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Possible causes of low temperature across evap. coil ?

Hello ,:

Very generally speaking ...... What might be the possible causes of a low temp. (10-12 degree) difference across an evap. coil on a 3.5 ton system that is properly charged by weight, where the coil has been removed and cleaned and the correct piston is installed and gauge readings @ the condenser are perfectly normal ?


  • TechmanTechman Member Posts: 2,144
    edited August 2012
    Perfectly normal

    I like that! If the readings were P.N. i think you wouldn't be here.Temps,press,W.B. S.H. S.C.
  • Charles JohnsonCharles Johnson Member Posts: 24
    Need more numbers

    All numbers seeming normal? Then low temp drop means too much airflow.

    Too much airflow is quite rare.

    If you didn't take entering air WB and SH and SC, you don't know that pressures and temps are normal. :)
  • Empire_2Empire_2 Member Posts: 2,343
    I 2nd Techmans comment.

    Something in your readings are not normal.  With a 10* drop, I'll bet your SH is sky hi and you may just be undercharged.  The only way for us to help you is for you to post some readings.  It's almost like saying my car makes a ticking sound, tell me what's wrong with it...

    Mike T.
  • EdyLogicMstr.EdyLogicMstr. Member Posts: 58
    Too much air maybe so?

    Thanks Charles, Here are my readings from this system. : ambient temp. = 82 F ,dry bulb entering air = 74.6 F , wet bulb entering air = 63.2 F, superheat = 15 F, subcooling I believe does not apply since it uses a # 80 piston orifice. At the condenser the gauge pressures are low side = 64 psig, high side = 186 psig (by the way it's R-22) and I agree it's unusual to find a case of too much air but my company recently rebuilt the blower assembly here with a new multi speed 1/3 h.p. motor and the squirrel cage was completely cleaned. Perhaps if I am given time to return here I will try a lower speed on this motor and see what happens.
  • Empire_2Empire_2 Member Posts: 2,343
    edited August 2012
    Sounds good.

    Yes air may be the problem.  Just a thought,...was a new blower wheel installed (different size) and maybe now moving too much air?  Also, make sure that the baffle plate is in place on top of housing.  Yea I agree that something has changed.

    Mike T.
  • Charles JohnsonCharles Johnson Member Posts: 24
    edited August 2012
    Check for bypassing air.

    Is this coil in a plenum above a furnace? If so check for air bypassing the coil around the sides. That would lead to the symptoms you are having.

    And subcooling always applies! It is valuable diagnostic even if you have fixed metering!
  • TechmanTechman Member Posts: 2,144
    edited August 2012

    That hi side press is a little low.It should be more like 240-225psig.And subcooling always applies, I agree!
  • Empire_2Empire_2 Member Posts: 2,343
    Good catch Terry.

    Head of 230 is what I see.  Didn't have my T/P chart but yea should be higher.  Sub cool can at least get you in the ball park with say 12* just as an example.  Still leaning towards an undercharge and wondering about blower from what I stated earlier plus what Charles had to add.

    Mike T.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,340
    head pressure

    The head pressure ought to be more like 200. It's only 82 deg. ambient. The head is low because the suction is low. Sounds like to much air on the evap or air bypassing. How are the compressor amps. Probably low
  • TechmanTechman Member Posts: 2,144

    I think a 200psig head is better suited for a 75* day.
  • Can you get us a subcooling number?

    Your evaporator saturation temperature (37 degrees) is right on the money, given the 50% relative humidity number you gave us (74.6 FDB, 63 FWB). Your superheat, however, is higher than desired.

    The subcooling number will definitely be useful here, especially since your condenser saturation temperature (96 F) is considerably lower than desired at an 82 F ambient. Low subcooling, in this case, would point toward a deficiency of refrigerant on the high side of the system.

    Excessive airflow through the evaporator will cause high superheat as well as a high evaporator saturation temperature. Can you to measure the airflow rate through the coil for us?

    Inquiring minds would like to know!
This discussion has been closed.


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