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Thread: low subcooling

  1. #1
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    low subcooling

    if a piston orifice is overfeeding, stuck, etc, would the system still work..just not as designed? Our system has always struggled to cool house if outdoor temp gets above 92 or so. Has been checked numerous times. 1 thing that stands out is..the superheat is always normal but subcooling is always real low, usually 2 or 3 degree subcooling. And the delta across coil is only 15 degree.

  2. #2
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    Arrow What is the relative humidity in your home?

    Quote Originally Posted by jrgreene1968 View Post
    if a piston orifice is overfeeding, stuck, etc, would the system still work..just not as designed? Our system has always struggled to cool house if outdoor temp gets above 92 or so. Has been checked numerous times. 1 thing that stands out is..the superheat is always normal but subcooling is always real low, usually 2 or 3 degree subcooling. And the delta across coil is only 15 degree.
    Maybe it has a larger tonnage evaporator & they didn't change the piston size to the condenser's tonnage.

    It would have a higher suction & coil temperature if piston is too large; better have it checked.

    If there is a high latent./humidity load on the evaporator coil it would have a lower delta, if high enough could be as low temp-drop as 11 or 12-F.

    If a lot of the small liquid line is in a hot area "well above outdoor ambient," insulate it; at higher 'attic' temps it could begin to flash-gas & reduce capacity.
    Also, are the condenser coils & fins clean with OEM rated airflow?

    At 50% relative humidity the temp-drop split should be (at 450-cfm per-ton) around 19-F or (at 350 to 400-cfm per-ton of cooling) around 20-F.

    What is the relative humidity in your home?
    Reduce home air infiltration rate, check for enough insulation & reduce solar gain. - Darrell
    Last edited by udarrell; 06-27-2010 at 01:46 PM. Reason: Added other options...

  3. #3
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    thanks for the reply. Coils have been cleaned, filter changed. Pressures checked. With normal superheat, it only has 2 degrees of subcooling. Humidity in house is right at 51%.

  4. #4
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    Quote Originally Posted by jrgreene1968 View Post
    thanks for the reply. Coils have been cleaned, filter changed. Pressures checked. With normal superheat, it only has 2 degrees of subcooling. Humidity in house is right at 51%.
    What about the liquid line, how is it run?

    Is condenser clean, or is it recirculating discharge air, or doesn't have full air flow?

    Tech might try adding little more refrigerant if it doesn't get Superheat in a liquid flood-back zone.

    There could be some mismatches in the refrigerant system.

    What is the actual indoor airflow CFM, - too high?
    At 450-cfm per-ton should get around 19-F drop at 51% RH, lower CFM 20-F.

  5. #5
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    Quote Originally Posted by udarrell View Post
    What about the liquid line, how is it run?

    Is condenser clean, or is it recirculating discharge air, or doesn't have full air flow?

    Tech might try adding little more refrigerant if it doesn't get Superheat in a liquid flood-back zone.

    There could be some mismatches in the refrigerant system.

    What is the actual indoor airflow CFM, - too high?
    At 450-cfm per-ton should get around 19-F drop at 51% RH, lower CFM 20-F.
    the house sits on pier and beam, and the lineset runs under the house and then through the floor into a closet. The air handler is installed in a closet off the floor. No return duct, just a big grill.

    Condensor is clean,and it has 18 degree air temp across it.

    Air flow does feel strong. I do know the speed tap is on the high setting. I have no way to check actual air flow though. I just checked the split at the registers, and getting 15.5 degree,. Could the actual airflow be to high? I always read, id be doing good to have enough.

    Thanks

  6. #6
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    Quote Originally Posted by jrgreene1968 View Post
    the house sits on pier and beam, and the lineset runs under the house and then through the floor into a closet. The air handler is installed in a closet off the floor. No return duct, just a big grill.

    Condensor is clean,and it has 18 degree air temp across it.

    Air flow does feel strong. I do know the speed tap is on the high setting. I have no way to check actual air flow though. I just checked the split at the registers, and getting 15.5 degree,. Could the actual airflow be to high? I always read, id be doing good to have enough.
    Thanks
    There is no substitute for being there, additionally, I can't see the many prior posts, so I'm not getting a clear view of the situation.

