Compressor Valves / Internal Relief Diagnostics

I was recently asked the question, “How can I determine if I have bad valves in a reciprocating compressor or if the internal relief on a scroll compressor is going bad?” These are good questions and I’ll address each individually!  Whether the system is R-22 or R-410A doesn’t make a difference.  The classic diagnostics for bad valves in a compressor are:

  1. Suction pressure will be higher than normal.
  2. Head or liquid pressure will be lower than normal.
  3. Super-heat and sub-cooling could be non-existent or high, depending on how bad the valves are.
    1. If valve are damaged badly, both super-heat and sub-cooling will be high due to the compressor not pumping efficiently and the refrigerant stays in the coils longer increasing super-heat and/or sub-cooling.
  4. And amp draws will be low (usually more than 20% of the RLA of the compressor)

So how can you be sure?  A quick way to diagnose bad valves in a reciprocating compressor is to do the following:

  1. To check the discharge valves, turn off the condenser fan motor and turn the unit on.
    1. The head pressure should climb very quickly if the valves are good.
    2. If the pressure does not climb, chances are the discharge valves in the compressor or “shot”.
      1. Keep in mind that while you are doing this, YOU are the high pressure safety.  Watch your gauge pressures and keep you hand on the disconnect.  If the discharge valves are good, you need to shut off the disconnect or hope the high pressure switch (if the unit has one) shuts the unit off.
  2. To check the suction valves in the compressor, turn off the indoor blower motor and turn the unit on.
    1. you should see the suction pressure start to drop and continue to drop.
    2. If the suction pressure does not drop, chances are the suction valves in the compressor are bad.

Combine these diagnostics with the classic diagnostics and you can be pretty sure as to the condition of the valves in the reciprocating compressor.

The most common cause of compressor valves going “bad” is liquid slugging. Liquid slugging is caused by:

  1. Not enough super-heat.
  2. Over charge
  3. Low evaporator air flow
  4. Incorrect metering device or over-feeding TXV.
  5. Operating the unit below the manufacturer’s recommended temperature ranges (this is not a refrigeration unit)
  6. Crankcase heater not working

With scroll compressors, there are no valves in the compressor but the orbiting scroll or fixed scroll could be “worn” due to liquid slugging. This would appear very much like bad suction valves in a recip compressor.

If pressure or temperature starts to build up in these compressors, they have an internal relief valve that opens and “dumps” hot discharge gas into the shell causing the compressor to shut off on internal overload.  The internal reliefs are set at [+/- 50 lbs] R410A – 600 lbs,  R22  -450 lbs ‘Differential” pressure.

Some Scroll compressors have an internal thermo-disc discharge gas temperature protection. This thermo-disc opens a gas passage from the discharge port to the suction side near the motor protector when the discharged gas reaches a critical temperature. The hot gas then causes the motor protector to trip shutting down the compressor.

Some Copeland Scroll compressors built in October 2004 and later (04J) have the addition of the Advanced Scroll Temperature Protection (ASTP). Advanced Scroll Temperature Protection is also a temperature sensitive thermo-disc that acts to protect the compressor from discharge gas overheating. Once the discharge gas reaches a critical temperature, the ASTP feature will cause the scrolls to separate and stop pumping although the motor continues to run. After running for some time without pumping gas, the motor protector will open.

To identify compressors with Advanced Scroll Temperature Protection, a label has been added above the terminal box.

As the label says: NOTE: Depending upon the heat build-up in the compressor, it may take more than one hour for the ASTP and motor protector to reset! 

 As always, once you have diagnosed the problem, you need to determine what caused the compressor valves to fail or the internal relief to become weak.  This goes back to dirty filters, dirty condenser coils, indoor blower motor bad, indoor blower wheel dirty, bad capacitors, etc. You may also read my posting on “Discharge Temperature as a Diagnostic” as this will help in diagnosing the ASTP in some Copeland scroll compressors.  In General, the main causes of High Discharge Temperature are:

  1. High condensing pressures
  2. High system superheat
  3. Low suction pressures
  4. High compression ratios.

Just replacing the compressor (or the whole unit) without first determining why the previous compressor failed, will only lead to the new one failing eventually also. It still comes back to good service practices!

About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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18 Responses to Compressor Valves / Internal Relief Diagnostics

  1. Thank you
    The information you shared is very informative.

  2. Walt Bireley - Indiana says:

    So witha scroll compressor I would shut down indoor blower, turn unit on and watch what suction pressure does? At what rate should I see pressure dropping that would suggest a bad compressor?

    Thanks for any help on this matter.

    • Wally — keep in mind scroll compressors do not have valves — If you shut down the indoor blower, what do you think would happen? No air across the indoor coil, suction pressure should start to drop, ice should start to form on the coil, superheat will eventually be noon existant and you will run the risk of slugging the compressor.

