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Old 09-20-2020, 08:58 PM   #1
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Failing A/C Roof Units

We have three Dometic 15,000 KW A/C units on our roof. I replaced the rear one several months ago. It now appears that the other two are also failing. Symptoms: Coolness of air through our vents has greatly diminished to basically room temperature when the rear (NEW) unit is blowing VERY cold.

Iíve a very intelligent neighbor (electrical engineer servicing the medical industry) who believes that rather replacing these units I should, instead, replace their capacitors. Saying that the capacitors are the typical failing point. At about $100 each, this Ė if it were true Ė would be a far more favorable alternative. But, since you cannot return electrical items if they donít solve the problem, I was wondering if thereís a way to check this theory out before I buy?

He also said that this is a technically challenging task, better left up to a tech. Thatís fine with me.

Anyone out there dealt with this situation before? If yes, what did you find/do?

Thanks.
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Old 09-20-2020, 09:58 PM   #2
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The compressor has a start capacitor that provides a power boost to start the electric motor that runs the compressor, and soft start, or “easy start“ capacitors help it start with less power. A hard start capacitor is used to start an aging, uncooperative compressor.

So if you hear the compressor running and it’s not blowing cold, a new capacitor will not address the issue. They just provide enough power to get the thing going. If you hear the fan running but the compressor isn’t coming on, then yes, the first place to start is the start capacitor. Also they cost around $15 ($35-$40 if they printed Dometic on it), not $100. Easy start capacitor kits for RV ac units are the high priced ones.

It’s far more likely that you have a leak somewhere and lost your charge. There is a very small diameter tube (forget the name of it) that is a common fail point but difficult to fix. Beyond that these units have no recharge ports.

So basically the HVAC trade does not service these things and if they did, they would charge so much you’d be close to the cost of a new unit. Also new units should be more efficient, but in the RV appliance market, efficiency improvements seem to be way behind residential.

So check to see if the compressor is coming on before messing with a start capacitor. You always want to diagnose before you start swapping parts.
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Old 09-20-2020, 10:20 PM   #3
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The compressor has a start capacitor that provides a power boost to start the electric motor that runs the compressor, and soft start, or ďeasy startď capacitors help it start with less power. A hard start capacitor is used to start an aging, uncooperative compressor.

So if you hear the compressor running and itís not blowing cold, a new capacitor will not address the issue. They just provide enough power to get the thing going. If you hear the fan running but the compressor isnít coming on, then yes, the first place to start is the start capacitor. Also they cost around $15 ($35-$40 if they printed Dometic on it), not $100. Easy start capacitor kits for RV ac units are the high priced ones.

Itís far more likely that you have a leak somewhere and lost your charge. There is a very small diameter tube (forget the name of it) that is a common fail point but difficult to fix. Beyond that these units have no recharge ports.

So basically the HVAC trade does not service these things and if they did, they would charge so much youíd be close to the cost of a new unit. Also new units should be more efficient, but in the RV appliance market, efficiency improvements seem to be way behind residential.

So check to see if the compressor is coming on before messing with a start capacitor. You always want to diagnose before you start swapping parts.
WOW!!! Interesting diagnosis. Thanks.

A point of clarification for me - being a total non-tech. When you state "So if you hear the compressor running and itís not blowing cold, a new capacitor will not address the issue." I hear the unit start - like I would in normal operations and or use of the heat pump. Is this what you're referring to?
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Old 09-20-2020, 11:38 PM   #4
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WOW!!! Interesting diagnosis. Thanks.

A point of clarification for me - being a total non-tech. When you state "So if you hear the compressor running and it’s not blowing cold, a new capacitor will not address the issue." I hear the unit start - like I would in normal operations and or use of the heat pump. Is this what you're referring to?
Two things happen when you turn on the AC. The fan for the evaporator coil runs and then the compressor turns on with a different distinctive noise - more of a deeper sound of an electric motor putting out the effort necessary to run the compressor combined with the sound of the compressor working. I guess you’d have to listen to the unit that’s working properly running by itself to hear the difference.

