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A/C Cooling Question

Yahoo Message Number: 79445
Hi,

My bus has two A/C rooftop units (they function as heat pumps too) @ 15,000 BTUs each. They are both RV Products model 9024-876. The front unit doesn't cool quite as well as the back unit does. I was up on the roof yesterday and took off the cover to the evaporator coil and cleaned only a small amount of dust from it. Also perked up all the aluminum tape to ensure no air leaks were diluting the cool air. Everything else looked fine visually. I keep the units covered in the winter months so the guts are pretty clean. My cleaning and taping efforts failed to facilitate any improvements. I measured the exhaust air temperature of the back unit @ 40F and the front unit @ 52F. Outside air temps were 80F at the time I took measurements.

Has anyone else had a weak cooling problem? If so, what steps were taken to correct the problem. I'm thinking the front unit needs to be recharged perhaps. It's been like this for about 4 years and hasn't gotten any worse. Help, advice, suggestions much appreciated. Thanks.

Best - Scott

2006 Allure 430 #31349
(Bus-Stead Lemon)

Re: A/C Cooling Question

Reply #1 –
Yahoo Message Number: 79449
Scott, I had a similar problem with one of my A/C's, I did not check the actual temp that it was putting out, but knew something was wrong. I though like you, that maybe it need coolant, but I call RVP and they told me that the Coolant level hardly ever need attention, turns out it was a Starting Capacitor, they shipped to me and I installed, worked like a Charm. This may help you, worth checking out.
Marvin 07 Allure #31459

Re: A/C Cooling Question

Reply #2 –
Yahoo Message Number: 79455
To all roof A/C experts,
Here is another question:

My front A/C operates and cools fine. However, intermittently during very hot outside temps, it will suddenly stop cooling. The fan continues blow strong but no cooling. If I shut the system down and wait for a few minutes then, switch back on, it will begin to cool again. What is causing the cooling to cut out? Could the coils be icing over and shutting itself down as a result? Could this also be a sensitive starting capacitor problem? How can I diagnose and fix this?

Daron Hairabedian, 98 Allure 30226

Re: A/C Cooling Question

Reply #3 –
Yahoo Message Number: 79458
My guess is that the thermal safety in the compressor motor windings is opening because the compressor is running hot. I would take a look at your condenser coil to see if it's dirty. If the coil is clogged, the system cannot get rid of the heat. The compressor is actually cooled by the refrigerant coming from the cold evap coil. If the condenser coil is dirty it cannot get rid of the heat, the system will not cool properly causing the refrigerant to enter the compressor warmer than it should be causing it to run hotter, opening the thermal safety. By turning it off for a while the compressor windings have a chance to cool causing the thermal to close allowing the compressor to operate again. If it were the starting capacitor then it wouldn't start at all. Remember its just a guess.

Steve & Jane

1998 Intrigue

#10557

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Re: A/C Cooling Question

Reply #4 –
Yahoo Message Number: 79459
Marv - Thanks. Perhaps an aging capacitor might cause the compressor motor to run a tad slower. That makes sense and would reduce cooling efficiency.

At 04:26 PM 5/22/2012, you wrote:

Re: A/C Cooling Question

Reply #5 –
Yahoo Message Number: 79464
A start capacitor is only used at start up to get the compressor to speed. If it were weak the motor would not get to run at all. If a motor does not get to speed it acts like it is overloaded and would cut out on the thermal protection.

Ron Baran 09 Magna7025

Re: A/C Cooling Question

Reply #6 –
Yahoo Message Number: 79466
I agree with Ron's comment. One day, on our front unit the fan would run but there was no cooling or heating. Eric Olstrom's shop in Tucson diagnosed the problem most likely as a bad capacitor, which it was. Once the capacitor was replaced all was well. Happily, this is not an expensive fix.

Lee
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Lee (leozbrowski@...) 2007 Country Coach Intrigue 12153
CAT C-13

Re: A/C Cooling Question

Reply #7 –
Yahoo Message Number: 79470
Ron - Thanks. That motor has both start and run capacitors (7.5 MFD 370V & 45 MFD 370V). It wouldn't hurt to replace these as a preventative maintenance item. I'll report back with details.

At 05:52 AM 5/23/2012, you wrote:

Re: A/C Cooling Question

Reply #8 –
Yahoo Message Number: 79471
Agreed. On mine there are three. Eric Olstrom said if one goes the others are effected so he replaced all 3 . . . still not expensive.