  3. Wally says:

    Then I misunderstood your post. What exactly do I do to confirm or deny a bad scroll compressor. Would I just watch that the suction pressure drops at a quick rate from a equalized state. My data is pointing to either bad compressor or over feeding txv. I have reason to believe it could be either or both. Just want to eliminate bad compressor before I change txv.
    Thanks for any advice and guidance

    • if it is “pumping” it is probably good – pressure differential on you gauges is a good sign it is working — what sort of pressures/temperatures are you getting?

      • Wally says:

        I appreciate your responses, this system is to say the least perplexing. I have reason to believe that from the beginning proper procedures were not taken.

        SLP – 80 psig
        SLT – 53 deg
        LLP – 182 psig
        LLT – 85 deg
        ODB – 75
        Temp drop across RA / SA is 12 deg
        TXV – R22
        SH – 6 deg
        SC – 10 (was about 5 before adding charge)
        Cleaned both coils
        I am leaning toward txv blockage causing over feeding.
        I know for a fact that the guy that brazed this system in did not run nitrogen through it

        Again, I appreciate your thought and any guidance you can give.

      • in viewing your data — I do not see a bad compressor. I see 1 of 2 issues — Either a TXV over feeding or too much air flow across the coil (see post on Air flow Effects on Air Conditioning – Revisited ) Check the TXV Bulb for position, tightnness, insulated, etc. Check CFM across the coil

      • Wally says:

        Cleaned Coil (wasn’t very dirty but I knew filter wasn’t being changed regularly) measured cfm and it is correct. TXV is positioned ok and insulated. Maybe I will do an ice water, warm water test to observe superheat movement. If txv is not closing completely what will superheat do?

        Your insight is highly appreciated.
        Thanks, Wally

      • if the metering device is over feeding — you should see low superheat, low subcooling, high suction pressure and low head pressure

      • if the metering device is over feeding — you should see low superheat, low subcooling, high suction pressure and low head pressure

  4. Bob says:

    Hello…. Hope my question can be answered.. Went to a service call today unit is 7 yrs old compressor won’t start checked voltage to compressor terminals have proper voltage.. Checked compressor to ground no short.. I think it’s a locked rotor as I can hear a (hum) and then a (click) overload tripped right away.. Unit was off on disconnect all day. Tried hard start kit no good. Even tried smacking compressor with a hammer no good… Customer said it made a loud noise a couple of days before. Will not trip breaker just internal overload in compressor… What do you think. I do not want to damage new one… I will be installing hi- lo pressor switches as it doesn’t have any now.. checked oil, no sign of acid at all…. Thanks.

    • You are describing “locked rotor” of the compressor. When you say you tried a hard start kit — was it a manufacturer’s recommended start relay/start capacitor or was it a “super boost” like an SPP5? Sometimes the SPP’s don’t have enough “kick” to free a stuck compressor.
      Regardless, if the compressor has locked up, the pressures are equalized, and you keep tripping the internal overload, you really should replace the compressor or unit since the compressor is probably out of warranty.

  5. Ches says:

    Aloha Sir,
    I am having issues with a scroll compressor and would like your expertise.
    york 5ton
    ambient 81
    SLP- 71psi
    SLT- 52
    LLP- 175psi
    LLT- 91
    Non TXV
    superheat 8
    R-22 factory charged system but believe line set is about 75feet
    Mahalo/Thank you in advance.

    • 3 things, (1) what is the indoor “wet bulb” temp (2) what is the model number of the indoor coil and outdoor unit so I know what the efficiency of the unit is and what orifice is needed and (3) do you know the orifice size ? It almost appears to be an “over feeding metering device but need more information

      • Ches says:

        Aloha Sir,
        Sorry for not replying sooner.
        outdoor M# H3RE060506B S#W0L7407503
        Indoor M# F2FP060H06B S# XCMS05253
        WB 70F
        Amb. 83F
        SLP. 63psi
        SLT. 63F
        LLP. 175psi
        LLT. 83F
        Sorry I could not remove the Orifice because it seems like its connected to the evap coil.
        Again Much Mahalo/Thank you.

      • Ches — with 70 degree wet bulb and 83 degree outdoor temp you should have ~23 degrees of superheat. You have a suction temp of 63 degrees and 63 PSI suction pressure which is ~38 degree so 63′ – 38′ = 25 degrees of superheat — very close to where you need to be. You have a liquid line pressure of 175 = 92 degrees and a measured line temp of 83 degrees so you have 9 degrees of subcooling. I do not see any problem here.

  6. Aneesh says:

    7.5 hp scroll compressor suction and discharge pressure showing almost same such as, suction is 50. Discharge also same as 55or 60.

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