Here’s another good clue though - if it’s blowing air that isn’t really cold but is cooler than ambient, the compressor is working and the charge is low. A temperature gun is very handing for working with heat and air as you can point it at the vent and compare that temp to something else in the room, like a wall or whatever.
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Old 09-21-2020, 11:08 PM   #5
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Two things happen when you turn on the AC. The fan for the evaporator coil runs and then the compressor turns on with a different distinctive noise - more of a deeper sound of an electric motor putting out the effort necessary to run the compressor combined with the sound of the compressor working. I guess youíd have to listen to the unit thatís working properly running by itself to hear the difference.

Hereís another good clue though - if itís blowing air that isnít really cold but is cooler than ambient, the compressor is working and the charge is low. A temperature gun is very handing for working with heat and air as you can point it at the vent and compare that temp to something else in the room, like a wall or whatever.
OK, I'll check on this tomorrow.

In case the problem is the "low charge." Does that mean replacement time?
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Old 09-22-2020, 12:18 AM   #6
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WOW!!! Interesting diagnosis. Thanks.

A point of clarification for me - being a total non-tech. When you state "So if you hear the compressor running and itís not blowing cold, a new capacitor will not address the issue." I hear the unit start - like I would in normal operations and or use of the heat pump. Is this what you're referring to?
Try turning it on in Fan only mode, then Cool, you should hear a difference.
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Old 09-22-2020, 04:30 AM   #7
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OK, I'll check on this tomorrow.

In case the problem is the "low charge." Does that mean replacement time?
OK first the small copper tube I mentioned earlier is the capillary tube. It is somewhat fragile and vibration can cause it to fail over time. If this is the case you’ll see some minor corrosion on it - it’s obvious.

To answer your question, yes they can be fixed but you have to find a qualified AC repair person willing to do it. Basically you need a friend that does AC because, as I mentioned before, the HVAC trade isn’t interested in fixing these units. The units are sealed with no ports to add refrigerant, so they have to first fix the leak, and then solder in two Schrader valve fittings (or install piercing valves), pull a good vacuum, and then recharge the unit.

It’s not that difficult, and can be done in a couple hours or less, but finding an AC contractor to take time out from condemning and replacing residential or comercial units (a very high profit endeavor) to climb on your RV roof and do a little technical work for a couple hundred $ is the hard part.

Doing a quick search, I found this thread where the author had one unit fail due to a leak (capillary tube failure) and another not working because the compressor was not starting. https://www.rv.net/forum/index.cfm/f...print/true.cfm . Read through it and you’ll have a pretty good understanding of both issues.

I replaced a 2 year old Penguin unit on an earlier coach in Memphis at an RV park (Agri-Center - good place!) one summer. It failed in Utah (capillary tube) and as we drove through city after city on the way to Memphis, I called every AC repair place I could find, and no one was interested. They were busy doing more profitable work. So somewhere in East Arkansas, I bought a new 15k btu unit at a Camping World for around $500, and when I got to Memphis and was set up in the park, did the swap and was cool once more. If it had failed while I was near home, I would have repaired it. The DIY or AC friend routes are the only cost effective and logistically feasible ways to fix these, unless you can find an otherwise unoccupied AC tech interested in a small job.
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Old 09-22-2020, 04:57 AM   #8
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as mentioned if you are low on coolant for the unit trying to find someone to work on the unit might not be that difficult. If you you find a place near you that works on commercial refer units for box trucks and trailer's like ( thermo king ) they should be able to re-charge it. My dad had it done on his unit years ago not sure what he paid to have it done as he's passed on now but i'm sure it was cheaper then replacing the unit. good luck keep us posted on the out come....
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Old 09-22-2020, 08:05 AM   #9
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as mentioned if you are low on coolant for the unit trying to find someone to work on the unit might not be that difficult. If you you find a place near you that works on commercial refer units for box trucks and trailer's like ( thermo king ) they should be able to re-charge it. My dad had it done on his unit years ago not sure what he paid to have it done as he's passed on now but i'm sure it was cheaper then replacing the unit. good luck keep us posted on the out come....
Agreed. Additionally you can find someone who fixes bar refrigeration equipment. They repair units identical to these all the time.
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