Hush hush. Quieting down the furnace

[Jammer]

Well, folks, I like to have my traylah be a place of Peace and Tranquility.

The addition of the pic-a-watt electric heat has raised the overall level of Peace and Tranquility on those cool nights when the furnace would otherwise required. Particularly for sleeping on the Gaucho or dinette where the noise from the furnace is truly Motel Worthy. Who among us has not had their slumber disturbed at some point by unitized motel heat pumps that even a jet aircraft mechanic would find loud?

I have been listening to my furnace. It seems to me that most of the blower noise issues forth from the air return opening under the sink, and not from the hot air outlets.

I am thinking of applying some 3/4" thick sound-deadening foam to the return air area, and assume that somewhere in the broad lands of the Airstream Forums, someone has already done this.

So, how did it turn out? Any suggestions?

[Jammer]

I did some poking around this morning and did some LISTENING.

There are three sources of noise.

1) Fan and air movement noise from the return air.
2) Transmitted rumble and vibration due to the rigid mounting of the furnace to the floor.
3) Air movement and burner noise from the hot air outlets.

To quiet down the furnace all three of these will have to be addressed. I'm going to start with the return air because it's loudest and easiest to fix.

The furnace return air is drawn in directly by the blower input with no baffling, filtration, or enclosure in the furnace itself. Therefore, a good deal of noise is generated. By using sound-absorbing materials in the air path from the trailer interior to the blower, it should be possible to attenuate the noise considerably.

Return air is drawn from two sources: through a baffle from the kick space under the undersink cupboard, and from the grills above and below the microwave. Most of the noise comes from the kick space. I believe that by applying self-adhesive acoustic form to the baffle system and the cabinetry casework, the return air noise can be reduced considerably.

The transmitted vibration seems to come mainly through the floor mount. Airstream builds a little plywood shelf, which is rigidly attached to the floor. Then, the furnace is attached to the shelf using semi-rigid sheet metal brackets. I believe the most straightforward way to control the vibration is to allow the shelf to float from the floor using vibration isolators:

Vibration Mounts eStore Cylindrical Rubber Anti Vibration Mounts Rubber Neoprene Urethane Sorbothane Silicone Gel Ring Mounts

There's plenty of clearance and it may even be possible to change the way the shelf is mounted without removing the furnace.

That leaves the hot air supply. The ducted supplies leading to the aft portion of the trailer do not pose a problem because the flexible duct attenuates the noise sufficiently. I'm thinking that it might be possible to design a replacement magazine rack that incorporates a baffle box that covers the duct outlets and is lined with sound-absorbing material.

[Jammer]

I bought some foam and plan to install it this coming weekend. Unfortunately I do not have any kind of noise meter so I'll only be able to share subjective results.

[Jammer]

Awkward place to take pictures since the cabinets weren't disassembled for installation so words will have to Do.

I lined much of the cabinet area surrounding the furnace with 1/2" and 3/4" acoustic foam. The foam I used has a self-adhesive backing for ease of installation. I did not apply any foam to the furnace itself and was careful to maintain reasonable clearances from parts that get hot.

I used the 1/2" foam where clearance was limited and at the front of the cabinet where I was trying to maintain as much return air space as possible. I removed the microwave and used the 3/4" foam on the outer wall of the trailer and the bottom of the Corian countertop surface to control the furnace fan noise that comes out from the top of the microwave.

The fan vibration/rumble now predominates if listening from the dinette, and the air and fan noise from the heat outlets predominate if listening from the gaucho. Overall it is somewhat quieter, and the character of the noise is less irksome, although the difference is subtle rather than "wow."

I am thinking of boxing in the heat outlets, with the box open towards the floor, and lined with a flame retardant material with good acoustic properties. Remounting the furnace on anti-vibration mounts to deal with the rumble would be a much larger project and I may skip that.

[Jammer]

Well gang having finished the "not enough heat in the bedroom" project (q.v.). I'm now experimenting with a little more noise reduction.

A conclusion of the "not enough heat in the bedroom" project was that only one of the forward-facing registers is necessary (or indeed desirable). This provides a noise reduction opportunity.

I placed 1" thick, flame-retardant acoustic foam on the inside of the cabinet behind the duct outlets and rerouted the register still in use (the other being blocked off right at the furnace, by a piece of tin where the collar was) in a serpentine sort of shape. The photo shows the route of the duct and a sample of foam, only before I put everything inside the cabinet.

This attenuates the air movement noise emanating from the forward register considerably. The only significant source of noise left is now the blower rumble.

Some reduction in airflow accompanies this change and I am retesting the heat distribution to be sure that there is still enough heat being delivered to the front.

[Jammer]

Folks I did get a PM a while back asking me how this all turned out.

The short of it is that I don't find the furnace noise to be objectionable any more. I never did put in vibration isolators under the furnace, but the foam and the S curve in the forward duct have, together, made enough of a difference to make this worthwhile.

[Friday]

I use a sound-deadening material that is used in vehicles... sometimes called 'Acoustimat' or varieties of. It can't go on hot items, but it deadens a lot of vibrations if you stick it on sheet metal...

[dznf0g]

I don't have the written info with me, but IIRC Dynamat (sound deadener) is good up to 210*. Haven gotten out the IR thermo, but I don't think the blower housing and plenum get that hot. May just have to check that out.
It is made for underhood auto applications (some of the materials...keep reading)

[rltaylor]

I've used "Dynamat" for vibration- and sound-deadening the (interior) door areas in my 2002 Jeep Grand Cherokee, for improved audio speaker performance. Pretty effective for improving the music system in my Jeep at highway speeds. But the Dynamat material is very heavy and I don't know whether it would be particularly helpful for the moving air issues of an HVAC duct.

[dznf0g]

There are different Dynamat materials. The one I am talking is more of a dense foam, not the heavy "rubber" door panel mats. The foam is not that heavy and affects different frequencies than the stuff I assume you are talking about. They have different materials for different areas of the car as well. Here's the site:
Dynaliner

To be accurate, it's Dynaliner, by Dynamat.

The spec sheet says up to 200*. I actually used this stuff on the inside of my Honda Gennies cases. Very time consuming and you need 2 thicknesses to keep adequate airflow through the case, but (subjectively) did quiet the gennies a little more than OEM. No melting nor deforming of the Dynaliner.

Looks like the hoodliner is good to 225*