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  1. #1
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    219

    new system questions!

    Okay, so the new HP is running.. I love this unit already, holy crap, is it quiet! My old AC unit used to make a heck of a racket! Anyway, it was cool yesterday and he got a late start so the tech said he would come back on a warmer day and finalize things, said the superheat (14) and subcool (8) were both a little high.
    Anyway, I have a few questions, of course..
    In AC mode, first stage, the temp drop was 25 degrees across the evap! I thought newer systems ran warmer? Then again, it was cool outside with no humidity. This morning I couldn't resist running the heat.. on low heat the temp rise was 30 degrees, is that normal? I have no clue on outputs of heat pumps.
    Why can I see the outside coil temp if I scroll through my t-stat menus? Is there something a homeowner can gain by knowing this?
    Thanks, Mike

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,025
    In AC mode, first stage, the temp drop was 25 degrees across the evap! I thought newer systems ran warmer? Then again, it was cool outside with no humidity.
    The first stage usually has very low velocity of airflow going through a larger coil area per heatload on it, thus perhaps a colder Evap-coil with a lot of coil & fin area with which to drop temp!

    The humidity level in the conditioned area of your home has the biggest affect on temp-drop; low humidity means a bigger tem-drop.

    It will have a larger temp-drop at higher dry bulb/sensible temps in your home; say 80F coupled with a low relative humidity. It could be a drop of 25-F...low airflow will increase temp-drop, however in a fairly normal range; - not nearly as much as big changes in humidity.

    Having a humidity gage is a must for checking indoor temp-drop performance of the system; then check the outdoor condenser temp-rise above the outdoor temp.

    The normal condenser temp-rise is dependent on the SEER rating of the system; higher the SEER the lower the temp-rise at specific conditions.

    The indoor humidity level also affects the condenser temp-rise, because the higher the indoor humidity the more latent heat-load will show up in the condenser's temp-rise.

    We can look at charts & determine if the indoor Return Air system is drawing hot humid air from an unconditioned source; such as the attic or garage, etc.
    Last edited by udarrell; 05-05-2013 at 12:50 PM.

  3. #3
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    219
    Darrel: It's been absolutely perfect weather here in NE Ohio, the temp and humidity inside my house yesterday was 68 deg and 38% humidity, so I am thinking that all the energy was going to temp drop, hence the reading.
    So this morning I was googling superheat and subcool, the readings he got late yesterday were 14SH and 9SC, according to the manual the unit wants 7-9SH and 5-7SC. From what I can find and comprehend, I am not getting the best efficiency with these readings, but should not be stressing the new unit. Am I understanding this right? Should I be concerned? How far outside suggested ranges raises a red flag? I just want to know if I can run this thing if we get a good hot spell (or cold spell) before the tech comes back to tweak... thanks for any advise. Mike

  4. #4
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    Yes run the thing and have it rechecked when loads are higher.

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,456
    7-9 superheat seems low. The 14 you got is more in line what we normally see residential equipment wanting.

  6. #6
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,025
    Heat pumps have a suction line accumulator to catch any liquid flood-back, & therefore some mfg'ers tend to operate them in the cooling mode superheat range he listed...

  7. #7
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,668
    The unit will be fine to run until he gets back to check when it warms up. Your current SH and SC readings are within range as they are.

  8. #8
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,456
    Quote Originally Posted by udarrell View Post
    Heat pumps have a suction line accumulator to catch any liquid flood-back, & therefore some mfg'ers tend to operate them in the cooling mode superheat range he listed...
    The indoor coil is the same on almost all brands between HP & cool only so their superheat is same on either product. And most heat pumps these days don't have accumulators. They cost too much! Still say under 10 SH is low for residential cooling.

  9. #9
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,371
    Quote Originally Posted by BaldLoonie View Post
    The indoor coil is the same on almost all brands between HP & cool only so their superheat is same on either product. And most heat pumps these days don't have accumulators. They cost too much! Still say under 10 SH is low for residential cooling.
    I've noticed that it isn't uncommon to see 5-8 deg superheat when dehumidifying at 320 cfm/ton with the Hyperion (the TAM8 gives you a superheat reading). Of course under normal scenarios and load conditions 10-15 is more common. Does that sound right to you, Bald? Just what I've noticed and assumed was typical. The EEV supposedly protects against low superheat, so I suppose that's normal and just typical of running a lower airflow, particular when the total coil load isn't very high. I'm pretty convinced EEVs are the way to go (all the more reason to braze with nitrogen though). They throttle to get the coil as cold as possible without freezing, and are pretty dynamic to compensate for a wide range of load conditions. Cooling down to 50-55 deg outdoor ambient temp doesn't seem to be a problem either even with a low indoor load.

  10. #10
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    219
    Okay, so I will run it and he will drop by on a good day to use AC and see where it's at, and he can do what he wants... But I'm gathering from the replies here that you all are mixed on what the superheat should be. But just so I understand things, am I correct to think that the lower superheat makes it more efficient but gets closer to the danger zone of bringing liquid back to the compressor, and the higher superheat means less efficient but safer? And what role does subcooling play? That number was much closer to the specs. Thanks, Mike.

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    The correct numbers will be in the installation manual. IF he weighed in the charge, it will be close enough until he can return when it's 80F outside so the ouse has some load and it can be run on high stage for 10 minutes.

    In mild weather over 55F, depending on you controls and the airflow settings, a 30F temp rise would not be unusual, especially in 1st stage. My 2 stage heat pump was running just a little over 35F temp rise in low yesterday when it ran for a while while it was 63F outside. Its' set to a comfort airlfow mdoe, so it runs lower airflows than typical. the airflows are dependand on indoor and outdoor temperature (system capacity).

  12. #12
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    Have him take quick line temp of the suction line leaving the indoor coil and compare it with line temp at outdoor unit.

  13. #13
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    219
    Based on what I've read and understood, taking the temps at both places with see how much change there is in the lineset, correct? Not sure what he will do when he comes back, but it is a 13-foot line set, so it is not a long run. I know he wants to check the air flow, static pressures, SH and SC, after I manually switch the dampers from heat to cool. Right now the system "chose" 1199 cfm airflow, of course that is theoretical (it is a 3-ton system) but may need tweaking, will find all this out soon enough.

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