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Modified my furnace return air design, any sound engineers here?


John E Davies

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I have been really mad at how noisy my furnace is, it drives one of my dogs completely nuts. I plan to do an extensive thread on the changes, but I am not done yet, and I have some return airflow testing to do using HVAC smoke matches.

I used a free sound meter app on my iPhone 13  Mini, so the actual numbers are probably not real accurate. 

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I used these settings:

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I set my iPhone on the street side bed platform with the mic facing the furnace grill, and I put the two fiberglass access covers and the mattress in place over the furnace. (10” thick optional mattress.) I started recording, walked to the thermostat,  started the furnace, let it run for about a minute, then let it shut off automatically before turning off the recording. There were no extraneous outside noises, the test was done inside my RV garage. Here is the initial test:

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Here is the final test:

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Before the mods, the furnace was irritating and screechy. Afterwards, it sounded a little quieter, but more importantly, it was much more even and mellow.

I am nor sure how to interpret the graphs, but it appears that the peak red sound levels (dBa) are reduced, and the peak blue max frequencies are HUGELY affected, the peaks drop from 3000 hz all the way down to 300 hz. (!!!) And the green average sound level goes from a crazy pattern to a mellow even one, way lower…. I think that change is what I am hearing. My dog will like it.

Comments welcome, especially from an engineer.

FYI, if you want to get a feel for how much nicer it can be, find a high density foam pillow and stuff it hard over your return air grill. My mods are quieter…

John Davies

Spokane WA

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SOLD 07/23 "Mouse":  2017 Legacy Elite II Two Beds, Hull Number 218, See my HOW TO threads: https://olivertraveltrailers.com/topic/john-e-davies-how-to-threads-and-tech-articles-links/

Tow Vehicle: 2013 Land Cruiser 200, 32” LT tires, airbags, Safari snorkel, Maggiolina Grand Tour 360 Carbon RTT.

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A few pics

John Davies

Spokane WA

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SOLD 07/23 "Mouse":  2017 Legacy Elite II Two Beds, Hull Number 218, See my HOW TO threads: https://olivertraveltrailers.com/topic/john-e-davies-how-to-threads-and-tech-articles-links/

Tow Vehicle: 2013 Land Cruiser 200, 32” LT tires, airbags, Safari snorkel, Maggiolina Grand Tour 360 Carbon RTT.

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14 hours ago, John E Davies said:

I am nor sure how to interpret the graphs, but it appears that the peak red sound levels (dBa) are reduced, and the peak blue max frequencies are HUGELY affected, the peaks drop from 3000 hz all the way down to 300 hz. (!!!) And the green average sound level goes from a crazy pattern to a mellow even one, way lower…. I think that change is what I am hearing. My dog will like it.

John,

For sure this is a huge difference, especially in the area of “human” hearing. I am not a sound engineer but have been in communications (wireless and wired) most of my life and read graphs like this on a regular basis. 

The scale, db (A), is what is called “A” weighted used to match human hearing. We typically have a perfect hearing spectrum of 20-20,000 hz (perfect hearing), but it is not LINEAR. In other words, when you listen to music at a low volume, the bass and treble are perceived to be less and the ear focuses on the voice range. That is why some sound systems have a “Loudness” curve to boost bass and treble frequencies under low volume but curve them down as you raise the volume.  Just a little background.

By smoothing out the curve, you have removed many of the sounds in the normal speaking range, which could interfere with you processing voice and actually irritate you. You have smoothed out the lows so they will not be as disruptive. All that will be good, especially for you (humans).

Dogs have a different hearing curve and they mainly don’t like the irritants in the voice and especially really high frequency (e.g. dog whistle) which you don’t seem to have.

Did you do anything to the actual fan motor in the furnace?  Like lube the bearing and such?  Just curious.

Very nice work. I have sound panels all over my radio room and my home theater/listening room and might just follow suit on those lower compartments.  I will be getting the Truma and it is VERY quiet but I never liked the location of the air return with NO FILTER.  I got the strangest looks down at Oliver when I asked why the air return didn’t have a filter…mine will!

 

Cheers,

Brian

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Brian, I did nothing to the furnace before the tests. I did open the inside service panel for a look at the flame pattern afterwards.

Thanks for the analysis, what do you think of those acoustic panels? I chose that type because they are just 1/2” thick, were very cheap and got good reviews. I only used 5 complete panels, the adhesive sticks very well but I taped the edges for a little extra long term security and for maybe a little extra shielding. That big raw wall was one big sounding board before applying the panels.

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There is still some fan noise coming out the heat register closest to the fan - shutting it helps, but that is sort of counter productive.

I think that one big blue 3000 hz spike in the first graph shows the ignitor “snapping” - it will be nice to have that much more muffled at night for every time the furnace relights.

Thanks again for the helpful comments.

John Davies

Spokane WA

  • Like 3

SOLD 07/23 "Mouse":  2017 Legacy Elite II Two Beds, Hull Number 218, See my HOW TO threads: https://olivertraveltrailers.com/topic/john-e-davies-how-to-threads-and-tech-articles-links/

Tow Vehicle: 2013 Land Cruiser 200, 32” LT tires, airbags, Safari snorkel, Maggiolina Grand Tour 360 Carbon RTT.

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3 hours ago, Brian and Brandelyne said:

I got the strangest looks down at Oliver when I asked why the air return didn’t have a filter…mine will!

Please post when you install your return air filter.  I considered a home-made filter mounted on the inside of the return air grill, but opted instead to tape filter fabric over the intake housing on our Truma Varioheat furnace.  I did this thinking that, given the distance from the intake housing to the return air vent, much of the air pulled in to the intake housing comes from the open area around the furnace, not just from the return air vent.  I figured that filter fabric taped to the intake housing would be more effective at filtering all of the air sucked into the furnace.

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I will be keenly interested in your filter design.  I welcome any improvements over my crude installation.

 

Hull #1291

Central Idaho

2022 Elite II

Tow Vehicle:  2019 Tundra Double Cab 4x4, 5.7L with tow package

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6 minutes ago, Rivernerd said:

opted instead to tape filter fabric over the intake housing on our Truma Varioheat furnace.

That is a great idea, I think I will do both!  My rationale, with that large vent opening and no intake filter, it will suck in lint, pet hair, human hair, even insects could get sucked in there and then, right into the Truma opening.

It just doesn’t seem like it should all be wide open like that.

I will certainly detail what I come up with but I do like your solution as a first step!

Great idea.

Brian

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I purchased my furnace filter fabric from Amazon:

https://smile.amazon.com/gp/product/B077SP23FP/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

The 4-square-foot piece is more than I will ever need, but it cost less than seven bucks. 

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Hull #1291

Central Idaho

2022 Elite II

Tow Vehicle:  2019 Tundra Double Cab 4x4, 5.7L with tow package

ARCOIDNMOKORTNTXUTsm.jpg

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3 hours ago, John E Davies said:

what do you think of those acoustic panels?

@John E Davies I really like those panels. I will be looking into them for sure. I use a company called Felt-Right for some of my applications and am thinking about putting a nice design on the ceiling of my Oliver to cut down on some of the reflective sounds inside. 

I think the ones you chose are more appropriate for the application you used them for. 

The reason I asked about the fan motor was I had an air handler in my HVAC system years ago that had a squeak in the fan motor. You could hear it at every vent in the house. I opened the unit up and lubed the bearings with some spray white lithium grease and it was totally silent again!

Regards,

Brian

 

 

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