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How Cold Is Too Cold?

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[#1] Posted: 01/21/2008 - 7:00:43 PM
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I'm being lazy, because I'm sure someone here knows this off the top of their head:
At roughly what temperature/range do heat pumps become ineffective and need back-up heat to keep up? I'm sure it can depend on certain variables, but I suspect there's a general rule of thumb of some kind.

Brian G.
Why Are They Called Heat Pumps Instead of Cool Pumps?


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How Cold Is Too Cold?
[#2] Posted: 01/21/2008 - 7:24:59 PM
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I don't know that there is an exact number, but I seem to recall that their heat output isn't really adequate/ efficient at some point in the 40's. You had better have backup heat if you expect colder temps than that.


Kevin

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How Cold Is Too Cold?
[#3] Posted: 01/21/2008 - 8:16:41 PM
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Efficiency begins falling off in the low 40's and the colder it gets the less efficient they become. I had a unit years ago that I actually disconnected the backup strip heaters since I used wood heat when the temperatures got consistently cold enough to keep a fire going; it would keep up until the temperatures were down in the upper 20's.
Somewhere around 30-35 the COP will fall too low to get any advantage of continuing to run the heat pump.

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How Cold Is Too Cold?
[#4] Posted: 01/21/2008 - 9:23:39 PM
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There is no specific temp. or even a range. It's simply when the heat pump passes the thermal balance point (when the building's heat loss is greater than the heat pump's capacity).

It's 15° here right now and my heat pump cycles on and off a few times every hour with no supplementary heat kicking in. Ok, I'm cheating. I have a Quadrafire wood stove burning away, but my refrigerant line is still hot.

There's also recent advances in air source heat pump technology. Read about some of the new low-temp. heat pumps. There's a company in Maine (that's right - MAINE) that builds a heat pump that doesn't have or need, ANY supplementary heat (even in friggin' Maine)!


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How Cold Is Too Cold?
[#5] Posted: 01/21/2008 - 11:18:27 PM
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Quote: Originally posted by Brian G

I'm being lazy, because I'm sure someone here knows this off the top of their head:
At roughly what temperature/range do heat pumps become ineffective and need back-up heat to keep up? I'm sure it can depend on certain variables, but I suspect there's a general rule of thumb of some kind.

Brian G.
Why Are They Called Heat Pumps Instead of Cool Pumps?



Warning! I'm going from memory here. . .

Heat pumps must produce cooling in the summer and heating in the winter. Because their indoor and outdoor coils are fixed sizes -- they can't change sizes when they switch seasons -- the designer has to design the system for either the heating or cooling load. If he were to design it for the heating load, the system would be too large for the cooling load (over most of the U.S.). So they design it for the cooling load. This means that the system is too small for the heating load. (If they didn't have to worry about cooling in the summer, the designers could design a heat pump to work at pretty much any temperature without auxiliary heat.)

So, the balance point will vary by region. In my area, it seems to be about 37 degrees.

The fancy systems that Kibbel is talking about use, I believe, multi-stage compressors (or perhaps they have more than one compressor) to vary the capacity of the system.

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How Cold Is Too Cold?
[#6] Posted: 01/22/2008 - 07:18:49 AM
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I saw one of the new all season heat pumps recently at a home show. I don't have the specs anymore, but it was supposedly able to provide heat (without a backup source) down to below 0 degs F. Of course, the thing is pretty expensive right now.
Matthew Steger
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How Cold Is Too Cold?
[#7] Posted: 01/22/2008 - 7:41:57 PM
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You can make a unit for cold weather operation but the hill is just as steep for any unit.
75 degrees indoors and 10 degrees outdoors is a pretty steep hill. Think about an air conditioner that has to deal with a 65 degree TD. 75 degrees indoors makes 140 degree outdoor temperature! There is a limit to the practical application of heat pump technology.
If you spend $1 in electricity costs to produce $3 worth of heat, that is a good deal.
But if you spend $3 for that same amount of heat, electric resistance heaters are just as good and last longer with fewer repairs.
That is why you look at the COP (Coefficient Of Power) ratings and at what temperatures they are rated for at the COP.
Cold wet air also means the units must spend more time in defrost mode.
Now if you are talking about water/ground source heat pumps, forget everything I just said.

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How Cold Is Too Cold?
[#8] Posted: 01/22/2008 - 8:10:29 PM
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All day long, it was niggling at the back of my crusty subconscious but I couldn't pull it out. Then, there it was. This thread had me remembering this guy.

http://www.ornl.gov/sci/btc/ap...bhp.html

OT - OF!!!

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How Cold Is Too Cold?
[#9] Posted: 01/22/2008 - 9:31:33 PM
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NOW that is a horse of a different color. I can remember back in my youth hearing about natural gas absorption cooling units, but by the time trade school and college days came along, they had gone the way of the dinosaur. I have never dealt with absorption units.
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How Cold Is Too Cold?
[#10] Posted: 01/23/2008 - 09:43:06 AM
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The heat pump for my office switches to supplemental heat at 25f to 30f, my system does it automatically.
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How Cold Is Too Cold?
[#11] Posted: 01/23/2008 - 6:39:45 PM
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Quote: Why Are They Called Heat Pumps Instead of Cool Pumps?

Brian--
I forgot to add this since I thought you were joking. Maybe you were. However, no one else has taken it up, so...

It's not called a cold pump because it doesn't pump cold...in either season. Cold is merely the absence of heat. You can't really pump "cold" into an area any more than you can pump a vacuum into an area. A heat pump absorbs heat from outside and transfers it into your home in the winter. No matter how cold it feels to us, until you reach absolute zero, there is still technically some heat outside.

Kevin

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How Cold Is Too Cold?
[#12] Posted: 01/23/2008 - 8:36:36 PM
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Originally posted by AHI in AR

I forgot to add this since I thought you were joking. Maybe you were.

I was, but what the heck.

Cold is merely the absence of heat.

I've heard that before and I realise it has a serious scientific basis, but it sure sounds silly in a way. A critical, non-scientific mind can't help immediately wondering why it isn't equally true that heat is merely the absence of cold. If I recall correctly, the reasoning is that heat must be generated somehow; not so for cold (in the universe as a whole, not in Mississippi in the summertime).

No matter how cold it feels to us, until you reach absolute zero, there is still technically some heat outside.

True. As long as we're wandering around hot and cold for kicks, I have to mention a show I saw years ago about a guy who had a business freezing things. His computer-controlled freezers took items down to absolute zero, for a fee (waaaay below "normal zero", but I can't recall the number). At absolute zero all the molecules will spontaneously realign themselves to be exactly the same distance from each other, no matter what the material. This results in certain desireable improvements, most notably durability. He did everything from expensive machine parts to complete Harley motors. Way cool.

Brian G.
"Freeze Master" Franchises, Anyone?

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