Chamberman
Senior Member
I really dislike how noisy the stock Dometic 11k is that came in our 2019 E19 and I've thought about installing the Houghton 9500 quite a bit over the last year since we bought the trailer used. However, I'm one of those people that hate to throw away anything that I've paid good money for that is working and this Dometic cools really well, its just really noisy while doing its job.
So along those lines, I've recently begun to work on quieting the original Dometic 11K AC that came on our E19. I had plans to document the before/after results with a dB meter. However Murphy and his law joined me and my generator didn't want to start when I got out to the trailer so I couldn't run my before & after tests of the initial work. It turns out the ethanol blend fuel really tore my carb up with corrosion. The stabilizer didn't help with this at all. Oh well, that's another thread in and of itself.
I started my Dometic work by following in Rotorbudd's footsteps with the excellent work he documented in this thread. I installed the same durometer 50 Sorbothane bushings from Isolateit.com. This helped to reduce the vibration and drone of the compressor substantially (I wish I had a SPL test to tell you how much). I also tried installing high density self-adhesive damping sheets inside of the lower plenum and this didn't make much of an audible difference IMO. A few weeks later I had the trailer at home and was running the Dometic to get a "seat of the pants" idea of how much of a noise reduction I was getting. The improvement was substantial due to the lack of low frequency compressor rumble that the Sorbothane bushings helped to minimize, but there was still a good bit of low level whistling and a white noise type of hissing sound that was audible. Further investigation led me to the down facing vents and the two side facing vents that I keep closed. Just putting my hand over these caused an audible drop in this hissing white noise. I took a SPL test and was seeing 75dB with the fan on high speed. The plenum has so much air pressure in it with the blower on HIGH speed that any gap or crack in the cover will hiss.
I got out my blue painters tape and went to work on every vent and gap that I didn't want air leaking out of. For the two side vents that I keep closed, I installed some wedges of foam into the vent hole and closed the vent door on them. I then taped across the lower portion of the side vent doors. I included a pic of what it looks like now with the painters tape on it. Yeah I know its not pretty, but its only a test to see if there's a substantial enough improvement to apply a permanent fix. I could immediately tell that there was less sound being created. I ran the same SPL test with the fan on high speed and it was now showing 72dB after isolating off all of the air leaks. So I dropped -3dB off of the noise levels by doing this mod. The white noise type of sound is now almost completely gone.
What I plan to do is to remove the lower plenum and seal the gaps up in the vents that face downwards from the inside so its not visible, they're not needed in this application IMO. For the side facing vents I'll install a dam of high density foam inside of the plenum that'll keep those two side vents from receiving any air pressure from the plenum.
The real test of the total improvement came when my wife came inside for the 1st time since my corrective actions on the Dometic. She was sitting at the dinette and I was laying sideways in the bed at the rear of our 19 with my head propped up directly under the rear window. I was laying there staring at the Dometic lower plenum and theorizing how to fix it. We talked for a couple of minutes and then she suddenly said "You know what, it is a lot quieter in here! We're talking in our normal speaking voices and I can hear you fine from the back of the trailer. Before we always had to basically yell at each other if we were on opposite ends of the trailer when the AC was running!". Yes that's right we can have a normal conversation now and the AC noise while still present in the trailer isn't nearly as intrusive as it was. Even sitting across from each other at the dinette and talking prior to the repairs required raising your voice slightly. Now that is completely corrected.
I believe that there's substantially more noise that could be removed by redesigning the lower plenum. Specifically adding an aerodynamic wedge type baffle to redirect the air front to rear would likely help versus the brute force method that Dometic chose to employ which is a flat bracket that sticks out into the airstream from the blower to help split up the airflow. However my desire was just to tame it to the point that it becomes an acceptable component that doesn't detract from our RV'ing experience and its definitely reached that level now.
I'll post another image of the repairs that I perform to the inside of the lower plenum once I finally have time to do them.
I must say that for the little bit of money that I have in parts, mostly for the bushings which are expensive for fancy rubber washers, this was completely worth doing. If you've been thinking about replacing your old noisy Dometic 11k then you should look into doing the isolation washer mod in the thread I linked to earlier and silencing the lower plenum air leaks as I outlined in this thread. Once you're done you may decide to hang onto it for a little while longer. Its not as quiet as a Houghton or a residential Mini-Split, but its far more acceptable once the mods are performed.
