heat pump uses to deliver... yes, just that same one unit of heat energy. Even in this nice case (of no challenging environmental conditions) the heat pump approximately matches a furnace.

The calculator also mentions the use of electric resistance auxillary heat with air-source heat pumps. That can make a heat pump look like it is working at very low temperatures, when in fact it has shut down the pump and is just running as an electric furnace (a.k.a. toaster in a vent).

Of course, in an RV the choice is electricity versus propane... but I doubt the either the RV air conditioner / heat pump or the RV furnace is nearly as good as the home equivalent.


Now, this thread was supposed to be about moving heat out of the food storage box, not into the residence...
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Old 10-20-2014, 05:22 PM   #8
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Nope - no auxillary heat used with ours when heat pump is working. My point was that modern air sourced heat pumps can work well below freezing. I won't take up anymore of space here as this is indeed off topic.
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Old 10-20-2014, 07:11 PM   #9
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You will note that the posts on home heat pumps have been moved to their own thread ( at the request of a member ). Thanks a lot. Forced me to reread all the posts and try to determine what was relevant to the OP.
Hoping that nobody complains.

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Old 10-21-2014, 11:00 AM   #10
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Since Glenn went to all that work, I might as well post a couple of links about new heat pumps. Most of the discussion on this thread has been about the early generations of heat pumps that were not efficient enough to work well in anything other than very temperate climates. Since then, new technologies have been developed to made them dramatically more efficient.

Ours is now about 4 years old and since then even more efficient types have been developed.

As I was reminded of by the energy people I worked with before installing ours, any temperature above absolute zero means that there is some heat energy present. Zero in C or 32F are the freezing point of water, but have no real meaning in terms of a heat pump - other than that was about the temperature the early models stopped being efficient enough to extract any heat.

Everyone I spoke with said that those early generations of heat pumps were generally reviled by anyone in cold climates and that that had made it very tough to get anyone to listen about the new ones.

New heat pump technology can warm homes, even in cold New England winters - Business - The Boston Globe

Architectural Record | Can a New Kind of Heat Pump Change the World?
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