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  1. #1
    Join Date
    Aug 2009
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    My contractor recorded 245 Head Pressure. Is that too low for R-410A system?

    Hi all. I am trying to figure out if my HVAC company is a bunch of idiots or if I just misunderstand subcooling.

    For my small Baltimore rowhouse I just got a newly installed 2 ton, 16 SEER split Carrier central air conditioning system that uses R-410A (cost me $x, ductwork was already present). I am worried the techs who installed it did not charge it enough. It has TVX valves so you need to use subcooling to measure the charge.

    They recorded a 245 psi Head pressure and 115 suction pressure on my installation receipt. He also wrote "10 degrees subcool" on the receipt, which I don't see how that is possible when a 245 psi equals 83 degrees on an R-410a P/T chart.

    The small, copper liquid line (when measured near the compressor valve) doesn't seem to heat up much at all (does not feel warm to the touch). When I measure it with a thermometer I get temps a few degrees above ambient. Once I measured it at 95 when it was 90 outside. A second time, I measured the liquid line at 87 degrees when the outside ambient temp was 83.

    Does it sound like I'm low on refrigerant?

    Is a 245 psig pressure just flat out too low for any system?...it seems it would be impossible to get 10 degrees of subcooling at that pressure since it is equal to 83 degrees... to get the 10 degrees of subcooling your liquid line would have to be like 73, which would be colder than ambient (isn't the liquid line supposed to be warm/hot?).

    The system actually cools ok with a healthy temperature split approaching 20 degrees. My concern is the system is not working as efficiently as it could and/or is being subject to excessive wear and tear due to a low charge.

    Oh, I don't want to make this post too long but on the inside of my compressor's service panel there is a chart used for measuring subcooling that I don't fully understand. Here is a recreation of that chart:

    ------Required Subcooling Temp------
    Pres | 6 ,| 8 ,| 10 | 12 | 14 | 16
    251 | 78 | 76 | 74 | 72 | 70 | 68
    259 | 80 | 78 | 76 | 74 | 72 | 70
    266 | 82 | 80 | 78 | 76 | 74 | 72
    274 | 84 | 82 | 80 | 78 | 76 | 74

    Since this chart provides pressures from 251 - 274, does that mean my pressure should fall within that range?


    Thanks alot for any advice! I've been browsing this site for awhile and it's been great for learning.
    Last edited by jrbenny; 08-10-2009 at 11:26 AM.

  2. #2
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    What was the outdoor temp the day they took their readings.
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  3. #3
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    Quote Originally Posted by beenthere View Post
    What was the outdoor temp the day they took their readings.
    Outdoor temp was about 78 - 80 when those readings were taken.

    Do pressure measurements vary based on the outdoor temp?

  4. #4
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    Yes.

    The condensing temp changes, so the pressure changes.
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  5. #5
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    Quote Originally Posted by beenthere View Post
    Yes.

    The condensing temp changes, so the pressure changes.
    Thanks for replying.

    Ok, so if the temp was 78 -80 when that 245 psi reading was taken, does it sound like I'm low on refrigerant?

  6. #6
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    Quote Originally Posted by alkydrinker View Post
    Thanks for replying.

    Ok, so if the temp was 78 -80 when that 245 psi reading was taken, does it sound like I'm low on refrigerant?
    ask your contractor.

  7. #7
    I saw the 245 head pressure and before I even read the suction pressure I thought to myself, "it may be an R-22 system:.
    Then again I work in southern AZ and never even see R-22 head pressures that low in the summer unless the unit is not cooling!

    If I'm not mistaken on the carrier condensers they have the subcooling value stated on the sticker along with the other info like model # and serial.

  8. #8
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    Your system is probably charged fine.
    You have a good temp split.

    Unless its having trouble cooling the house. I don't think you have anything to worry about.

    If its having trouble cooling, call them back and let them know.
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  9. #9
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    Thanks for the additional replies. For my own peace of mind and greater understanding, I wish I could wrap my head around this subcooling thing.

    If my pressure was 245 psi when it was like 79 degrees outside, and a 245 pressure equals 83 degrees on an R-410a P/T chart, then the temp of my liquid line would have to be COLDER THAN AMBIENT (i.e., 73 degrees) to get 10 degrees of subcooling??? How could it be colder than ambient?

    Also, is it not a major red flag that my liquid line does not seem to be getting warm at all near the condenser hook up?

  10. #10
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    A high SEER unit on a mild day won't have much heat in the liquid line.

  11. #11
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    The line shouldn't feel warm. When its 80 or less outside.

    The actual outdoor temp when he measured may have been cooler then you think.

    An udercharge, would not cause the liquid line to be cooler then the outdoor temp.
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  12. #12
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    Arrow Not much to Worry about Yet

    Quote Originally Posted by alkydrinker View Post
    Thanks for replying.

    Ok, so if the temp was 78 -80 when that 245 psi reading was taken, does it sound like I'm low on refrigerant?
    If OAT was 78-F subtracted from 83-F condensing-temp (CT) - that is only a 5-degree temp rise, should probably be around 12-F.

    From Goodman 16-seer TXV R-410A mfg'ers Data:
    Two-ton @800-cfm; 75-IDB; 63-IWB; 75-F outdoor ambient (OAT) |
    R410A; 275-psig = 90-F, your -78-F OAT would be a 12-F condenser discharge-air temp-rise.; Suction 135-psig= 47-F; your Tech had 115-psig or 83-F CT.

    We don't know what the indoor humidity was? Goodman's subcooling is listed at 7-F +/- 2-F therefore, a 5-F subcooling for their unit would be within specifications.

    Maybe your unit is supposed to have 10-F Subcooling?
    With a proper digital TH, you can take the condenser discharge air temp & then the liquid line temp & ballpark its Subcooling over different temps & heavy load conditions. Let your Tech know what those readings are, & he'll tell you if he needs to recheck it, add more refrigerant. Also, every user should have a humidity gauge indoors!

    Does appear to be some low on charge, but you need to track the subcooling over different conditions; then you may need to call your Tech. User's always need a licensed Tech to correct the problems!.


    "The small, copper liquid line (when measured near the compressor valve) doesn't seem to heat up much at all (does not feel warm to the touch). When I measure it with a thermometer I get temps a few degrees above ambient. Once I measured it at 95 when it was 90 outside. A second time, I measured the liquid line at 87 degrees when the outside ambient temp was 83."
    That part is okay, it should sub-cool the liquid somewhere near the OAT. - Darrell
    Last edited by udarrell; 08-09-2009 at 10:43 PM. Reason: Some additions & clarifications

  13. #13
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    UDarrell - thanks for the info.

    So you are saying you can get a ball park subcooling measurement by measuring the temp of the liquid line subtracted from the temp of the air blowing up from the condenser?

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