So along those lines, I've recently begun to work on quieting the original Dometic 11K AC that came on our E19. I had plans to document the before/after results with a dB meter. However Murphy and his law joined me and my generator didn't want to start when I got out to the trailer so I couldn't run my before & after tests of the initial work. It turns out the ethanol blend fuel really tore my carb up with corrosion. The stabilizer didn't help with this at all. Oh well, that's another thread in and of itself.
I started my Dometic work by following in Rotorbudd's footsteps with the excellent work he documented in this thread. I installed the same durometer 50 Sorbothane bushings from Isolateit.com. This helped to reduce the vibration and drone of the compressor substantially (I wish I had a SPL test to tell you how much). I also tried installing high density self-adhesive damping sheets inside of the lower plenum and this didn't make much of an audible difference IMO. A few weeks later I had the trailer at home and was running the Dometic to get a "seat of the pants" idea of how much of a noise reduction I was getting. The improvement was substantial due to the lack of low frequency compressor rumble that the Sorbothane bushings helped to minimize, but there was still a good bit of low level whistling and a white noise type of hissing sound that was audible. Further investigation led me to the down facing vents and the two side facing vents that I keep closed. Just putting my hand over these caused an audible drop in this hissing white noise. I took a SPL test and was seeing 75dB with the fan on high speed. The plenum has so much air pressure in it with the blower on HIGH speed that any gap or crack in the cover will hiss.
I got out my blue painters tape and went to work on every vent and gap that I didn't want air leaking out of. For the two side vents that I keep closed, I installed some wedges of foam into the vent hole and closed the vent door on them. I then taped across the lower portion of the side vent doors. I included a pic of what it looks like now with the painters tape on it. Yeah I know its not pretty, but its only a test to see if there's a substantial enough improvement to apply a permanent fix. I could immediately tell that there was less sound being created. I ran the same SPL test with the fan on high speed and it was now showing 72dB after isolating off all of the air leaks. So I dropped -3dB off of the noise levels by doing this mod. The white noise type of sound is now almost completely gone.
What I plan to do is to remove the lower plenum and seal the gaps up in the vents that face downwards from the inside so its not visible, they're not needed in this application IMO. For the side facing vents I'll install a dam of high density foam inside of the plenum that'll keep those two side vents from receiving any air pressure from the plenum.
The real test of the total improvement came when my wife came inside for the 1st time since my corrective actions on the Dometic. She was sitting at the dinette and I was laying sideways in the bed at the rear of our 19 with my head propped up directly under the rear window. I was laying there staring at the Dometic lower plenum and theorizing how to fix it. We talked for a couple of minutes and then she suddenly said "You know what, it is a lot quieter in here! We're talking in our normal speaking voices and I can hear you fine from the back of the trailer. Before we always had to basically yell at each other if we were on opposite ends of the trailer when the AC was running!". Yes that's right we can have a normal conversation now and the AC noise while still present in the trailer isn't nearly as intrusive as it was. Even sitting across from each other at the dinette and talking prior to the repairs required raising your voice slightly. Now that is completely corrected.
I believe that there's substantially more noise that could be removed by redesigning the lower plenum. Specifically adding an aerodynamic wedge type baffle to redirect the air front to rear would likely help versus the brute force method that Dometic chose to employ which is a flat bracket that sticks out into the airstream from the blower to help split up the airflow. However my desire was just to tame it to the point that it becomes an acceptable component that doesn't detract from our RV'ing experience and its definitely reached that level now.
I'll post another image of the repairs that I perform to the inside of the lower plenum once I finally have time to do them.
I must say that for the little bit of money that I have in parts, mostly for the bushings which are expensive for fancy rubber washers, this was completely worth doing. If you've been thinking about replacing your old noisy Dometic 11k then you should look into doing the isolation washer mod in the thread I linked to earlier and silencing the lower plenum air leaks as I outlined in this thread. Once you're done you may decide to hang onto it for a little while longer. Its not as quiet as a Houghton or a residential Mini-Split, but its far more acceptable once the mods are performed.
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My next project.
