Original message
| sliderack
 |  "Compressor cutting off hi psi"
, posted Mon 24 May 18:53  
Hello, had a question. Was out looking at a 10 ton unit consisting of two five ton compressors. The one compressor in question kept trying to start but the psi went up to 400 in a matter of seconds and hi pressure switch cut power to the compressor. The compressor would drop down in psi and tried to restart but cut off on hi head again. This process kept repeating. Wanted to bypass the switch but was a little afraid to. I wanted to see if anyone knew possible causes of high pressure so quickly upon start, a restriction perhaps or compressor not pumping right. I know you can't diagnose a compressor without looking at it but I was wondering if anyone ran into this same problem before. The unit was on a rooftop with one side practically hanging off the side of the roof so it was just about impossible for me to access the controls. Many thanks and excellent forums.
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Replies:
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| demandcooling
| "Re(1):Compressor cutting off hi psi" , posted Thu 15 Mar 19:36
what brand is it? custom made or the popular brands. some manufacturers use solenoid valves for staging ( variation of loads) and pumping down. check if it has solenoid valves?
DO IT RIGHT THE FIRST TIME SAVES TIME AND $$$$
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| cneiner
| "Re(1):Compressor cutting off hi psi" , posted Fri 25 Mar 16:56
Sorry I mean high superheat and low pressure
R&D Hussmann
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| cneiner
| "Re(1):Compressor cutting off hi psi" , posted Fri 25 Mar 16:55
Service valves need only to be cracked to read pressure.
Check the head pressure on the other compressor. If the system is over charged it should be high as well.
Overcharge is not likely the problem if the faulty compressor is instantly cutting out out. It would likely take at least 10 seconds.
Usually when they cut out instantly there is a restriction. Maybe the service valve is broken.
If it was a liquid filter dryer plugged on a parallel system the pumping action of the second compressor would pump all of the refrigerant into the condenser. In that case an overcharge would cause it to cut out on high head even with only one compressor. If it's not overcharge, a plugged drier would cause the evaporator to starve causing low superheat and possible low pressure cut out.
R&D Hussmann
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| cneiner
| "Re(1):Compressor cutting off hi psi" , posted Fri 25 Mar 16:45
Did you front seat a service valve instead of just cracking it to check pressure.
If it's a new system I have seen a few cases where the rubber plug was shoved in the copper tube a few inches before the guy brazed it and he didn't see it.
OVERCHARGE condition possible, but if the compressor is parallel with another they would both be tripping on high head. Non condensibles could also be in the system.
R&D Hussmann
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| sliderack
| "Re(1):Compressor cutting off hi psi" , posted Mon 24 May 19:22
Um, wanted to bypass the hi press switch for testing only!
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| | Xenos
Webmaster 
| "Re(2):Compressor cutting off hi psi" , posted Mon 24 May 19:27
Test what? It's hitting 400 instantly, not building up or anything. if it was building up I would say soak it with water.
Xenos.
The best way to escape a problem is to solve it.
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| Xenos
Webmaster
| "Re(1):Compressor cutting off hi psi" , posted Mon 24 May 19:03
I actually think it's overcharged from the winter. Yes it could be a restriction but it would have to in the discharge line.
To expand. If it was charged in the winter it would have been easy to overcharge since the condenser could have healed lots of liquid, now that summer is here there is no room for expansion.
Xenos.
The best way to escape a problem is to solve it.
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| | sliderack
| "Re(2):Compressor cutting off hi psi" , posted Mon 24 May 19:32
Yes, that probably would have been the logical choice to follow. Guess the best way is to recover, vacuum, and recharge with virgin R22. I thought it was a restriction in the liquid line dryer but only had a fraction of a degree off between inlet and outlet. With compressor cycling like that I don't know if that was an accurate reading. Sure got the discharge line hot as anything though.
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| | Xenos
Webmaster
| "Re(3):Compressor cutting off hi psi" , posted Mon 24 May 19:46
Assuming it was a restriction in the drier your head wouldn’t hit 400. The liquid would just log up there in the condenser and since you wouldn’t be picking up any heat in the evaporator the head would be low since it wouldn’t be rejecting any. You have a hydraulic pressure occurring most commonly it’s an overcharge or a restriction in the first part of the condenser reducing it’s effective area. Remove some gas. You won’t need to evacuate,\ unless you let air in, just remove it and put some back in. could be non condensable but you need to start with the charge.
Is the condencer pluged? on the air side? just reread the last post and you wrote it as if it was not instantly hitting 400.
Xenos.
The best way to escape a problem is to solve it.
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| | sliderack
| "Re(4):Compressor cutting off hi psi" , posted Mon 24 May 20:20
I checked the coil and it didn't look too dirty, although fins were mashed in in several places. I did consider a dirty coil driving the head up but it cut off so quickly on hi head that I thought it was another problem. But it may be a combo of things causing this compressor to cycle on hi. Many thanks for your responses. I really appreciate the insight.
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| | Mastertech
| "Re(5):Compressor cutting off hi psi" , posted Fri 25 Mar 17:15:
Sliderack, Chemically clean those coils before u do another thing to it. Ive had some units do that to me. If there are a double coil tear it about and clean in between the coils. If u peg 400psi instantly then clean the coil then recheck it.
That should always be the first thing u do to a unit.
[this message was edited by Mastertech on Fri 25 Mar 17:38] |
| | wreddog123
| "Re(5):Compressor cutting off hi psi" , posted Fri 4 Jun 11:09:
hi, is this unit a heat pump.If so it sound like a bad exp. valve or metering device.
wreddog123
[this message was edited by wreddog123 on Fri 4 Jun 11:21] |
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