Mod #42: Soundproofing a Noisy Furnace

Mod #42: Soundproofing a Noisy Furnace

Submitted on: 07/19/08

     Category: featured, heating
Mod Rating: 12345

(42 ratings)

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Mod Description:

In a previous mod, we discussed how to supplement your RV heating needs with a catalytic heater in order to save propane used by the forced-air furnace. A side benefit of that mod was a reduction in furnace blower noise since the furnace doesn’t have to cycle as often, if at all. We all know how noisy these furnaces can be, right? What if you have no supplementary heat source? How can you make your noisy furnace a little more tolerable? Simply. Just add some soundproofing!

Mod Difficulty:

If you have a larger RV, it will have a higher output furnace which requires a larger fan to effectively distribute heated air throughout the RV. Even smaller RVs can still suffer from excessive blower noise. And since most RV furnaces are stuck in an enclosure built out of thin luan, they can transmit a lot of blower noise through the thin walls of the enclosure, and the noise can even be amplified by the enclosure the furnace is housed in. Provided nothing is mechanically wrong with your furnace, adding soundproofing material to the inside of the furnace enclosure can provide an order of magnitude reduction in noise output.

This is actually quite easy to do. Start by removing the air return register and inspecting the inside of the furnace enclosure. With a tape measure, measure the dimensions of the top, back, and sides of the enclosure. For example, the top of the enclosure may measure 24″ x 24″. Tha’s 2′ x 2′, or 4 sq. ft. What you are doing is measuring the total square footage of the enclosure interior where you will be placing the soundproofing material. Typically, about 6-8 sq. ft. is required, unless you have a really big enclosure, like the inside of a dinette bottom storage area.

Next, get out your furnace manual. Find out how many square inches of return air area the furnace requires to operate effectively. For example, a 30,000 BTU Suburban furnace requires 54 sq. in. of opening area for the air to flow through the return grill. This size furnace usually will have a grill that has 4 sets of louvers. Only 2 are actually required to support the return air requirement, which in this case is about 60 sq. in.

Why are you doing this? Because part of this mod is also adding some soundproofing material to the back of the register. Much of the blower noise is transmitted through the front of the register, so it makes sense to try and reduce noise here as well. In the example above, you should apply soundproofing material on the back of the register over the middle two of the four louver columns. This is OK to do since you are still meeting the return air requirements of the furnace.

Finally, cut the soundproofing material to fit your measurements of the furnace enclosure and affix to the walls (sides, top, and back) of the enclosure. The material suggested in the parts source link below can be had with a peel-and-stick backing, making the attachment of the material very easy. It costs a little more than the non-sticky stuff, but you won’t have to use any glue if you choose this type.

You can expect a reduction of about 6dB of noise level. How much is this? The human ear perceives sound levels of 3 dB as being twice as loud or quiet. For example, if you measure your furnace noise output, it might be 61 dB at 5′. Reducing the noise level to 58 dB would be perceived as twice as quiet. With this mod, using the 3/4″ material, your furnace could be a much as four times as quiet! Now you won’t have to turn up the TV every time the furnace kicks on!

ModMyRV recommends these parts for this mod:
Mr. Gasket 11027 G-Sport Titan-Lite Sound Dampening Material
Dynamat Dynaliner One 54″ x 32″ sheet 1/4″ thick

General Reference
Wikipedia Article on Sound Pressure

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12 Comments For This Mod

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  1. jmt

    Does this really help? Can you actually hear the difference?

  2. ModMyRV

    Absolutely! I’ve personally “done this mod”. The pictures are from my installation and so are the sound measurements. Each heater enclosure is a little different in every RV so results may vary. The biggest sound loss was made from adding the sound proofing material to the grill. I’m also thinking of adding a finished board the size of the grill mounted about 2″ off the grill, with sound proofing material on the back of it. This will help kill some of the fan noise even more by absorbing and reflecting the sound back toward the source. Give it a try. Pretty low cost mod…

  3. Portagie68

    ANybody ever try and install a filter on these furnaces?

  4. D. J. McBride

    I used the “hammock” material sold at Menards, folded over twice for four layers and several inches larger than the intake grille, then used a bebnt up coat hanger to hold it tight to the furnace. It seems to be catching quite a bit a dust.

  5. Jim-Bob

    Yes, I added a return air filter to my furnace. I would remove the grill and measure the opening. Make sure it fits tight so no dust gets around it. Get a good quality filter from Home Depot or Lowes. Chances are you won’t find the exact size of the opening. If you can’t find one that fits get one slightly larger and cut it down to size. Take a look at the one you are buying and see if it can be easly sized to your opening. I used a hot glue gun an made my own to fit. I am a plumbing and heating contractor, and I would never run a furnace without a return air filter. All of the dust that gets sucked in will eventually coat everything including the heat exchanger in the furnace.

  6. daveinwa

    I also added a filter behind the return air grill. Hard to believe the RV didn’t have one. Dust can cause all types of problems!

  7. MRLCMPS

    Completed this mid last year, and man what a difference! I can now here myself think, and I believe that the furnace performs better as well.

  8. Wayne

    Want material is best to use for the soundproofing material inside the enclosures?

  9. James

    I just completed this mod using Dynamat that I purchased at an automotive parts store, it is used for sound proofing on car restorations and stereo installs.

    My furnace enclosure did not have walls on either side and was open to the hot water tank on one side and the shower base on the other. I used the Dynamat to make the walls.

    I left the backing paper on except for a 1″ strip along all four sides to stick to the wood framing that was there for both side walls. I was careful to make a complete enclosure for both side back and ceiling. I also cut strips for any exposed floor as well as the front wall that the gill attaches to. In other words I covered as much of the compartment as possible, as well as tow of the front grill louvers.

    The result was FANTASTIC!! More than a 50 % reduction in noise to my ears anyway. I highly recommend this mod to anyone that has a furnace that sounds like a jet taking off.

  10. RVRob

    The noise of the furnace is a major problem in my 38 foot Class A - I am woken every time the furnace comes on. After reading about this mod it is definitely something I am going to do. Question: I looked at the Dynamat and it appears to be foil on the side towards the furnace - won’t that just make the sound reflect within the enclosure?

  11. Carl Kenny

    I re-routed the cold air intake to a different, farther away location which wasn’t right in front of the furnace. I used a 4″ hole saw to drill 2 large holes and then a 3″ for 2 smaller holes through the side panel which led into the cupboard next to it. From there I created a new intake from under my drawers and there is also ventilation holes from other places since the sink and many other non air tight cabinets and drawers lead into the new opening. I’d say the noise is 80% less than what it was previously without the risk of soundproofing materials being inside a furnace enclosure. I also added a cold air return filter over the holes in the walls I drilled. This was great stuff with adhesive velcro strips to place where you want and put the material right over it.

  12. Hammer_64

    Great article. I have been thinking about making a modification like this. Working for 28 years as an acoustics expert, I would like to clarify some technical points about dB which are often misunderstood. A 10 dB increase in sound pressure level (SPL) equates to a doubling of the perceived loudness of a noise. A 3 dB increase is associated with a doubling of the sound power. That being said, a 6 dB increase is a significant improvement, nice job!

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