Originally recorded June 23, 2016.

My 4-ton Rheem unit with its Copeland Scroll compressor exhibited an interesting issue, and I lost cooling for a short time. There happened to be a power blip. Normally that’s not much of a problem. However, these Copelands can sometimes make loud startup sounds, yet others do not. They can also make a shutdown sound. Yet others do not. If they make a shutdown sound, it is because the pressure in the system spins the scroll backwards for a moment when power is lost.

Now, if power happens to be reapplied at the moment the scroll is going backwards, the compressor will start running in REVERSE, and as such, the air expelled outside is cold and the air inside is warm.

That’s exactly what happened. Power was lost, the scroll spun backwards, and while that was happening, power was reapplied, resulting in a backwards-running compressor. There are lockout devices available to prevent this sort of thing from happening. This is the first it ever happened (that I’m aware of) in 10 years, so I expect it’s a pretty rare occurrence.

Should the compressor be left to run in reverse, no damage will occur, however the head pressure will get very high until the compressor cannot spin anymore. At that point it will thermal overload out. Upon restart, it will resume operation in the normal direction.

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32 thoughts on “Copeland Scroll Compressors Can Run In REVERSE!

  1. Keith C says:

    I have a Lennox Condenser Unit model HS26-048-5P with a Copeland scroll compressor and I have witnessed this happening at least three times with the latest incident occurring this month. The incident occurred even though I had installed a time delay relay with a “delay on break” function some eight months ago. The time delay relay is a “ICM Controls ICM203 Delay-on-Break Timer with 0.03 minutes-10 minutes” that was installed in the 120 VAC AC line circuit that feeds the 24 volt control transformer located in the blower. When time allows I need to check the relay again to make sure it is still functioning properly, but I suspect that it is still possible for a temporary dip in the AC supply to the house to be just right so that the relay or the time delay function in the White-Rogers digital thermostat does not react to prevent the reverse operation of the compressor. The compressor tripped out on thermal overload and would not restart until it was allowed to cool down for approx 1.5 hours. I did not want to use water to cool the compressor and wet the compartment etc. Took the condenser unit cover off and let it cool with ambient air flow.

    Statement from the Lennox HS26 Series Unit Information:
    The time delay performs no other functions. Without the delay it would be possible to short cycle the compressor. A scroll compressor, when short cycled, can run backward if head pressure is still high. It does not harm a scroll compressor to run backward, but it could cause a nuisance trip of safety limits (internal overload).

  2. Dave V says:

    This just happened to my unit. We had a quick power outage while the AC was running and then the unit started to make a vibrating sound as if something was out of balance. Turns out it was the Compressor running backwards just as was described here.

  3. 17Vintage says:

    On a 3 phase compressor If the discharge valve isn't fully seated when it shuts down, the discharge pressure can push the scroll backwards. When the motor turns back on while it's turn backwards it can run backwards. There are no red valves in the scroll compressors to keep refrigerant from gong back into the compressor. There is a floating seal that acts as a valve. Seven the compressor shuts off the seal disengages from the muffler plate this allows refrigerant in the discharge chamber to flow into the suction chamber of the compressor. It is possible for the seal to not separate from the muffler plate and cause the discharge to flow back into the scroll and turn the motor backwards

  4. Pierre Segui says:

    A compressor runni g in reverse will never change from cooling to heating, he will just make noise and displace less gaz. If the airco switch from cooling to heating after a short outage, it means the 4 ways valve did switch.

  5. Slyte Lee Acidic says:

    At 1:43 you said p[umping warm air inside and cold air outside? because the compressor was running backwards? Reversed rotation scroll compressor will not magically create a heat pump ! Not even if the units is a heat pump! Yes a 3phase scroll can be electrically reversed but it wont pump if it's running the wrong direction period. a single phase scroll can assume a reversed rotation if the scroll is rotating backwards due to pressure equalization during the off cycle and turns on while rotating backwards. (That's why the delay timer is there) It doesn't crate a a heat pump when this happens it just wont pump.

  6. Alexi Rodriguez says:

    My 1 year old American Standard packaged unit does the same thing. It has a scroll compressor. Every time I have a very short power surge, the a/c begins to hum loudly outside and it stops cooling the air that is going into the house. I automatically turn it off at the thermostat and wait 6 minutes. I turn it back on and it runs normally again.

  7. HighVoltageMadness2300 says:

    my Amana with a Copeland scroll has this a few times when there was a power cut. Once I came home during a heavy thunderstorm to a loud humming/buzzing noise which I thought was the sump pump. However, it was not. The air coming out of the vents was warm. Then when I went outside I found it was the condenser. There was a powercut in which this exact same thing occurred and caused it to restart in reverse. Installing a hard start kit may help with this issue.

