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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8766
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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What is the likely cause of heat pump compressor running in

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What is the likely cause of heat pump compressor running in a vacuum with frost on bottom third of compressor?

Phil :

All work to be done by licensed professional. WE KEEP GOING UNTIL YOU HAVE THE INFORMATION YOU NEED, I come and go. This is step ONE.

Phil :


Phil :

Refrigerant boiling out of the oil when the compressor goes into a vacuum will cause the bottom of the compressor to frost up to the oil level.

Phil :

That will only continue however until all of the refrigerant boils out of the oil

Phil :

For that to continue, there has to be a flow of refrigerant into the compressor. that is through the suction line, if thats happening the suction pressure will not be in a vacuum

Phil :

Tell me a bit of what you are doing, I can offer a lot more information.

Phil :



The unit is not cooling. The low pressure cut off valve trips. High pressure is good. Just replace indoor TXV and LL dryer.

Phil and other HVAC Specialists are ready to help you
Hello again, thanks for accepting. We can continue on the same dime so to speak

with the suction pressure in a vacuum there is not enough refrigerant flow to run the head pressure to normal levels... it should be running quite low...... unless... the system was grossly over charged with refrigerant to get the pressure up.

we can discuss all that and the many possible causes.

a blocked refrigerant metering device, feeding the evaporator coil (the outside coil in heating mode, and the inside coil in cooling mode)... almost certainly the cause. 99% chance of that.

Why the refrigerant metering device is clogged or not working is another issue.

There can be many causes for that.

I need to ask you a bunch of questions around that topic

- Had you changed the compressor recently?
- is the new drier the type made for heat pumps
- tell me about the flow arrow(s) on the drier
- tell me if the system had a refrigerant leak and had been running in a vacuum earlier before you did this work

- tell me how long you evacuated the system for and at what micron range.

Im looking for moisture in the system or a bad mix of POE and mineral oil that sometimes happens with compressor changes.

We can go from there, Phil
Customer: replied 5 years ago.
3 ton heat pump split system running normally for 4.5 years, no previous problems. Original compressor, no charges changes made since initial start up. 13 seer, txv indoors and out, R-22. Initial symptom was 'lock out on low pressure' while in cooling mode. Heating mode functions normally.
First, we verified indoor airflow was normal, then jumped out the low pressure switch while monitoring pressures. Within a few seconds (10 or 15) the suction pressure had dropped into a vacuum, and the head pressure was in the 140 psi range. Frost appeared on the bottom of the (scroll) compressor, but no other frost was observed. We cycled the reversing valve several times,in an effort to determine if it were stuck, or partially travelling - it appeared to function correctly. At this point, we suspected a severe restriction in the liquid line or indoor txv. With no visual evidence of the location of the suspected liquid line restriction, we assumed the indoor txv was at fault.
After recovering refrigerant, we changed the indoor coil (txv factory installed on coil, obtained as an assembly), brazing over a nitrogen flow. Next we installed a new bi-flow filter drier in the liquid line, sized for 1 to 8 tons of R-22. The flow arrows point both ways, naturally. Leak tested with nitrogen, evacuated to 29 inches mercury, refilled with nitrogen, evacuated for 20 plus minutes after reaching 29 inches the second time, no micron gauge was available.
After the second evacuation, we refilled the system with the recovered refrigerant, and restarted the system (in cooling mode, still).
We obtained exactly the same symptoms as before: suction pressure less than zero, head around 140 - 150, frost on bottom of compressor. Normal operation in heating mode. Addition of slight amount of additional R-22 (a few ounces) did not change the observed pressures, and with normal operation in heat mode, we do not suspect a severe mis-charge. Further, since the system had never previously been opened since start-up, we do not suspect a problem with moisture or incompatible oils.
At this point we suspect a stuck check valve, but we are wary of changing another component without a second opinion. Further, we are having a hard time locating a suitable replacement.
Ruud UPNE 036 JAZ, matching 3 ton air handler.
Your input would be appreciated.

Hello again, so far you have done everything right, your logic is flawless, and your observations are useful.

We can assume there was no refrigerant leak, due to the fact of the amount of refrigerant you recovered. thus no water in the system.. and no need for a more thorough evacuation than what you did.

If you had a only a restriction in liquid line to the evaporator there would have been a visible frost line at that point as but there was no visible indication as you noticed and alluded to.... its a pretty good guess that the check valve is not opening at all. that was an important observation.

There is not much else it could be if virtually no refrigerant is getting out of the condenser coil to feed your new TXV and thus back to the compressor.

I would try warming up the check valve to about 120F or so, and putting a flat block of wood against it so that it does not dent when you hit it crisply with a heavy screw driver handle .... as the compressor is operating.... and as you have a pressure differential across the valve.

See how that goes, and if you can detect any change.

Tell me what Ruud says when you try to order the check valve from them,. they should have that part. check valve failures are not entirely uncommon, You may have to ask for a more experienced counter man. We can get it one way or the other however.


Customer: replied 5 years ago.
After checking with Rudd the technician he still thought the indoor txv was the likely problem even though the whole coil unit was just replaced. He suggested taking temperature readings on the low side exiting outdoor unit and just before entering indoor unit and on the low and high entering/exiting the indoor unit. Since I do not have specialized equipment for that I bought several small $2 outdoor thermometers. I marked the thermometers with a sharpie to represent the various temperatures listed on the little surrounding plastic case and then removed the thermometer from the plastic case. Broke a couple in getting them out which is why I bought extras. Once the thermometer was out I used black foam insulation around the pipe and thermometer to reduce the effects of the air temperature on my readings. Ran heat for 20 minutes to make sure the pipes were warm. Measured temperature on high side leaving the outdoor unit and the temperature just before entering the indoor unit. The temperature were the same and around 65 or so (the thermometers were not super accurate no two read the same but were within 2 degrees of each other). Repeated with thermometers reading high and low at the indoor unit. High dropped to around the 65 but high stayed at 85 or so. Looks like a bad indoor txv. Going to replace it to see if it fixes the problem.
Hello again.

Given the history, chances that the new TXV is bad are under 1%
Lets wait on replacing it.

Correct me if I have forgotten, but the system heats great, blows warm air in the house when its cool outside... but when you put the system into cooling mode, the suction pressure *measured at the compressor suction... drops into a vacuum.... as you read 140 psig or so head pressure.

From that we know the service valves are in the correct position, but that no liquid is getting from the outside condenser coil to the evaporator coil .. and the only control valve you can see is the TXV at the evaporator and a check valve that flows refrigerant around the TXV at the outside coil when the heat pump is in cooling mode.

You mentioned earlier that you thought it might be stuck...thats a good guess. Had you tried to shake it loose by tapping on it with a block of wood and screw driver as suggested earlier?

The Ruud tech had you check for a temperature difference on the theory that check valve was stuck...**but leaking some refrigerant through, which is common. Yours may stuck or its port blocked 100% closed.

in that case there will be no temperature difference... because the blocked check valve is not leaking any liquid refrigerant through, so is not evaporating any refrigerant at that restriction...

.... with no evaporation there is no temperature change.

(key principle it may be worth reading over a few times)

The reason I suspect that, and not the new TXV is that old TXV operated the same way... the only common factor is that check valve... as you had suspected earlier.

Had you tried tapping on it... what did the most experienced Ruud parts man have to say about ordering you one as discussed briefly earlier... for them to tell you its not available makes no sense, its something a brand new counter man might say.