Quieting the Dometic Penguin A/C

[Ray Kimsey]

We have just returned from our month long trip out West and was beginning to put together a list of what I wanted to order to replace our Dometic  Penguin II A/C with the Dometic Freshjet (see following article:)

I pulled down the interior plastic air distribution panel to get a picture of the exact model number and verify that mine could be replaced before I put together a list to order. My son-in-law and I were going to do it. What I discovered was some flexible ducting that the jetstream of air was being routed through that had to be vibrating, rattling, creating all sorts of air disturbance. So I decided before ordering maybe it would be worth it to try and see if I could do anything to make a difference.  The worst I could do would be to break something, then I would have another excuse to order the Freshjet. 

Anyway, I left the tubing in place and lined it with styrofoam blocks I cut out on my table saw and band saw (they work so much better than trying to cut with a box knife).  My goal was to seal any cracks and make the path for the air to be as straight and clear as possible to prevent air turbulence.  I included pictures of before and after below.

So the results are not as great as what I expect you would get from the Freshjet, Houghton, or Truma, but I was surprised what a difference it made. Now I am at least going to wait another year and see how this works out. I forgot to get a picture of my phone of the sound before, but it was averaging between 74&75 dB. Afterwards under the same conditions (vent openings the same, phone in same location and orientation, and no background noise) the sound level had dropped to 67 dB which you could easily tell an audible difference. I am not sure how my phone app would compare to a real dB meter, but I suspect the 7 dB drop is not that far off. The other thing that was eliminated was the vibration the A/C seemed to have. 

If you have questions, feel free to ask, and if you have other suggestions, please do so. I am not a sound engineer and I started this just playing around so I am very open to other suggestions.

 

Edited October 20, 2023 by Ray Kimsey
typo

[Jim and Frances]

Hmmm....I will be looking into this for sure!  Thanks for the info.  👍

[John E Davies]

Very cool. FYI moving air doesn’t like squared corners at all. At the downstream side especially you get low pressure and turbulence, which decreases the flow rate and makes extra noise.

Try putting nice smooth curves wherever possible, for example use a 1” or larger router bit, followed by hand sanding. Tape all seams and openings. 

Any reduction in area in a duct will reduce the airflow cfm, so it is important to have the shape be as aerodynamic as possible.

John Davies

Spokane WA

[Ray Kimsey]

18 minutes ago, John E Davies said:

Very cool. FYI moving air doesn’t like squared corners at all. At the downstream side especially you get low pressure and turbulence, which decreases the flow rate and makes extra noise.

Try putting nice smooth curves wherever possible, for example use a 1” or larger router bit, followed by hand sanding. Tape all seams and openings. 

Any reduction in area in a duct will reduce the airflow cfm, so it is important to have the shape be as aerodynamic as possible.

John Davies

Spokane WA

Will do, I really like the idea about the rounded corners with the router bit.  I started this yesterday not really expecting it would amount to any significant change.  Now that it seems to be a possible solution I will probably rework what I did, but I wanted to get some other ideas before I start.  So thank you very much for the tips. I thought closing all cracks and crevices with the tape should help. The varying thickness of the foam board was to come straight down from the sides of the fan opening.

[Coastal Aggie]

this was something i had been thinking about laying in bed a few weeks ago listening to the AC. it really seems like the compressor itself doesnt make an unreasonable amount of noise at least to me with the soft start. the main issue is with the air blasting like a jet engine against the interior panel. my brother has a 3d printer and i have some CAD experience from college so i think im gonna try and design something to smooth out the airflow and hopefully quiet it down some.

[Gliddenwoods]

What holds the styrofoam in place? 

[topgun2]

10 hours ago, Gliddenwoods said:

What holds the styrofoam in place? 

Friction, spray glue, and a little bit of foil tape?

[Ray Kimsey]

21 hours ago, Gliddenwoods said:

What holds the styrofoam in place? 

Currently, for me the friction, foil tape and the grid that you see in the picture. I started this really intending to put in the new Freshjet. When I saw the flexible ducting, then I decided to see if there was a way to make it quieter. I did not actually think it would make as much difference as it did. Now I am thinking, maybe as others toss out ideas it might just work to go this route. I probably will rework it some and improve upon what I did. Or maybe I will go back to the Freshjet option. The improvement is very discernable, but not nearly as quiet as some of the newer A/C models.

[Ray Kimsey]

22 hours ago, Coastal Aggie said:

this was something i had been thinking about laying in bed a few weeks ago listening to the AC. it really seems like the compressor itself doesnt make an unreasonable amount of noise at least to me with the soft start. the main issue is with the air blasting like a jet engine against the interior panel. my brother has a 3d printer and i have some CAD experience from college so i think im gonna try and design something to smooth out the airflow and hopefully quiet it down some.

I would be interested in hearing how this turns out.  

[jd1923]

Good work Ray! 🙂

You'd be surprised what only a little foil tape will do to close off wrong paths, keeping the cold air going where it's supposed to go. I've been into a couple of these, and after being a professional installer back in the day, it depresses me to observe the average installation these days. I spend hours correcting shoddy work on everything I own (now that installs are now more a hobby).

