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:
- Suction pressure will be higher than normal.
- Head or liquid pressure will be lower than normal.
- Super-heat and sub-cooling could be non-existent or high, depending on how bad the valves are.
- 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.
- 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:
- To check the discharge valves, turn off the condenser fan motor and turn the unit on.
- The head pressure should climb very quickly if the valves are good.
- If the pressure does not climb, chances are the discharge valves in the compressor or “shot”.
- 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.
- To check the suction valves in the compressor, turn off the indoor blower motor and turn the unit on.
- you should see the suction pressure start to drop and continue to drop.
- 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:
- Not enough super-heat.
- Over charge
- Low evaporator air flow
- Incorrect metering device or over-feeding TXV.
- Operating the unit below the manufacturer’s recommended temperature ranges (this is not a refrigeration unit)
- 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:
- High condensing pressures
- High system superheat
- Low suction pressures
- 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!