Aaon OAU compressor killer

[Burnedout]

Hey guys need some help on an Aaon oau. Unit killed 3 comp. (locked up) last year and one so far this year. It is on an high rise apt. complex. The problem is when temps reach 95f the head pressure on all four circuits is about 400-410. suction is about 90-95, R-22. superheat is about 25 and subcooling is about 15. Pressures are higher when it gets above 100f. I have washed it, checked the motors, the fan blade depth, no recirculating air, etc.. Room setpoint on wattmaster is 74, it cools the hallways. It will sometimes reach it but comp. are getting crushed. I also have the same problem at another site with the same units. I looked at the blueprints and unit was designed for 90f outside air, which is a joke. Unit has supply vfd which runs at reference, have tried to slow it down which did work, but not good enough.Has anyone else had problems with these units?

[ryan1088]

First things first...are all the fan motors running. AAON are notorious for having non oem fan motors being installed and tripping on thermal after 15-20 minutes run time. Secondly they are pretty hard to get really clean in my experience. What model unit is this?

I made a sprayer out of some 3/4, 1/2, and a ball valve that works really well drill small holes on the end of a cap soldered on the end works really well for us.

I know compressor manufacturers will say 20 degrees superheat but I personally prefer less than that. 10ish is where I try to get to unless otherwise specified by the manufacturer.

[BillDuffy]

From the info you gave it sounds like your units are over charged.

[DNMechanical]

when working on a system that have problems like this. I normal purge nitrogen when brazing, flush the system really good, pull really good vacuum to 500....be really careful, because everything adds up,,,a piece of problem here and there will cause lots of headache

[Six]

If you're running sustained 400 psi and over head pressures with high super heat you're going to keep eating compressors. Imagine your amp draw.

Having to slow down supply air lower head is just a last ditch response to poor planning on who ever put the unit on that roof.

Look for maybe alternative aftermarket solutions for high ambient scenario's. It's probably your only option.

[hvac wiz 79]

You check to see if the hot gas bypass is leaking by? ... I'm not certain all models are equipped with one but every one I've seen is.

[Six]

Even if it had a HGBP it would not be on all circuits.

I think he mentioned that.

[hvac wiz 79]

If you're running sustained 400 psi and over head pressures with high super heat you're going to keep eating compressors. Imagine your amp draw.

Having to slow down supply air lower head is just a last ditch response to poor planning on who ever put the unit on that roof.

Look for maybe alternative aftermarket solutions for high ambient scenario's. It's probably your only option.

You think unit is beyond it's design capabilities so slowing down supply is likely the only alternative to the high evap load?

[hvac wiz 79]

Even if it had a HGBP it would not be on all circuits.

I think he mentioned that.

Oh ok. I misread. Thank you for steering me away.

[beenthere]

Sounds like indoor air flow was set to high from the start.

[maxster]

CCHs? did you recover the freon during a run comparing the inlet air into the condenser on the head pressure for 22.where is the HP control at 400psi running head..and none where burn outs?put up a model number..and you have scroll compressors on that hot gas it won't start opening till the suction goes below 55PSI on the suction with LLS and a adj.regulator.usually first stage circuit is considered the last level of cooling getting near the setpoint

[crymtide]

I have had a lot of trouble out of these units and they can be compressor killers. The condenser coils are notorious for blocking. You need to mount an all out assault in cleaning mode from both sides.

[Burnedout]

Well I doubt it is overcharged, I just think it has too much load for the unit. All the motors are oem. I have washed them with a booster pump, plus you can see sunlight through the coil. The company who put the unit in is no longer in business either, wonder why? All four compressors are overamping due to the load. Also the high pressure cutout is 425 psi. I dropped Vfd speed to 40% but don't want to drop it too much. Do you guys know of any aftermarket parts to drop the head pressure?