Thinking heat pump undersized
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  1. #1

    Thinking heat pump undersized

    Hi there,

    I'm new to this forum and this is my first post.

    I have a new house and I acted as the general contractor during the build. We moved in in June of 2011. I decided to have a Luxaire Acclimate 2 stage heat pump and variable speed air handler installed. The reason I decided on the Luxaire unit was due to the fact that the guy selling the unit was very competent. He sized the unit to a 4 ton. I'm heating approx. 4650 sq ft with the heat pump, have 9 foot ceilings on the main floor, cathedrial ceilings in both my sunroom and master bedroom. I have in floor electric heating in all my tiled areas, which is maybe 40% of my total floor area of the main floor and upper floor. We also have three propane fireplaces installed as well.

    My issue is that my aux. heat, which is 15kw, is cutting in way too often. It's consuming between 100-150kwh per day. I have an energy meter installed on my air handler/aux heater. I have spoken to the guy numerious times, and he has come back and installed a switch so I could turn off 10kw of heat. The problem is that the house cools off too much and even with the 5kw of heat going so now I leave it on.

    He says he has done a heat lose calc and come up with 44,000 btu.

    I live just outside of St. John's, Newfoundland, Canada. The temperature doesn't get too cold. The past few nights it has gotten down to 14f, but last night it was 25 and it was still using the full 15 kw of heat.

    I've had my infloor turn off for a while to see if the heat pump could keep up and it can't so I've turned my in floor back on and it's not really helping.

    When the guy's installed the system they installed a couple of remote sensors, one in my living room and one up stairs. We were finding the up stairs really cold so the guy came back and turned the sensor on upstairs. So now the termostat is reading that sensor. Were still finding the upstairs cooler than the main floor so we have been closing the grills on the main to try and force more air to the upper level. Now the guy is insisting on coming back to install manual dampers in the basement to close off the main floor ducts. I think it's a waste of time considering the grills are almost closed.

    Could someone please give me your opinion on why my aux heat is kicking in so much? Is it an undersized heat pump or a balancing issue?

  2. #2
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    If you have a bigger heatpump,will be too big in ac season.
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  3. #3
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    You want to balance your airflow at branch where it attaches to main,not @ diffuser.
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  4. #4
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    14f is pretty cold. I don't know your area but it seems like it could be undersized. Without knowing much more about the house like windows insulation etc, we can't tell you if it's undersized. For that many sq feet in Portland Or(lows in the high 20s less than a few days per year) we would be wavering between a 4 and 5 ton. Remember a heat pump puts out approximately 50-60% of its rated capacity at 17f. So if your heat load is 44,000 BTUs and your heat pump is rated for 48,000 your getting less than half of your needed heat at 14f.
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  5. #5
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    Tank

    Have you actually seen the load calc in writing on software letterhead? If not, I would ask for it.

    Several things

    I would want to see an independent load calculation performed for comparison purposes. You can perform one yourself-see HVAc Calc above. I believe the charge is around $50 or pay another dealer to perform one. You might be undersized.

    Refrigerant charge on system should be checked.

    And make certain second stage heating on condenser is working properly.

    Your 15 KW heat strip should be staged.

    That would be a good starting point to get to the bottom of your high electric usage.

    IMO

  6. #6
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    A heat pump should be sized for the cooling needs, backup for the heating needs. So a house in a cold climate (ours is colder) will need a lot of backup heat when it gets frosty. We size to -10 to be safe, I think our design temp is 0. Around here, any house with a 4 ton pump will have AT LEAST a 20kw backup, an older system could be 25 and we have seen 30. Hopefully you really built the place well being new. Got foam insulation???

    Anyway, I'd agree with openclass, if you tried to size a heat pump for the heating load or a good majority of it, you would be way oversized for cooling, even with a 2 stage. That technology is 75-80% capacity on low.

    Somebody crunch a summer heat gain, that's what the HP should be sized for. If he thinks 44K for a loss, then he's sized your backup just fine.

  7. #7
    Join Date
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    Quote Originally Posted by BaldLoonie View Post

    Anyway, I'd agree with openclass, if you tried to size a heat pump for the heating load or a good majority of it, you would be way oversized for cooling, even with a 2 stage. That technology is 75-80% capacity on low.

    Somebody crunch a summer heat gain, that's what the HP should be sized for. If he thinks 44K for a loss, then he's sized your backup just fine.
    while I agree somewhat I also slightly disagree. The Trane two stage units like the XL20i are 50% of capacity at low stage and a XL16i is 60%-70%. We have installed systems and locked them into first stage cooling to meet both loads accurately but I also know we don't have the same extremes in Portland that you do in Indiana. This is why I like the 20i so much is that it would be a perfect unit for this example. 5 tons of heating and only 2.5 tons of cooling requiring less backup heat and more efficiency while still dehumidifying in summer. Then there are the variable units like the iQ drive and green speed.
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  8. #8
    Thanks for the responses guys!

    The house is 2x6 wall construction with R-20 batt insulation in the walls, ceiling is R-50 blown in and windows are low-e coated with argon gas.

    The first step that the guy wants to do is the dampers in the basement. My fear is that the coldest part of the winter will be over and we'll run out of time in getting this resolved this year.

    I haven't seen the load calcs myself and I was going to download the software to do my own check. I've also been in contact with my utility company to see if they can provide me some assistance.

    Does anyone know how to tell if and when the second stage is cutting in?

  9. #9
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    Amp draw ,heat rise increase
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  10. #10
    I don't mean aux heat. I mean the second stage of the heat pump.

  11. #11
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    Heat rise
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  12. #12
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    Mar 2006
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    Tank

    Keep in mind that a true 15 KW heat strip yields about 51 KBTUs.

    IMO

  13. #13
    Yea, I know. And that's why I'm concerned. Last night it was 25f and 15kw (51,000 btu) of heat going along with the 48,000 btu from the heat pump. Maybe a little less due to the temperature. 99,000 btu of heat to counteract 44,000 btu's of heat loss. Something is wrong there.

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