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Old 06-29-2021, 12:51 PM   #1
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Loud rear A/C, Is that a common problem?

I am putting this in Class A because I wonder if it is a construction problem. The question: Are the rear AC's louder in some RV's due to the RV's construction? Ours is and I have now talked to two other people who's are. If so why? I also read a post by someone who found thinner roofing ply in the rear of their RV. Below is background if desired. Our rear AC has worked great for three years, 2 of which are full timing. We use it as the main unit unless we are back in the bedroom. Cold air. Always works.



We have a 2008 National Tropical DP purchased used. Both AC's were the same and probably original. Duo Therm's. I have had both lids off and they appear identical. The rear one made a lot of noise but worked fine. The front one was as quiet as a roof unit gets. The front has since been replaced with the same.



That rear AC noise is from the compressor. It sounds like the vibration comes through the roof.It is loud. I replaced the crush gasket with an aftermarket. I did not make the bolts too tight. Not much change. Eventually I got some carpet samples and put those under the corners. I removed the rubber dampers from under the compressor. I tried to get new but these original ones are larger than the replacements and the new ones don't fit well or damp. They are loose. Matching not available from Dometic or any source I could find. I found rubber washers and shimmed the new type but no improvement. The tubing does not contact the frame and damping it does no good. The fans are not out of balance. I would try the original crush gasket but it is not available and I doubt it would change much.
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Old 06-29-2021, 01:21 PM   #2
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In my old '94 Bounder, the rear was around the same loudness as the front only since it was in a smaller room, it seemed to be louder. What I'd do during heatwaves is run whichever AC wasn't in the same room as I was in. Get up in the morning, LR would be running, switch on the BR and the LV off. Then I had 2X 12" fans that I'd use to direct and stir the cold air.

That scheme would work fine until the late afternoon when I'd need to switch on the LV unit.

Now, I own a Journey with basement air and I can hear that run but it's much quieter then roof mounted. Both rigs were/are diesel pushers, both have dual pane windows but the Journey uses ceiling venting. I like the Journey better in these 115ᵒ F days we are having, and the fact there's little worry of the A/C freezing up from constant use. Basement air just doesn't seem to do that.

As far as the rear A/C being noisier? I wouldn't buy that it is unless you've measured the dBs output by each using a sound level meter at the same distance and it's the same. I think it's because of room size it only seems louder.
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Old 06-29-2021, 02:34 PM   #3
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I would try two new run capacitors.
You may be able to find better isolators than basic rubber mounts. Is it just a bolt and rubber pad or true isolators, threaded hole or studs vulcanized the rubber at both ends. McMaster Carr has many isolators .
The air noise is louder in my bedroom AC because of less vents /more static pressure. I haven't noticed more compressor noise. Maybe the roof is more ridged near rear cap and maybe stronger rear roof frame than more middle of front AC. Even the frame of is piece of motorized equipment like industrial blower will have a resonant frequency if you hit it with a hammer. If the vibrations of blower are near the same frequency due to speed of it, it can bounce resonate and create destructive resonation . Not saying it will self destruct but it might be resonating.
I would try two new run capacitors to see if smooths out if it's really bothering you.
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Old 06-29-2021, 06:55 PM   #4
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Quote:
Originally Posted by 153stars View Post
I would try two new run capacitors.
You may be able to find better isolators than basic rubber mounts. Is it just a bolt and rubber pad or true isolators, threaded hole or studs vulcanized the rubber at both ends. McMaster Carr has many isolators .
The air noise is louder in my bedroom AC because of less vents /more static pressure. I haven't noticed more compressor noise. Maybe the roof is more ridged near rear cap and maybe stronger rear roof frame than more middle of front AC. Even the frame of is piece of motorized equipment like industrial blower will have a resonant frequency if you hit it with a hammer. If the vibrations of blower are near the same frequency due to speed of it, it can bounce resonate and create destructive resonation . Not saying it will self destruct but it might be resonating.
I would try two new run capacitors to see if smooths out if it's really bothering you.

Thanks. Some new stuff to try here. This one has a single run capacitor which will be really easy to check since I have a tester. The isolators that it has are a simple spool shaped rubber pieces with a metal tube down the center. A bolt through the tube holds it together. I found McMaster Carr and I'll see if there is anything to fit. The compressor is definitely the source and it does move around a bit but it does not hit any metal. The old rubbers are out of shape. I'll check that capacitor and let you know.
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Old 06-29-2021, 07:21 PM   #5
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Quote:
Originally Posted by Jim_HiTek View Post
In my old '94 Bounder, the rear was around the same loudness as the front only since it was in a smaller room, it seemed to be louder. What I'd do during heatwaves is run whichever AC wasn't in the same room as I was in. Get up in the morning, LR would be running, switch on the BR and the LV off. Then I had 2X 12" fans that I'd use to direct and stir the cold air.

That scheme would work fine until the late afternoon when I'd need to switch on the LV unit.

Now, I own a Journey with basement air and I can hear that run but it's much quieter then roof mounted. Both rigs were/are diesel pushers, both have dual pane windows but the Journey uses ceiling venting. I like the Journey better in these 115ᵒ F days we are having, and the fact there's little worry of the A/C freezing up from constant use. Basement air just doesn't seem to do that.

As far as the rear A/C being noisier? I wouldn't buy that it is unless you've measured the dBs output by each using a sound level meter at the same distance and it's the same. I think it's because of room size it only seems louder.

We also use the AC that is farthest from us, swapping ends morning and evening. You have given me an idea. I measured sound levels a foot from each and they are within 1 DB. That is because it is the fan that is making the sound. The sound that is disturbing us is a low vibration transmitted through the ceiling that even makes the bathroom fan buzz sometimes. Not something the meter will measure. I will go up top tomorrow and see if they sound different from up there but I am guessing that it is the transmission of the vibration and a difference in damping between the front and rear units. Both are seated on a foam gasket and, for a while, they both had the same gasket as I replaced both. Both are connected to the roof from beneath by a 4 long thin bolts. It has to be damping on the compressor, excessive shaking of the compressor or a difference in roof construction allowing more transmission (thus the question I posed.)
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Old 06-30-2021, 06:14 PM   #6
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I am not sure how your MH is laid out, but on mine the front AC is in the living room area but the rear is in the hallway going into the bedroom.

My AC's are all ducted together but the rear AC mainly feeds the rear bedroom hallway and bathroom which takes a 90 degree duct turn, while the front feeds the 3 vents in the living room and drivers area.

I can turn on either AC and get air at all of the vents. The Rear AC sounds louder, but it is close to both hall walls. The front AC sounds quieter, but is is probably 3 feet from the nearest walls.

Both my AC's are new so I suspect they are close in sound level.
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