HP w/Scroll

[subzero*psia]

I remember seeing a post a very long time ago in the refrigeration section about scroll compressors "back gassing" or at least that is what I would call it. The compressor makes a groaning or hissing sound when after satisfying the call. I have seen this happen on one heat pump condensing unit. Anybody have any knowlege on this?

By the way.... no... I'm not dead.... just been away for awhile!

[Andy]

Hi, Subzero
nice to see you back!
I have heard scrolls doing this, usually just prior to failure.
Never was allowed to gut one to see, as they were usually under warrenty, but I have all ways assumed it was the sound of them running backwards after the outlet check valve had failed.
Which came first the chicken or the egg?, did the check valve fail allowing the scroll to run backwards on equalization, wrecking it's self or did bits break off the comp leading the check valve to pass?

Anyway this things I worked on were 25 and 30hp and quite expensive, never known one to fail after being replace provided the contactors and safeties were properly checked out.
Regards. Andy.

[bhrobin]

Scroll compressors do have a check valve essential for proper operation but do run backwards just a bit on shutdown. That's why they so frequently have time delays on single phase models. If the compressor were re-energized during this moment, they would continue to run backwards until the overload popped or the system was satisfied. A brief run backwards doesn't hurt them a bit, in fact, we've seen them run for days backwards with little or no damage.

Rather unlike most other compressors, the discharge valves are needed only to keep liquid from migrating into the compressor from the high side of the system. It will continue to operate in this failure mode until the liquid either stops it or destroys it. We have seen shattered scrolls as a result of this problem. Therefore, it is our position that if the discharge valve fails, the compressor should be replaced at once.

[subzero*psia]

Its a 2 1/2 ton 15 SEER system. When I installed the system it was warm (July).... only required the A.C. It handles the load fine, superheat checks with mfg. specs. No sweat at the compressor, the suction line has less sweat than on recips.... amps check okay... just kinda freaky to hear it on shut down when you aren't used to it I guess. I like how quiet they are while running but the whirring hiss on shut down really catches your attention because it IS so quiet during the run mode. The homeowner is happy with it he mentioned that he has never heard a system so quiet.

This is the first scroll I've put in myself so I have been hawking it. Its been in service for 6 months, no complaints and it gets the job done fine. It gets heavy usage here as the temps are fairly moderate in winter.... its 42F outside now. Anything over 50F outside = Heat Pump. Anything less than 36F = Furnace.... anything between those two numbers the HP is the first stage and trys to raise the indoor temp by 2F within 20 minutes.... "if the HP can't" then the Furnace takes over.

I installed the Invensys 9620i tstat.... one of the best tstats I've seen yet, also installed a Honeywell power humidifier system. Both the tstat and humidity controls require outdoor sensors since it is two stage heat. Installed a new H.E. furnace and all the ductwork too.... they originally had an old gravity furnace with no ductwork to the second floor.... YEEEEHAA! LOL!! When all was said and done... it looked pretty.... we ran a main supply through coat closet on the first floor and in a bedroom closet on the second floor... and finished with drywall. Not shabby at all for a couple of cowboys..... hehehhehheh! The owner liked it so well he had us install a whole house water filter system, a new water softener AND a new hot water heater.... well ... he decided we might as well replace his old water lines at the same time too. LOL! The homeowner is remodeling their upstairs bathroom and has asked me to come over... I keep telling him "I'm not a plummer!" LOL!!

I put this same type of control system and a heat pump onto an existing heating system in another house if I dare call it that.... 4400 square feet!! The homeowner of the larger didn't want the system with the scroll compressor though... its a five ton 12 SEER system and it has been performing well since July too.

I'm just not used to the scrolls yet... I know the charge is more critical so I am very careful with them on start up.

Anybody else heard of scrolls running backwards like this on shutdown.... maybe it is more peculiar to heat pumps as the only time the unit I mentioned does this is at shutdown after a call for heat...