    At 51% RH, it would take a lot of additional airflow to lower the split that much to 15.5-F; a High humidity load could easily do it. Higher than normal airflow could somewhat raise the superheat & decrease subcooling.

    How large is the evaporator coil?
    It appears 'there there could be' a mismatch between a higher capacity evaporator coil & compressor, matched with an insufficient condenser capacity to condense enough of the vapor fast enough- so it can Sub-cool to a desired temperature. (Could be numerous causes...)

    In addition, if the piston were oversized the condenser would not be able to keep up to the volume of hot vapor to be condensed.

    If the condenser motor &/or fan were changed, the replacements may not be OEM, & may not be moving the condenser's Rated airflow.(?)
    That lower airflow condition could result in an 18-F condenser air discharge temp rise, even though there might be a little liquid flashing near the piston & lowered BTUH.

    Some units operate with a very low subcooling, such as window units & some self contained units.
    Also saw where, some new high efficiency equipment runs as low as 4-F subcooling.


    Also, perhaps you're not getting a proper measurement of the subcooling.

    Actual operating airflow should always be checked. - Darrell
    Last edited by udarrell; 06-27-2010 at 08:37 PM. Reason: Clarifications...

  7. #7
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    Quote Originally Posted by udarrell View Post
    Tech might try adding little more refrigerant if it doesn't get Superheat in a liquid flood-back zone.
    • Airflow that is too high is so rare that this is almost surely not the problem. With 51% humidity I doubt excessive airflow which tend to raise coil temperature and humidity levels.
    • With only 2 degrees subcooling you may have liquid flashing or boiling in the liquid line due to pressure drop due to friction and a small liquid lift.
    • Also the split measured at the registers may be considerably lower than the split across the evaporator because of heat gain in the ductwork. Check it out.
    • We seldom have all the information that we need to nail a more complex problem. That is why we ask questions and make educated guesses in lieu of more information. I agree with Udarrell "that there is no substitute for being there". However, I have an extended list of data that I require our techs to collect of every maintenance, service call or installation. You would be amazed at the difference it makes. The data is entered on the invoice and the office can use my own excel spreadsheet to analyze it for problems. If I am looking at the data I just analyze it in my head using the knowledge and logic that I built into the spreadsheet.)


    1. I would first recommend simply stopping the indoor fan and observing the freeze pattern on the return bends of the evaporator. All circuits should frost at the same time and frost fully. (A circuit that is not fully active will cut system capacity, reduce the TD across the evaporator and effectively make the coil too small for the orifice; that is, it will overfeed the other circuits.)

    2. IF the evaporator coil is fully active I agee with Udarrell and would suggest adding slightly more refrigerant as a reasonable test. My experience tells me that the subcooling for a properly sized orifce on a split system normally is about 10to 15*F. (That is because the orifice will not feed correctly if the liquid is flashing in the liquid line due to pressure drop.) I would add refrigerant with that target to see if the superheat drops into a flooding condition. If the superheat is still OK...I would be satisfied.

    Neither of these tests takes much time and expense. This testing and observation is the kind of thing that can only be done by "being there".

    3. If the coil starts to flood and the airflow is 400 cfm per ton (per ARI testing airflow), I would suggest a TXV because you then charge by subcooling and the valve controls the superheat under all load conditions. And added advantage is that it makes the system operate safely and more efficently over a wider range of operating conditions...including refrigerant undercharge and overcharge and dirty filters.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  8. #8
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    Arrow It is Time for "Verified System Performance"...

