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Mod #42: Soundproofing a Noisy Furnace

Posted By Mark Corgan On July 19, 2008 @ 7:33 am In featured, heating | 12 Comments

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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