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  1. #1
    Join Date
    Aug 2008
    Location
    Raleigh
    Posts
    43

    Typical temp raise with HP?

    I'm in Raleigh, NC and it's unusually cold (23 degrees right now). My house is 4 months old, very well insulated (energy star cert). The upstairs is about 1700 sq ft. I have a lennox XP-16 - 3 ton unit. Plain old lennox air handler. Right now t-stat set at 72, the return air temp is 72, and the supply is only 79. Should I be seeing more than a 7 degree increase? Seems low to me. Also, last night it got to about 15 outside and even with the 5kw resistance heat, the temp dropped to 68 inside. Is 15F cold enough that the HP and aux heat shouldn't have been able to keep up?

  2. #2
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,903
    Yes - too low.

    If the ductwork is in the attic, check for missing duct insulation and leaks. If problem persists, get the system checked.
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  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,898
    It does sound low.

    The low temp rise could be caused by the heat pump its self having a problem, or it could be a duct issue.

    Or, the method you used to measure the temp difference.
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  4. #4
    Join Date
    Feb 2004
    Location
    Pacific NW
    Posts
    567
    Delta air temp at 23F outside for a 3 T unit should be around 11 or 12 F, still pretty low.

    Typical 3T at 23 F may be putting out only 15,000 BTU/hr.
    Typical 3T moves approx 5000 pounds of air per hours, or say 1250 BTU/degF. That gives only about 12 F air temp rise.

    If your blower is set for highest speed you may be moving 7000# per hour, which would give you only about an 8F rise.

    So, either you are really moving a lot of air (goodness), you have air leaks someplace (badness) like amd and beenthere say, or the Lennox heat output with temp drops faster than the rule of thumb I used for the 15K number.

    The 7F would be accurate if you are moving over 8000# of air/hour (over 1800 CFM, unusual for fastest fan on typical 3T, but you may be moving that much air)

  5. #5
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    it's 20 degrees out now i am getting a 14 degree rise out of my heat pump!



    .

  6. #6
    Join Date
    Jan 2008
    Location
    Lehighton, Pa.
    Posts
    178
    When I got up this am,it was 20 deg. here in SC. I also have the Lennox xp16, however it's a 4 ton. I've been having a lot of problems with my unit , but it appears that it is working much better now.

    As a comparison to your numbers, mine are: as stated, o/d -20 return-74, and supply-88


    ADDENDUM: By the way, your unit is two stage. Is it running in second? It may be in stuck in first stage.

  7. #7
    Join Date
    Jul 2005
    Posts
    5,509
    Quote Originally Posted by vtbuster View Post
    I'm in Raleigh, NC and it's unusually cold (23 degrees right now). My house is 4 months old, very well insulated (energy star cert). The upstairs is about 1700 sq ft. I have a lennox XP-16 - 3 ton unit. Plain old lennox air handler. Right now t-stat set at 72, the return air temp is 72, and the supply is only 79. Should I be seeing more than a 7 degree increase? Seems low to me. Also, last night it got to about 15 outside and even with the 5kw resistance heat, the temp dropped to 68 inside. Is 15F cold enough that the HP and aux heat shouldn't have been able to keep up?
    There are too many variables to diagnose whether there is a problem or not from here. The delta_T can drop quite a bit at lower ambients. Whether yours is too low or not cannot be determined from here, not without knowing a lot of things that you probably won't be able to determine on your own.

  8. #8
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,624
    5 kw heat strip for a 3 ton heat pump seems small it only puts out 15000 btu

  9. #9
    Join Date
    Aug 2008
    Location
    Raleigh
    Posts
    43
    XP16 is 2 stage, but from what I've read - below 40 ambient, it only runs at high capacity, regardless of what t-stat calls for. My air handler is a lennox CB26 3T (based on the datasheet it can move between 900-1400 cfm). Now that I'm actively monitoring the system (and the ambient is higher) I've noticed that even at 35 ambient, I'm only getting about 4-5 degree temp increase. I did find a duct leak (small one) and it's been fixed.
    When ambient is over 20 (which is 99% of the time) it always stays nice and warm. Had it not dropped that low I never would have started monitoring my system so closely. The house is still under warranty, so I called the hvac contractor that installed the system. He explained to me that the hvac system was only engineered for normal temperatures, blah blah. I understand that. But it seems like my HP has problems. Other than temperature differential, is there anything I can say to him to convince him that there is a problem?

  10. #10
    Join Date
    Jan 2009
    Location
    South Georgia
    Posts
    42
    Tell your tech that you want to keep your house 72 deg (this is not unreasonable) and during night time your house drops to 68 deg (this is not normal).

    Tell him that you are gonna keep calling until the problem is fixed, or call a better company and have him pick up the bill.

    You have a problem and a reputable company should gladly fix it.

    Ps I would have put 10kw into that system given your geographical location and normal winter time temps.

  11. #11
    Join Date
    Jan 2007
    Posts
    84
    I agree with the poster above. 5 kw is not enought auxillary heat . You need at least 10k and maybe 15.
    This will bring up your temperature a lot faster but with more cost.
    I have 20k in mine broken up for 10k above 20 degrees and full 20k when outside temperature is below 20.

    I had no problem when our temperature dropped to -4 the other night.My heat pump is also a 2 stage Trane Xl16i. The contractor can set it up to heat your house even below 20 degrees but he needs to have enough aux heat.

  12. #12
    Join Date
    Jun 2007
    Location
    The Twilight Zone
    Posts
    2,964
    Here is technical data for my 14 SEER, 3-ton, R22 Goodman heat pump:

    Outdoor-----Temp after
    temp -------indoor coil------BTU's

    55F-----------103.7F-------38,800
    45F-----------99.1F--------33,500
    35F-----------95F----------28,700
    25F-----------91.6F--------24,900
    15F-----------88.8F--------20,800
    5F------------84.8F--------16,600

    The "temp after indoor coil" is based on a return of 70F.

    Good luck.

  13. #13
    Join Date
    Aug 2008
    Location
    Raleigh
    Posts
    43
    Thanks so much guys. This is exactly the kind of info I was looking for. The contractor will be here in about an hour. I'll post the outcome.

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