Why doesn't roof AC run all the time?

[belfert]

I have two Carrier rooftop AC units that maybe have 20 or 30 hours on each one.

Anyhow, I had them both on yesterday, but the front of the bus was noticeably warmer than the rear of the bus.  I turned the front AC to max cold on the dial.  The problem is that the front unit didn't run the compressor much even though it was warm up front.  I would have expected the compressor to run all the time until it got cool up front.

Any ideas what I might check?  I noticed it not cooling too well on a trip last year, but didn't pay much attention then.

I had a travel trailer before and the compressor would run non-stop no problem when it was 113 degrees outside.  (It could only drop the temp inside to maybe 85 or 90.)

[oldmansax]

Sounds like a failing thermostat. If Carrier is like most, it uses a stat with a gas filled aluminum sensing tube. When the tube warms, the gas expands and closes an internal switch which actuates the compressor. If the tube loses its charge, there is no gas to expand and close the switch. If the tube loses some of its charge, the compressor actuates at a higher temperature.

There are other factors involved but I would check the thermostat first.

TOM

[belfert]

I assume the thermostat is in the ceiling assembly?  If I swap ceiling assemblies that should help in diagnosis, correct?

If this is in the rooftop portion, how do I check or replace it?  Do I need a professional?

[JackConrad]

    Not positive, but I am pretty sure the themostat tube is in roof top part of the AC. I think the only thing that connects between the roof top unit and inside cover is a couple wire connectors (and a 3rd if you have the heat strip option).  Maybe Nick will chime in on how to test the unit.   Jack

[Nick Badame Refrig/ACC]

Hi Brian,

The fact that the stat is cycleing the compressor, lads me to belive that it may be ok.

It sounds like the sensing tube is in the wrong place..

It's possible that cold supply air is comming in contact with it and prematurely cycleing it off.

Compair the placement of the tube with the rear unit.

Good Luck
Nick-

[belfert]

I believe I found the problem.  The sensing tube is mounted on the ceiling assembly.  The ceiling is not finished under the front AC unit so the ceiling assembly cover does not fit right and allows cold air to leak onto the sensing tube.

[Barn Owl]

Isn't there an adjustment on the stat? I have seen some if you remove the knob there is a screw you can adjust to modify the set points.

[rusty]

  Belfert  I have carrier roof top units and the compressor cycles way to much and is off more than it is on. The instruction say to place the tube in the evaporator fins. I thougth this was the problem because of putting it in the coolest place. I moved it to the intake duct but this did not help. This must be a problem with carrier. If you find the problem advise us Thank you Wayne

[Sojourner]

About compressor off & on cycling too often while trying to cool.

1)   If condenser's air flow is restricted and/or fan or blower is dirty or slow air flow....it will cause compressor's to work harder (high-head pressure) and the thermostat breaker switch on motor's housing to switch off until cooled somewhat to turn back on. Check for leave and etc. Check motor's bearing for lube dryness...or dose it spin freely.
2)   Check inlet filter restricted to cause icing build up on evaporator.
3)   Check the evaporator's fan or blower for slower than normal flow
4)   Make sure both evaporator and condenser are clean and fins are open instead of flatten. You can get fin straighten-er tool to open them.
5)   If thermo bulb is getting cooled air instead of room air...check for outlet (cooled air) is directed or too close to evaporator's room air inlet.
6)   For non wall type thermostat control...Thermo bulb should be on side of evaporator's room air inlet opening behind the grill.
7)   Wall type thermostat control should be mounted on partition wall (never on outside wall) and away from cooled air flow passage or redirect cold air duct.

FWIW

Sojourn for Christ, Jerry

[TomC]

I had the same problem on my front Coleman roof top.  My fix was to put one wrap of electrical tape around the thermostat bulb, and presto-the thermostat allowed the air to come down lower since it was semi insulated from the cold.  It has been running for 14 years this way.  Good Luck, TomC