Work ethic is needed, but not found less often today. Can't wait (kidding) to tear apart the A/C on our Oliver someday (1-2 years out). Thinking, did OTT seal the 14x14" standard opening, or does air flow into the area between the fiberglass shells. 🤣 I hope not, but we'll see soon.

I used this tape for prior RV A/C installations and several other projects since (like for the exhaust duct on a Jenn-Aire down-draft gas cooktop). Love this tape, when you need an air-tight seal: amazon.com/3M-Metallized-Flexible-Silver-109-6/dp/B00A7I5Q0E/ref=sr_1_3?crid=1OW71NIMMYN48&keywords=3m+foil+tape+3350&qid=1697936285&sprefix=3m+foil+tape+3350%2Caps%2C176&sr=8-3 

And PLEASE do yourself a favor and do NOT save a few pennies on ANY tape, or most glues for that matter, that is not 3M (Yay! Minnesota Mining & Manufacturing)!

[MobileJoy]

On 10/20/2023 at 9:58 AM, Ray Kimsey said:

the sound level had dropped to 67 dB which you could easily tell an audible difference

Ray, just curious, did this mod seem to quiet the unit outside the trailer?

[Ray Kimsey]

1 hour ago, MobileJoy said:

Ray, just curious, did this mod seem to quiet the unit outside the trailer?

No, I don't think so.  I didn't really test it because mine has never been what I would think as abnormally loud outside. 

[Ray Kimsey]

10 hours ago, jd1923 said:

Good work Ray! 🙂

You'd be surprised what only a little foil tape will do to close off wrong paths, keeping the cold air going where it's supposed to go. I've been into a couple of these, and after being a professional installer back in the day, it depresses me to observe the average installation these days. I spend hours correcting shoddy work on everything I own (now that installs are now more a hobby).

Work ethic is needed, but not found less often today. Can't wait (kidding) to tear apart the A/C on our Oliver someday (1-2 years out). Thinking, did OTT seal the 14x14" standard opening, or does air flow into the area between the fiberglass shells. 🤣 I hope not, but we'll see soon.

I used this tape for prior RV A/C installations and several other projects since (like for the exhaust duct on a Jenn-Aire down-draft gas cooktop). Love this tape, when you need an air-tight seal: amazon.com/3M-Metallized-Flexible-Silver-109-6/dp/B00A7I5Q0E/ref=sr_1_3?crid=1OW71NIMMYN48&keywords=3m+foil+tape+3350&qid=1697936285&sprefix=3m+foil+tape+3350%2Caps%2C176&sr=8-3 

And PLEASE do yourself a favor and do NOT save a few pennies on ANY tape, or most glues for that matter, that is not 3M (Yay! Minnesota Mining & Manufacturing)!

The purpose of the flexible ducting that descends down from the fan and connects to the ADB (1st photo) was to keep the cool air from flowing into the area between the shells. I left it in place and just put the styrofoam inside it in the picture.  It would allow me to remove it all and put it back together like it was originally in case this project did not work out. Like I mentioned before, I may redo this now that I think it is worth redoing. It would allow me to make a neater installation if the flexible ducting was removed. 

[topgun2]

2 hours ago, MobileJoy said:

Ray, just curious, did this mod seem to quiet the unit outside the trailer?

I'd have to imagine that it is quieter on the outside too - quieter is quieter - unless you are simply sealing off the "noise" from one area and not the other.  From what Ray is telling us it would seem that the reduced noise level he is sensing is the result of cleaner air flow (i.e. not the compressor and/or associated parts).  Can't wait to hear more!

Bil

[Steve and Cindy]

I  finally got around to giving this mod a try today.  I took care to cut and shape the foam to smooth out the airflow as much as possible.  I found that the foam cut very well with a tabletop bandsaw, and was shaped into rounded corners easily with a wood rasp.  I applied a generous covering of silver metallic tape to hold it all in place.  Unfortunately, I didn't get the reduction in noise.  74dB on low and 84dB on cool after the mod, same as before, using my iPhone.  I think it did eliminate a little bit of crackling that was caused by the flex duct vibrating.  Maybe it had to do with the thickness of the foam, where mine was generally thinner that what is shown in this post.   I took advantage of the opportunity to seal up the edges of the return airway as much as possible since it was fully exposed to the rough cut through the hull, and all the dust that comes with that.  Overall it was a good experience and I don't regret the time spent, even though I didn't get the hoped-for noise reduction.   It was worthwhile to clean up the return air path.  

[Ronbrink]

10 hours ago, Steve and Cindy said:

I took advantage of the opportunity to seal up the edges of the return airway as much as possible since it was fully exposed to the rough cut through the hull, and all the dust that comes with that.

This is a very good point! I did the same with a recent Atmos 4.4 install when replacing the DPll, the extra precaution taken with almost double the airflow of the new unit.