Single Phase Scroll Going Backwards?

[tuba]

So I diagnosed a leak in a system (small carrier rooftop, horizontal condenser fan), moved some of the parts to the identical system next to it to get some cooling in a computer room. A few weeks later the repairs were approved. I repaired the leak, installed a new blower motor, new dual capacitor, did the other stuff too. When I started it up with the refrigerant, the compressor made that 'backwards noise' and my pressures were equalized. I had to do a double take because I knew I saw suction in the initial diagnosis. So I shut it off, started it again and all was well. Has anyone seen this before on a single phase scroll? It has me scratching my head.

[IFRZAIR]

Yes, I have seen it on some of the first gen scrolls. Early Carlyle Milleniums

[tuba]

Now that you mention it, I think that is what this was. It had two wires coming out of the top center. I guess it is a thermal overload? Any idea what causes the reverse?

[IFRZAIR]

Yeah if it had the little sensor with two black wires out the top, that was first generation, If i remember correctly, It had to do with the scroll gears. If you added liquid into discharge side of comp during off cycle and then started it would spin backwards, once you shut it off and wait a few seconds and start it again it would spin correctly. I remember my first time....Good Times

[thermojohn]

Now that you mention it, I think that is what this was. It had two wires coming out of the top center. I guess it is a thermal overload? Any idea what causes the reverse?

I understand the earlier generation of scrolls did not have a check-valve on the discharge. If the power 'hicupped' while the compressor was operating, the hot gas would back-flow, causing it to spin backwards, and re-start in reverse. Since the indoor t-stat would not satisfy due to a lack of refrigerant flow (cooling), the compressor would run continuously until the IO would open and shut the compressor down. It would then cool, and start back up with correct rotation.

Another cause of the same problem is that top-mounted temp sensor in the compressor. A lot of Carrier TRA and YRA units with Copeland scrolls had them. The earlier scrolls used them, and they also hiccup the Y call when they fail. I've had to replace a few sensors that were opening prematurely. Those were 15-18 years ago or so, and they are old school nowadays.

You happened to catch it at that magic moment.

[tuba]

Thanks for the comments. That is exactly how it sounds. I started it after adding most of the charge to the liquid side. I appreciate the responses, now I can stop scratching my head.....