    Quote Originally Posted by lynn comstock View Post
    • Airflow that is too high is so rare that this is almost surely not the problem. With 51% humidity I doubt excessive airflow which tend to raise coil temperature and humidity levels.
    • With only 2 degrees subcooling you may have liquid flashing or boiling in the liquid line due to pressure drop due to friction and a small liquid lift.
    • Also the split measured at the registers may be considerably lower than the split across the evaporator because of heat gain in the ductwork. Check it out.
    • We seldom have all the information that we need to nail a more complex problem. That is why we ask questions and make educated guesses in lieu of more information. I agree with Udarrell "that there is no substitute for being there". However, I have an extended list of data that I require our techs to collect of every maintenance, service call or installation. You would be amazed at the difference it makes. The data is entered on the invoice and the office can use my own excel spreadsheet to analyze it for problems. If I am looking at the data I just analyze it in my head using the knowledge and logic that I built into the spreadsheet.)


    1. I would first recommend simply stopping the indoor fan and observing the freeze pattern on the return bends of the evaporator. All circuits should frost at the same time and frost fully. (A circuit that is not fully active will cut system capacity, reduce the TD across the evaporator and effectively make the coil too small for the orifice; that is, it will overfeed the other circuits.)
    Additionally if that does not reveal a problem you can also check temperatures with the blower operating to see if there is a variation in temperatures caused by an unbalanced airflow distribution through the coil circuits.

    That can cause a similar uneven heat-loading of the individual circuits reducing the capacity of the coil. Then a TXV, if installed would sense the lowered temp of the circuits delivering some liquid & lower the liquid flow rate again reducing cooling coil capacity.

    Anything causing an uneven airflow through the individual coil circuits such as a messed-up transition or bend of sorts can cause uneven circuit load problems...


    2. IF the evaporator coil is fully active I agee with Udarrell and would suggest adding slightly more refrigerant as a reasonable test. My experience tells me that the subcooling for a properly sized orifce on a split system normally is about 10to 15*F. (That is because the orifice will not feed correctly if the liquid is flashing in the liquid line due to pressure drop.) I would add refrigerant with that target to see if the superheat drops into a flooding condition. If the superheat is still OK...I would be satisfied.

    Neither of these tests takes much time and expense. This testing and observation is the kind of thing that can only be done by "being there".

    3. If the coil starts to flood and the airflow is 400 cfm per ton (per ARI testing airflow), I would suggest a TXV because you then charge by subcooling and the valve controls the superheat under all load conditions. And added advantage is that it makes the system operate safely and more efficently over a wider range of operating conditions...including refrigerant undercharge and overcharge and dirty filters.
    Comstock, that is an excellent post.
    I really like your spreadsheet use so the tech gets feedback from you.
    Most techs don't collect enough of the right kind of data to trouble shoot complex problems.


    In this era, there are excellent test instruments available, some at reasonable prices, that are not being acquired & used to reveal the factors robbing systems from their nominal rated delivery performance.

    I would hope other contractors would setup an XL spreadsheet & model your troubleshooting procedures.

    No-one can deliver "Verified System Performance" with insufficient & inadequate test data procedures. - Darrell

  9. #9
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    thanks for the replies

    had a guy out today, same one that was here last few times..so he is pretty familiar with the problem. He checked pressures again, along with sh and sc. Also checked airflow, and even slowed fan down to see how it acted. He seems to think the orifice is to big still. He has already mentioned that last time..but didnt seem sure, since the system is working ok..just not good. The system was bought new 3 yrs or so ago, and supposed to be a matching system.
    I told him id rather him have a correct piston to install, incase the piston is the wrong size, before he recovers refrigerant etc. So we wouldnt be without air during the heat of the day..this tx heat is a killer
    He did try adding a little refrigerant, but sh immediately went to 0 and sc didnt move much if any.

    the readings he got today

    suction pressure 75
    head pressure 225
    sh 8
    sc 2
    outdoor temp 95
    indoor db 76
    indoor wb 64

  10. #10
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    Yup. Time to pump it down and confirm yea or nay on the piston issue. I'd definitely be leaning toward a TXV in your climate with the variations you get down there.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  11. #11
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    Pretty classic symptoms of a slightly over sized piston.
    Unfortunately I see it all the time.

    I agree with others about putting in a TXV.
    IMO, fixed metering is pretty dumb in this climate.
    If more government is the answer, then it's a really stupid question.

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