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  1. #1
    Join Date
    Jul 2008
    Posts
    13

    Heat pump: any output below 0 degrees F?

    Does a typical (not special low-temp type) air-air heat pump provide any usable heat output below 0 degrees F? In looking at specs for popular brands, the lowest temperature for which data is given seems to be 0 degrees F. However in the area where I live, winter temperatures can drop to -20 F or even -25 F for a few days at a time.

    I am wondering if the heat pump will still provide output to complement the electric resistance coils at these temps? Will it at least keep operating at C.O.P. = 1 or 0.95 and deliver most of the electric power going into it as BTUs in the house? Or will it completely stop operating at extremely low temperatures and leave me with nothing but the capacity of the resistance coils to warm the house?

    The reason I am asking is that I want to make sure the HVAC contractor puts in large enough electric coils, if the heat pump doesn't give me any heat at all under extremely low temperature conditions.

  2. #2
    Join Date
    Mar 2008
    Location
    mesa arizona
    Posts
    309

    yo

    yes in a heatpump.......rreally dont work in cooooooolllld weather.......heatstrips are supplementary heat ur adittional heat

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,176
    Below 0, most heat pumps are still barely above 1 COP but their output is minimal. If you figure a timed defrost each hour, the net effect is hardly anything. I may be in the minority but I tell people to shut the thing off when it is that cold, especially with timed defrost. If you have a stiff wind blowing, you may run in defrost 5-10 minutes before you get that coil up to 70-80 to terminate defrost. That's a lot of cold air being put in the house!

  4. #4
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    From the Expanded Heating Data for my 3-ton Goodman R22 14 SEER heat pump:

    Ambient----------Air handler
    Temp-----BTU-----Temp---------COP

    10F-------18,700----86.2--------2.26
    5F--------16,600----84.4--------2.06
    0F--------14,400----82.6--------1.84
    -5F-------12,300----80.7--------1.61
    -10F------10,100----78.8--------1.35

    The "air handler temp" is the temperature of the air as it leaves the indoor coil, based on a 70F return air temp.

  5. #5
    Join Date
    Jul 2008
    Posts
    13
    BaldLoonie,

    I don't live very far from your location in Indianapolis...I'm in N. Kentucky near Cincinnati so my climate is similar to yours. Do most people with heat pumps in this region have additional heat sources besides the resistance strips to replace the lost output of the heat pump when it gets really cold?

    I have been looking into replacing my oil furnace/split AC with a packaged heat pump unit to recover some square footage in the basement taken up by the oil tank and furnace. Now I am worried that the 3-ton heat pump with 7kW to 10kW heat strips won't cut it when the outside temps fall into the -20s F.

    Thanks,

    Shran

  6. #6
    Join Date
    Jul 2008
    Posts
    13
    gary g,

    You're still getting some usable BTU at -10 F, even though the leaving air temp drops. Thanks for the data.

    Shran

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,176
    That's higher than I expected it would be.

    Up here, the average home with a 3 ton pump would have 15 or 20kw backup and heat just fine. In our area, all electric homes have lower overall bills than those with gas in them. Figure that somewhere up to $240 a year is just paying for the honor of having gas service then add usage on top of that. So having the one fuel saves.

    If your house is big enough to need 3 ton, 10kw won't do it for you.

  8. #8
    Join Date
    Jul 2008
    Posts
    13
    BaldLoonie,

    Excellent information - thank you.

    Shran

  9. #9
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    Quote Originally Posted by Shran View Post
    gary g,

    You're still getting some usable BTU at -10 F, even though the leaving air temp drops. Thanks for the data.

    Shran
    I would tend to agree with Bald Loonie: I would probably shut my heat pump off at those brutally cold temps because the heat pump would run constantly and go into many defrost cycles (I have time-initiated defrost).

    Take care.
    Last edited by gary_g; 08-27-2008 at 01:32 PM. Reason: Added defrost comment

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,087
    You should still get usable heat at -10°F.
    If its a heat pump with on demand defrost, you don't waste alot of electric in needless defrost.

    Shortly after -10°F, the heat pump isn't any more efficient then the strip heaters.

    Same as baldie though. A 3 ton around here would normally have a 15 KW strip heat package.
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  11. #11
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    Quote Originally Posted by BaldLoonie View Post
    That's higher than I expected it would be.

    Up here, the average home with a 3 ton pump would have 15 or 20kw backup and heat just fine. In our area, all electric homes have lower overall bills than those with gas in them. Figure that somewhere up to $240 a year is just paying for the honor of having gas service then add usage on top of that. So having the one fuel saves.

    If your house is big enough to need 3 ton, 10kw won't do it for you.
    I have a 1700 sq-ft, all-electric, split foyer built in '86. 3-ton heat pump and 15 kw strips.

    15kw = 51195 btu-hr

    BTU = 1.08 x cfm x Delta T

    Delta T = BTU / (1.08 x cfm)
    = 51,195 / (1.08 x 1400)
    = 33.86 degrees

    So, 70F return air temp + 33.86 = 103.86F air temp after the air handler with a fan speed running at 1400 cfm.

  12. #12
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    The data put up by gary_g is important. Notice that the COP is nearly 2, which is a good thing. You also see the downside. The 36,000 BTU heat pump is putting out less than 1/2 of that capacity at 0 deg F. This is the issue. The heat it is producing is nearly twice as efficient as electric resistance, but the capacity is down, and normally when it is colder the house's losses go up, so supplement must be added. Where the house's losses equal the capacity for a given temp is what is called the balance point. Below that temp you must add some heat to keep up. As was already stated, frequent, unnecessary defrosts do no good for the overall efficiency. This is where demand defrost pay off, but is available on certain brands and models.

    paul

  13. #13
    Join Date
    Jul 2008
    Posts
    13
    tecman - none of the reps has mentioned demand defrost to me. I will have to ask about that - thanks.

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