  8. Jeffery Brock says:

    That sound is the compressor running backwards for as split second on account of the discharge check valve leaking by. If the compressor runs backwards it doesn't heat the house that is your reversing valve that controls that function not the compressor running backwards as a pump running backwards will not cool or heat. Yes this a year or 2 too late… lol

  9. Charles Smith says:

    My scroll compressor will reverse if the power flickers just right. I put a delay relay into the 24v circuit but it did not prevent it from happening again. I can hear the growl in my indoor unit when this happens. I simply turn the system off for a minute and back on and it runs normally again. It is not a heat pump so there are no extra gadgets that can cause the problem. Someone told me there is a device that will detect a reversed compressor and prevent it. Don't know for sure.

  10. John P. Callan says:

    I have a client in a high rise condominium with a water source heat pump that did this routinely. Although in the case of his unit there must me internal check valves to prevent reverse refrigerant flow. It has a replacement compressor 2-3/4 ton model ZR32K5E-PFV-800. The power is interrupted, the scroll comes to a stop very quickly, then the scroll begins to spin backwards due to the internal pressure from the high side, then the power comes back on and the compressor runs in reverse for about 10 minutes until the external shell is 165 degrees, then it stops and sits for a half-hour to an hour. With no refrigerant flow to cool the compressor, it get really hot before it thermal trips. I replaced the original unavailable R22 recip compressor with a Copeland scroll. This unit, too, has ICM protective relay devices (ICN492 & ICM200) that didn't help so far. I am righting the fault duration limit to get better results – 2 seconds does not do it.

    Turns out the contactor's contact disks were badly burned and not making perfect contact. Combine that with some vibration and the thing would quit for a second or less. The Copeland scroll seems to have higher inrush current demand. It degraded the 30 amp contactor rapidly. It now has an up-rated 50 amp contactor that I hope will stand up to the high current surges at start-up. New contact seemed to work until it didn't..

  11. Edenio Gonzalez says:

    24. Brief Power Interruptions

    Brief power interruptions (less than 1/2 second) may

    result in powered reverse rotation of single phase

    Copeland Scroll compressors. High pressure discharge

    gas expands backwards through the scrolls at power

    interruption, causing the scroll to orbit in the reverse

    direction. If power is re-applied while this reversal is

    occurring, the compressor may continue to run noisily in

    the reverse direction for several minutes until the

    compressor’s internal protector trips. This has no

    negative impact on durability. When the protector resets

    the compressor will start and run normally.

    Emerson strongly encourages use of a timer which can

    sense brief power interruptions and lock the compressor

    out of operation for two minutes. A typical timer wiring is

    shown in Figure 16.

    No time delay is required on three phase models to

    prevent reverse rotation due to power interruptions.

    (Copied from Emerson Bulletin AE4-1299 R21)

  12. Ted Allen says:

    i commissioned a goodman condenser single phase M/N GSX130601BJ, S/N 2005155921. I observed a delay for a moment. my 410a pressure with a piston equalized. the rotation reversed. the copeland scroll shut down on thermal overload. i came back 24 hrs later with a sp-6 hard start kit. i started it up and it sounded like a bomb went off, loud bang. i shut it down and restarted the condenser with no further issue. in 25 years, i never had this happen. my partner and i looked at each other and said we heard a bomb and we're still alive. mind blowing.

  13. David Sobczynski says:

    Hello, this seems to be similar to what I'm experiencing. Had a blower motor replaced and literally the next day a grinding noise started. When the grinding noise happens the cold air with stop blowing and the grinding will continue for 15 minutes. After 15 minutes the noise stops, which i assume is the condenser shutting off, but the fan will continue to run for approximately 30 minutes. The condenser will then turn back on and start cooling. About 10 ppl have been out to look at it but we are unsure as to watch the issue is. This is a reheem ac unit and even the manufacturer is unsure

  14. Timothy Myers says:

    The 3 Phase Compressor won't run backwards on its own. What causes it to run in reverse is the 3 Phase Power Service from your Power Company. In the case of a power blip, power outage, the Power Company may make a quick repair to restore service quicker, but at the cost of temporarily reversing the Phase Rotation of their 3 Phase Service. Once their permanent repair is made, they check their Phase Rotation, which allows your Compressor to return to its designed direction…..any 3 phase motor or compressor can be reversed by swapping any two conductors on its starting circuit. Thats basically what is happening upstream of your residence in an emergency repair by the power company. They may be bypassing a downed transmission line, re-wiring a transformer configuration due to a bad transformer, any number of problems.

  15. Mario V Gomez says:

    It’s not the compressor running backwards. It’s the loss of power to your reversing valve. The valve reverses the flow of the REFRIGERANT cycle, not the compressor. The compressor rotates in one direction only on a single phase power source such as a residential structure. So, this entire video is incorrect.

  16. Applianceman600 says:

    Yep, that is totally normal. We had a Rheem air conditioner that would always do that on shutdown. The air conditioner was 12 years old when we moved in to our house, and it was 24 years old when we decided to upgrade. It always made that sound the entire 12 years I had heard it run.

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