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  1. #1
    Join Date
    Nov 2005
    Location
    central virginia
    Posts
    12
    is there a temp range where i should simply set the heat pump to run on emergency heat? I know it was in teh 20's and my heat pump ran all night. it kept the house in the low 60's .

    Is it advisable to turn it to emergency heat at some outdoor temp if I know it will be that temp or lower for awhile?

  2. #2
    Join Date
    Jun 2003
    Location
    Florida
    Posts
    9,548
    No,unless your backup heat source has enough btuh's to heat your entire home. at low temps, the heatpump needs help(aux heat), but is still effecient compared to other forms of heat.
    If everything was always done "by the book"....the book would never change.

  3. #3
    Join Date
    Apr 2004
    Posts
    7,680
    The back up heat is supposed to be sized to carry the entire load. (size heat pumps for cooling and auxilliary heat should be sized to heat the home, even though the heat pump doies the lions share, the heaters are to be sized to carry it.)

    And there is never a good reason to turn off the outdoor unit unless there is a service issue. Let it run, it's always more efficient for the percentage it contributes of the total output than just going straight to electric heat.

  4. #4
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by docholiday
    And there is never a good reason to turn off the outdoor unit unless there is a service issue. Let it run, it's always more efficient for the percentage it contributes of the total output than just going straight to electric heat.
    I didn't think this was true below the point when the coefficient of the performance of the heat pump drops below 1 (e.g. it takes more electricity to run than the equivalent heat that can be moved into the house).

    I'm not sure exactly when this happens though (e.g. how cold it has to be outside)

    [Edited by JoeSix on 12-08-2005 at 12:48 PM]

  5. #5
    Join Date
    Jan 2003
    Posts
    243
    The coefficent on a heat-pump never drops below 1. Let the system run. most systems will shut down usually around 0 degrees but only because the unit is not designed for that low of temperature. It is always better to let the heatpump run.

    The reason that it is running all night is because the heat pump is not producing enough BTU to heat the home. So then the aux heat will step in to heat when needed. It is fine for the system to run for long periods. It is acually more efficent running constantly than it is cycling.

  6. #6
    Join Date
    Apr 2004
    Posts
    7,680
    Joe

    I'm randomly picking a heat pump spec sheet. It really doesnt matter which one for the sake of the discussion.

    At 47 degrees you have a COP of 3.12
    At 17 degrees you have a COP of 2.22

    Now I'm not bold enough to suggest this is exactly linier but for all intents and purposes it aint that far off. you will be well below -20 before you even get close to a 1.0 on a properly operating machine.


  7. #7
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by docholiday
    Joe

    I'm randomly picking a heat pump spec sheet. It really doesnt matter which one for the sake of the discussion.

    At 47 degrees you have a COP of 3.12
    At 17 degrees you have a COP of 2.22

    Now I'm not bold enough to suggest this is exactly linier but for all intents and purposes it aint that far off. you will be well below -20 before you even get close to a 1.0 on a properly operating machine.

    Damn. They work even better than I thought. I stand corrected. (That's what happens when you don't double check your sources -- I recently read an article on heat-pumps that said the break-even point was in the teens. Clearly it was mistaken.)

    So what does this mean in reality? If the above heat pump is, say, a 2-ton, who many BTU's is it putting out at 47 degrees and how many do you get at 17 degrees?


  8. #8
    Join Date
    Mar 2004
    Posts
    1,042
    Heat pumps are generally at their full rated capacity at 47 degrees.... so you generally expect around 24,000 at 47 (to the extent that two tons is still 24,000 BTU these days). At 17 you are typically around 14,000 BTU for a two ton, and yeah, COP is still well above 2 at that point for anything on the market now.

  9. #9
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by wyounger
    At 17 you are typically around 14,000 BTU for a two ton, and yeah, COP is still well above 2 at that point for anything on the market now.
    I wish I had known this 4 years ago before I got talked into a 66K BTU furnace for a townhouse with a 15K BTU heat load. Sounds like I wouldn't even have needed much auxiliary heat...

  10. #10
    Join Date
    Apr 2005
    Location
    Kingston Ontario Canada
    Posts
    1,233
    Yes I have a 2.5 ton 410A system and it is heating my house no problem. The upper floor of my house is 1400 sq ft, and I have a full finished full basement. It was 5 F this morning and my heat pump heated the house to 70 F, although it may have run for approximately 3.5 hours to 4 hours to get it there.I have it set back to 68 F overnight. I did have a refridgeration mechanic tell me once that he had a older Goodman heat pump and he use to shut if off around the end of December and just use his emergency heat. I assume that this was to protect the integrity of the heat pump. I have read here on this forum by a few techs that heat pumps shouldn't be set back and that they should be kept at a constant temperature. So I have re-set my overnight temperature from 66F to 68F and my daytime temperature to 70F. My unit must be oversized as my house in quite well insulated and heats the house? Talking about insulation, I think that is the key in deciding to install a heat pump or not. In houses that have a significant heat loss heat pumps don't work so well, and houses that are well insulated and weather proofed, heat pumps are a good choice.

    Thorton

    [Edited by thorton on 12-08-2005 at 03:46 PM]

  11. #11
    Mathematics aside,
    If it's 10F outside, you will probably cycle the strips half-duty. That means for the other 12 hours a day you are blowing 85F air from the vents which can make you feel cold. So what do you do, you turn the heat up from 70F to 74F to stay warm.
    Alternatively if you just used em heat you could leave it on 70F and let it cycle. In other words, you feel more comfortable at a lower temperature.

  12. #12
    Join Date
    Jun 2003
    Location
    Florida
    Posts
    9,548
    Originally posted by docholiday
    The back up heat is supposed to be sized to carry the entire load. (size heat pumps for cooling and auxilliary heat should be sized to heat the home, even though the heat pump doies the lions share, the heaters are to be sized to carry it.)

    And there is never a good reason to turn off the outdoor unit unless there is a service issue. Let it run, it's always more efficient for the percentage it contributes of the total output than just going straight to electric heat.


    Maybe where you are....here you have a 5kw backup which won't handle the load at low temps, but it will keep you alive(emergency heat).
    If everything was always done "by the book"....the book would never change.

  13. #13
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    Originally posted by fortressofcomfort
    Mathematics aside,
    If it's 10F outside, you will probably cycle the strips half-duty.
    Depends on the house losses, etc. I have a 3 room 500 sq-ft addition with a 1.5 ton Goodman HP. Last night 16 degrees. Maintained 70 deg inside, no Aux, and 30-40 minute run time and 30 minute off.

    paul

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