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  1. #1

    heat pump sizing with fossil fuel back up

    I know it's ok to upsize a heat pump 25% when used for heating and cooling when using elec. em/aux heat. But when you are sizing for a dual fuel set up do the same rules apply. The reason I am asking is because I did a manual j load calc. on a new 1250 sq. ft. residential home and came up with a loss of 50k btuh and a gain of about 26K btuh. The general contractor told me he wants a 4 ton 13 seer heat pump! When I asked how he came up with this size he told me he received two other bids and both were quoting for a 4 ton. Because you cannot run the heat pump and furnace at the same time, is this the reason for oversizing in an attempt to reach the heat loss with just the heat pump or did I just miss the memo altogether? Also, I have seen evap coils installed on the RA side usually for less than ideal situations. Does this make a significant difference on blower and overall system performance of equipment? Thanks in advance for your professional input!

  2. #2
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  3. #3
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    Dec 2002
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    the coil in the return is a REALLY BAD IDEA, you will see premature heat exchanger failure.

    your general contractor needs to be replaced with someone that understands moisture levels and comfort levels, not to mention mold.
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  4. #4
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    What's the location? That's still a descent heat loss and gain for a small home. Where did you Get you're design outdoor temperature from?

    If you have propane, you are sometimes just as week going all electric. Then you can run the hp and aux together.

    In you're case you might consider a 3 ton 2 stage, but assuming the codec is right...2.5 is the highest id go for single stage. The unit will sorry cycle on mild weather and hurt efficiency and comfort. A 4 ton in that home will have very little humidity removal, be noisy and drafty. If be surprised if you ductwork to support that airflow.

    Sent from my DROID RAZR using Tapatalk 2

  5. #5
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    Jul 2006
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    Virginia
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    do not install the evap coil in the return air will rust out heat exchanger read the install guide. also if a new 1250 sq ft house needs 50000 btu of heat must be some lousy construction or very cold. probably the other contractors ars oversizing because new consruction is priced by the tonnage more tons more money .
    We really need change now

  6. #6
    Join Date
    May 2000
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    Indianapolis, IN, USA
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    I would not oversize a heat pump. No professional organization says it is OK to oversize 25% in any case. If you have any humidity at all, short cycles could compromise humidity control. Moving 1600 CFM through a 1250 sq ft home could be drafty and noisy. Some may say to oversize 1/2 ton but it really changes the balance point little. And in this case, if you go 2.5 ton you are already oversizing. If this is what he wants, this might be a job to avoid ... to avoid complaints when the HO gets in his butchered house.

  7. #7
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    SW FL
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    Quote Originally Posted by hvac/airplanes View Post
    I know it's ok to upsize a heat pump 25% when used for heating and cooling when using elec. em/aux heat. But when you are sizing for a dual fuel set up do the same rules apply.

    The reason I am asking is because I did a manual j load calc. on a new 1250 sq. ft. residential home and came up with a loss of 50k btuh and a gain of about 26K btuh.

    The general contractor told me he wants a 4 ton 13 seer heat pump! When I asked how he came up with this size he told me he received two other bids and both were quoting for a 4 ton. Because you cannot run the heat pump and furnace at the same time, is this the reason for oversizing in an attempt to reach the heat loss with just the heat pump or did I just miss the memo altogether?
    YOUR Manual J indicates that the "NEW" house thermal envelope REQUIRES 100% improvement.

    1,250 square foot residence targeted heat loss should be < 25,000 BTU/HR
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  8. #8
    I forgot to mention the outside winter design db here is 5 deg. I spoke with a guy I used to work with at another company with a similar climate and he said they size all their heat pumps for heat load which sounds like the other two bidders on this job did. It only gets really warm here ( 85-90) for maybe 1.5 months and the winters are long. Again my concern is fan noise @ 1600cfm and humidity/short cycling issues. But if sized for cooling load, were talking 24,000 btuh in the winter with a loss of 50,000- the gas furnace will run most of the winter! It almost seems like either way you look at it the HO is going to be pissed off. Any other thoughts on this subject will help, thank you.

  9. #9
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    Lotta morons out there. Just because another company sizes pump to heat loss doesn't make it right. Actually he's not sizing the heat pump to the loss. A 4 ton HP at 5 out is likely about 20K output. Or at least a Goodman 13 SEER is, I can get to their specs easily. So if you really are sizing their heat pumps to the heat loss, that 1250 sq ft house needs a 10 ton heat pump!

    My house is 1000 sq ft well built. 2 ton HP, should be a 1.5 ton but I like it COLD in the summer. I switch to gas at 20 out since I keep it cool in the winter too. We size to -10, a typical winter coldest temp. My gas bill hardly goes up in the winter, the HP does so much of the work. Despite the design temp, we don't spend that much time below 20.

    But I'm with Dan and the others, very pathetic construction with that kind of gain & loss in a new 1250 sq ft home! I still say if they are wanting you to size this stupidly, it's not a job you should want to get involved with.

  10. #10
    Join Date
    Jan 2004
    Location
    PA
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    68,805
    With your heat gain. A 2.5 ton heat pump. What is your winter design temp?

    As others have said, sounds like poor construction to have that much gain and loss. unless your summer design is 115, and winter design -30.
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