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  1. #1
    Join Date
    Feb 2007
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    999

    Question about delta-t

    I usually wait until the end of May to schedule my PM. We had a couple of warm days so I turned on the a/c. While the heat load was minimal, it was uncomfortable in the house, but I had to lower the set point but still wasn't comfy.

    The airflow is set to 400 CFM/T, condensate production was less due to lower RH. I checked the DT and found it to be 14* (was 18 at last year's PM).

    My unit has high and low pressure safeties, so I feel secure if I need to run it occasionally until its checkup.

    My question: If heat load and RH are lower, would I expect my DT to be lower, or could this be a sign of lost refigerant?

    Amp

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,905
    When the humidity latent load is low, then the delta T should be higher, not lower.

    The majority of the boiling refrigerant would be primarily absorbing sensible heat; not latent heat; which should have dropped the discharge air temp more than if the humidity had been well above 50%.

    If there were a very high humidity latent load then the delta T could have gone from 18F down to 14F.

    I'd have a good Tech check it.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,776
    Delta would be lower due to low humidity. Don't feel secure about using it because it has high and low safeties. If its low on charge, the compressor can be running too hot, and have a short life span. Even though the low pressure is high enough to keep the low pressure switch from tripping.
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  4. #4
    Join Date
    Jul 2006
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    1,091
    Quote Originally Posted by udarrell View Post
    When the humidity latent load is low, then the delta T should be higher, not lower.
    I read in the Manual S that the equipment will use 50% of the latent capacity not used for the latent load for the sensible load.... If I read it right.

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    Quote Originally Posted by rickboggs View Post
    I read in the Manual S that the equipment will use 50% of the latent capacity not used for the latent load for the sensible load.... If I read it right.
    Manual S is saying what I said, but stated differently.

    Here is a mfg'ers example with light outdoor & indoor conditions.
    At 400-cfm/Ton; orifice metering device; only 65F outdoors; 70F Indoor dry bulb; 67F wet bulb or 82.5% RH; Delta T 12F; Sensible only 0.40.

    At 59F indoor wet bulb or 52% RH; the delta T is 18F; Sensible 0.70.

    That was a 2-Ton condenser with a 3-Ton indoor A-Coil; the sensible ratio varies somewhat between coils & a little between high or low CFM.

    As beenthere suggested, if it's low on refrigerant the indoor delta T will be lower; the compressor depends on a cold suction vapor to keep it from getting too hot.

    Check the discharge air temp delta T, & see what you get; list all the temps.

    Higher SEER systems have lower condenser temp splits; provide the condenser's SEER.

    Record INDOOR ROOM temp & especially the humidity reading.

    With lower outdoor temps, I prefer a TXV, as it does a better job; than an office piston; providing proper superheat according to its consistent set point.

  6. #6
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,851
    You know "sometimes" if the conditions inside and our are very humid, I would probably try and charge the system the best I could, using SH,SC etc. and not concern myself to much about the delta. If you try and produce a good delta under harsh or humid conditions, you can almost bet your charge will be not be correct, delta is something that should be checked last, after the system has had ample time to run. Correct charge is always more important than delta.
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  7. #7
    Join Date
    Feb 2007
    Posts
    999
    O.K. guys, I've read your comments. If I feel the need to run the a/c before the really hot/humid weather comes in and have it checked, would it be safe to run it if the suction line is 'beer can cold'?

    Just for information, the unit is an RARL-048-JEZ, w/RCQD-4821A* coil, and RGFD-09EZCM furnace (ARI # 551175)
    47000 BTU, 11.45 EER, 14.7 SEER.

    Amp

  8. #8
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,851
    Quote Originally Posted by ampulman View Post
    would it be safe to run it if the suction line is 'beer can cold'?
    Maybe only if your trying to only cool one can, but if your trying to cool a case, it might be overcharged.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

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  9. #9
    Join Date
    Jul 2006
    Posts
    1,091
    Quote Originally Posted by udarrell View Post
    Manual S is saying what I said, but stated differently.
    Yeah, your post reminded me of what I read.

  10. #10
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
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    Get a tech out to check the charge. With low RH your delta should be higher than 14 degrees. Unless your air flow is set too high.
    Remember, Air Conditioning begins with AIR.

  11. #11
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,302
    Quote Originally Posted by ampulman View Post
    O.K. guys, I've read your comments. If I feel the need to run the a/c before the really hot/humid weather comes in and have it checked, would it be safe to run it if the suction line is 'beer can cold'?

    Just for information, the unit is an RARL-048-JEZ, w/RCQD-4821A* coil, and RGFD-09EZCM furnace (ARI # 551175)
    47000 BTU, 11.45 EER, 14.7 SEER.

    Amp
    If your a/c tech is sharp, he/she does not need to wait for a super hot/humid day to properly charge up your system. 85 degree weather, for example, is plenty fine for normal charging procedures.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  12. #12
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by Mr Bill View Post
    Maybe only if your trying to only cool one can, but if your trying to cool a case, it might be overcharged.
    Bill, joke aside, this will be the unit's 4th cooling season. Assuming it was correctly charged initially, with deltas in the 18+ degree range (had some air adjustments), could there be a situation where it would display symptoms of an overcharge?

    Amp

  13. #13
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by Kevin O'Neill View Post
    Get a tech out to check the charge. With low RH your delta should be higher than 14 degrees. Unless your air flow is set too high.
    Airflow is set at 400 CFM/T. Filter is also clean. Would you suspect unit has lost charge?

    Amp

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