A REALLY Loud Compressor

A while ago, I walked up on one of our managers who was talking to a junior tech diagnosing an intermittent controls issue on a pool heat pump.

In the background, you could hear an EXTREMELY loud compressor.

The junior tech had just been moving some wires around, and the next thing he knew, there was a clack and then the noisy compressor and equalized pressures.

Do you know what happened yet?

I instructed him to shut it off and asked if it was a scroll compressor.

Sure enough, it was.

What happened was an instantaneous short cycle caused by the loose connector being moved. In that split second, the high-pressure gas in the scroll forced the scroll plate in the opposite direction ever so slightly. Once the power came right back on, it was running backward.
Now, this really shouldn't happen, but when it does happen, it's because of one of a few reasons.

Three-phase miswiring 

When a new three-phase building is constructed or a new unit or compressor is installed, it is possible to miswire the phases, resulting in a compressor running backward. This is not a good thing, but it can be corrected by switching any two legs of incoming power.

Instantaneous short cycling

This is what happened in the case of my junior tech. In most cases, the time delay in a board, thermostat, or controller will prevent this from occurring. Sometimes, the cause is internal to the system due to loose connections, etc.

Miswiring or Failed Capacitors

In single-phase applications, the run capacitor applies a phase-shifted potential that helps get the motor running and keeps it running. If the capacitor has failed or the compressor is miswired, it can occasionally (rarely) result in it running backward.

Failed Discharge Check Valve

Most scroll compressors have either an internal or external check valve that prevents the discharge gas from forcing back through the compressor, causing it to spin backward.

Occasionally, you may find a scroll compressor that makes a loud whirring once it cycles off. That can usually be corrected by installing a discharge check valve or by replacing the compressor if you choose.

Finally, be aware that anytime a scroll runs backward, it can do significant damage. If you find one that's running backward, shut it off immediately and correct the cause.



Dale Bryant
Dale Bryant
2/20/17 at 09:26 AM

What is the picture at top?

Lionel Stoxstell II
Lionel Stoxstell II
3/15/17 at 01:52 AM

This is the info iv’e been looking for!! Thanks!!

5/9/18 at 08:14 AM

My first experience with a scroll compressor with a failed check valve was 1993 when a client called me and said his unit makes a horrible noise when it turns off. I went out and while the heat pump was running, I pulled the disconnect. When I did this, the compressor started spinning backwards and was also generating 288 volts to the outdoor fan motor, which was really spinning fast. As the pressures equalized, the voltage kept dropping until the compressor stopped altogether. Yes, when refrigerant spins the compressor, it becomes a generator. Really messed with me for a little until I found out that Rheem had snuck this compressor into this unit as an experimental test. The compressor was in an insulated box so you could not see that it was different. This model had always had a recip in it before. Rheem provided a new compressor. Brian.

Carroll Price
Carroll Price
12/1/22 at 04:28 AM

It appears that installing external check valves on scroll compressors (as a backup) would be a good idea.

    Tom Paparounis (coldman29)
    Tom Paparounis (coldman29)
    12/3/22 at 04:14 PM

    I wonder why the engineers overlooked that? Reciprocating compressors have discharge valves inherent in their designs that positively keep the refrigerant from flowing back into the compress. (Providing they aren’t leaking of course)

Chuck phelan
Chuck phelan
12/1/22 at 04:47 PM

A start kit has remedied this issue for me so far. I have seen this a few times and found it hard to believe that a single phase would do this. Was close to changing out the compressor on a so called “lemon” (customers opinion) when the thought came to me about a start kit possibly getting the jump on the correct rotation. This unit has a moderately long (65′) line-set that I thought might have a migration issue. After crankcase heaters had been applied the issue would still occasionally happen.


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