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  1. #1

    Underperforming heat pump

    I have a 5 ton Bryant heat pump that just hasn't been it's old self lately. The compressor and TXV were replaced last fall. Seemed to do OK during winter, but this spring coil began freezing up in AC mode. Tech comes out and sees low suct pressure and immediately adds freon. Schrader valve is leaking too so he replaces core. Coil still freezes up, mainly at night. Comes back out and replaces TXV. Suction pressure is still low and coil still freezes. He is stumped now. I ask about airflow and sure enough turns out the insulation in the air handler is coming loose and needs to be secured. Air flows are now back in the 400 CFM/ton range. He says "let's let it run a couple of days and see how it works. You might have a cap tube plugged."

    Being an engineer and tired of techs guessing, I've taken a few measurements. Based on these, what should I expect him to do or look for when he comes back out.

    Outdoor amb 87F, WB 77F, Suct 65psi/48.5F (11.5 superheat), Disch 241psi/88F (27 subcooling), Indoor return air 72F, WB 60F, Supply 58.6F, WB 51F, liquid line temp @TXV inlet 89.6F, TXV outlet 89.3F, Evap disch line temp 58F. Liquid line sight glass clear and dry.

    Equip documentation says target subcooling should be around 12F at these conditions. Refrigerant overcharge or something else?

    Thanks in advance for your advice.

  2. #2
    Join Date
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    Um, kinda hard to do a pressure-enthalpy diagram without knowing your refrigerant.

  3. #3
    Sorry, R-22.

  4. #4
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    Can't give you DIY here. However...

    You need an R-22 pressure-temperature chart and a DuPont R-22 pressure-temperature-enthalpy chart.

    You already have your measurements, subcooling, and superheat numbers.

    On the DuPont chart, create a temperature-enthalpy cycle with condensor on top, expansion on the left, evaporator on the bottom, and compressor on the right.

    When evaporator temperature falls below 40*F, the coil is in danger of freezing. You already know that lack of air, or lack of charge will cause that. You can verify that with the pressure-temperature chart.

    You have 72*F air coming into the evaporator and 58.6*F air leaving. That's a delta of 13.4*F. What should it be?

    That particular heat exchanger relies on the temperature of the air crossing it, the velocity and amount of air, the temperature and amount of refrigeration vapor and liquid crossing, the transfer coefficient of the medium. A clogged capillary tube (one of many) would do what? A dirty evaporator coil would do what?

    Regards

  5. #5
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    If refrigerant was added only based on low section pressure, you need to call anyother company.

  6. #6
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    Jan 2004
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    If your wet bulb temps are taken across the coil, you have a total BTU removal of 50,400 BTUs if your really moving 2000 CFM.

    If he pumped the 22 back to the OD unit, and then replaced the TXV, and didn't adjust charge. Good chance its a little over charged. Might also have have to clean the evap coil. No telling how dirty the coil is between the fins where you can't see the dirt. Also, the blower wheel should be checked to see if its dirty.
    Air flow should be verified.
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  7. #7
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    If refrigerant was added only based on low section pressure, you need to call anyother company.

  8. #8
    Join Date
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    Generally, overcharging a TXV will rise the subcooling. Conversely, undercharging will decrease subcooling.

    A meticulous engineer would allow 10-20 minutes for system stabilization between charge changes.

    Now you have a recorded baseline.

  9. #9
    Don't want DIY...unless you mean Duly Inform Yourself

    AMD, I agree with you, but I've got $2,000 invested in the current guys and I'm determined that they will return my unit to efficient operation if I have to stand over them and tell them every step to take.

    Coil and fan are clean. I measured air flow across both returns and flow is now close to 2000 CFM. Indoor readings were taken at the return register and inside the closest supply register.

    I took the measurements the day after the tech came out. And yes, I let the unit run for 15 minutes before taking readings.

    Thanks again for all the help.

  10. #10
    Quote Originally Posted by sparksandfan View Post
    Can't give you DIY here. However...

    You need an R-22 pressure-temperature chart and a DuPont R-22 pressure-temperature-enthalpy chart.

    You already have your measurements, subcooling, and superheat numbers.

    On the DuPont chart, create a temperature-enthalpy cycle with condensor on top, expansion on the left, evaporator on the bottom, and compressor on the right.

    When evaporator temperature falls below 40*F, the coil is in danger of freezing. You already know that lack of air, or lack of charge will cause that. You can verify that with the pressure-temperature chart.

    You have 72*F air coming into the evaporator and 58.6*F air leaving. That's a delta of 13.4*F. What should it be?

    That particular heat exchanger relies on the temperature of the air crossing it, the velocity and amount of air, the temperature and amount of refrigeration vapor and liquid crossing, the transfer coefficient of the medium. A clogged capillary tube (one of many) would do what? A dirty evaporator coil would do what?

    Regards
    Before I construct my Mollier diagram (haven't done one of those in a while ), I have two questions. Why is there no temp drop across TXV and why does the suction line temp drop from near 58F at evap outlet to 49F at suction valve? Obviously a kinked line or other restriction might cause that but pretty sure that's not an issue (will check that tonight however)

    Thanks

  11. #11
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    Only have a few minutes so will have to bang this out pretty quick.

    Subcooling should be 12*F because that is what the TXV design is targeted. Your subcooling is 27*F which indicates as previously discussed, an overcharge.

    Your ambient outside is 87*F. Your condensor will probably run from 25 to 35 degrees above ambient. Therefore, if your condensor temperature line on the p-h chart (not h-s chart) is 112*F(?), and adding 27*F subcooling which pushes it to the left and into saturated liquid on the chart, then drop straight down to 40*F evaporator design temperature, and would guess that you are barely out of saturated liquid area of the chart.

    Getting called , so that's it for now. Oh, but your superheat is 11.5*F and it should be 15*F...

  12. #12
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    Quote Originally Posted by sparksandfan View Post
    Only have a few minutes so will have to bang this out pretty quick.

    Subcooling should be 12*F because that is what the TXV design is targeted. Your subcooling is 27*F which indicates as previously discussed, an overcharge.
    Getting called , so that's it for now. Oh, but your superheat is 11.5*F and it should be 15*F...


    Not all systems want 10 to 12* of SC. Not all systems want the same amount of SH.
    And I am talking about TXVs.
    Your using rules of thumb.
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  13. #13
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    12*F supercooling based on Msg 1 equipment documentation.

    Don't have a complete thought on superheat here.. it just makes me wonder.

    I don't disagree with what you say.

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