Lee

Re: A/C Cooling Question

Reply #9 –
Yahoo Message Number: 79472
Lee - Thanks. Per the parts list, there is a fan capacitor (7.5 MFD 370V) and a run capacitor (45 MFD 370V). Together that's about $26 worth of parts and not a bad replacement job. Might not be a bad idea to check the supply voltage when I'm up there. I'll report back with details. Luckily it isn't blazing hot here (80F during the day) and the unit does cool. It just doesn't cool as well as the rear A/C unit does and I'm curious why.

At 06:01 AM 5/23/2012, you wrote:

Re: A/C Cooling Question

Reply #10 –
Yahoo Message Number: 79474
Its called a combination capacitor. The 7.5 microfared (mfd) is for the fan motor and the 45 mfd is the run capacitor for the compressor. It has both capacitors in one can. There are 3 terminals on it. They should be marked C for the common terminal F for the fan and an H for the hermetically sealed compressor.

Steve & Jane

1998 Intrigue

#10557

______

Re: A/C Cooling Question

Reply #11 –
Yahoo Message Number: 79475
If it had an actual start capacitor then the capacitor would have a black plactic look to it.

Steve & Jane

1998 Intrigue

#10557

______

Re: A/C Cooling Question

Reply #12 –
Yahoo Message Number: 79476
I just fixed the same problem on my Dometic front AC. Turned out to be a $30 part which the tech called a "Freeze Switch". Basically a little electric thermostat that monitors the compressor and shuts down the system if it senses the compressor is freezing up. He said that sometimes can happen in high humidity areas. In my case the humidity was very low which led him to replacing the "Freeze Switch".
On a similar subject he told me Dometic will soon be "upgrading" (corporate talk meaning "make obsolete) the five switch Comfort Control. Anyone needing to replace one in the future will also need to replace the motorboards in their units because, surprise, surprise, they won't be compatible.

Jim
10613

Re: A/C Cooling Question

Reply #13 –
Yahoo Message Number: 80305
Steve,

Daron here. I finally climbed up, removed the covers and inspected the coils. They are clean. Also, there seems to be a lot of vibration when the compressor is running. It appears that the thermal switch is is sensing high temp in the motor and cutting the power as you said....Or... is it a problem with the "Run" side of my combo "Start/Run" capacitor. I checked online sources that show several types of Start/Run capacitors @ $14 to $22. Do you think that is worth a try or am I on the wrong track? Meanwhile, I described the symptoms to a mobile tech who happened to be working in the area where we are presently camped. He said: "Your compressor is binding, drawing too much current and getting too hot. The thermal switch is then shutting down the power until the motor cools down. Considering the age, (13 yrs) you need a new A/C. The cost for a Dometic A/C, heat pump combo unit would be $1,000 - $1,200 installed with four-five day lead time to obtain the unit. Also the mother board in your rear A/C and the Comfort Control would probably need to be replaced."
Is this correct? Is this the only solution? Should I check the amps drawn by the compressor as suggested by Dave Rousey @ Styleworks RV? Should I bother fussing around with a replacement capacitor or just retire the tired old unit? If that is the case I will be very upset with myself as I replaced the fan motor and squirrel cage assembly just nine months ago! Oh well, It's only money....and a lot of sweat! Daron Hairabedian, 98 Allure, 30226

Re: A/C Cooling Question

Reply #14 –
Yahoo Message Number: 80332
The combo capicitor is not a start / run. They call it a combo because it it two start capacitors in one. The 3 terminal connections are labled "C" for comon, that is the 120v to the capicitor, "H" for hermetic which is the compressor and "F" fan motor. Chances are the vibration is from one of the blower wheels is out of balance. Either by dirt or the balancing weight came off. Could also be some kind of debris in one of the blower wheels. If it was drawing too much current the the 20 amp circuit breaker should be tripping. If the cap was going bad then it wouldn't start at all. If you can get your hands on a clamp on amp meter then you can easily see what the amp draw is. But it still sounds like you have a bad thermal overload. If you have a multi meter then you can take an ohm reading on the windings right after it quits. Be sure to turn off the circuit breakers before you you do that.

Steve & Jane

1998 Intrigue

#10557

______

Re: A/C Cooling Question

Reply #15 –
Yahoo Message Number: 80333
Steve,

Thanks for staying with me on this. The 20 amp A/C circuit breaker in the bedroom main power panel is not tripping.
I will buy a clamp amp meter somewhere, test amp draw while unit is cooling and report results.
Daron

Re: A/C Cooling Question

Reply #16 –
Yahoo Message Number: 80335
No need to spend a lot of money on a new one, just go to a pawn shop they usually have a few there to choose from, just make sure it works before you buy it. I decent one will have a volt ohm meter built in with it.

Steve & Jane

1998 Intrigue

#10557

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