Furnace return plenum "bangs" when blower motor starts/stops

The sheet metal return plenum on one of our 2 otherwise identical gas furnaces gives out a loud "bang" as the duct contracts inwards when the blower starts and again when the duct returns to its original conformation when the blower stops.

The other furnace that doesn't "bang" seems to have a relief "X" on its broad surface while the one that does "bang" doesn't have the "X".

The return plenum is 24"x8"x60".

I would prefer not to reduce the blower speed since the furnace is in the basement and needs to provide forced air all the way to the 2nd and

3rd floors and the ductwork is highly non-optimal (the previous owner had some hack do it with a maze of flex duct -- one day I will have it replaced but not now...)

I have heard about people using "banding" to stiffen up the plenum and reduce the movement but am not sure of the best way.

So...

  1. What is the recommended way to eliminate such movement and "banging"?

  1. If some type of banding is appropriate, what are the appropriate materials and techniques to do it in a professional and neat manner?

THANKS!!!!

Reply to
blueman
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Well there you go. Those Xs are there for a reason. Get the installer back to do the job right. Otherwise, you'll have to add some external stiffening.

Reply to
Ed P

Go to lowes and get a small piece of angled iron or aluminum. Can be

1/2" x 1/2". Cut to fit diagonally, drill holes, and put some screws into the sheet metal.
Reply to
jamesgangnc

Maybe returns and vents are to small

Reply to
ransley

Wish I could but it was installed 10 years ago - under the previous owner. External stiffening is probably the simplest solution though I could use some pointers on specifics...

(ultimately, I will need to redo all the ductwork to replace the flex duct hack)

Reply to
blueman

Probably not enough returns plus constriction in the awful mess of flexduct. But I can't replace it all now for many reasons so I need an interim fix that works.

Reply to
blueman

Do I really need angle iron? Wouldn't that stick out and look awkward (and potentially be dangerous by sticking out that way)

Would using thick enough iron banding work? I was thinking of making one or two bands wrap around the 3 accessible sides.

I imagine all I need to do is overcome the net suction effect of the blower (net of the air flowing through the returns and net of the static bending resistance of the sheet metal plenum).

Reply to
blueman

I think that the poster recommending use of angle iron meant to put it parallel to the plenum over a considerable distance to stiffen it. Or, if you know where the bowing occurs, the angle iron could be cut to length and placed at rightr angles to the plenum to stiffen it at that point. But if the plenum is in the ceiling rafters of the basement, 1/2 inch sticking down shouldn't make any difference unless you basketball team members using your basement.

Reply to
hrhofmann

Cutting a hole for a basement return might do it, I did it.

Reply to
ransley

That may cause problems depending on the proximity to the furnace with the return air duct drawing in combustion air. As for the angle iron idea, this doesn't have to be iron; aluminum may be used. It only needs to be strong enough to stop the sheet metal from flexing.

Reply to
Bob

Since when do furnaces need ducted returns, Ive seen no return ducts many times, just the furnace intake.

Reply to
ransley

In colder cliimates a return plenum is almost always necessary to circulate ari in rooms far from the furnace. Houses you saw must have been 40 - 50+ years old.

Reply to
hrhofmann

The plenum is actually vertically oriented, sitting on the floor adjacent to the furnace which is why I was concerned about having something sticking out that people could bang into (given that the space is a bit cramped there).

The bowing and banging (which I have been told is properly called "oil-canning") occurs very prominently across the middle of the "belly" of the plenum so I'm not sure that angle iron along the edges will help enough since the problem doesn't seem to be along the edges per-se.

I bought some more "ess cleat" last night and it actually seems to be about as stiff as 1/8" flat iron (note "drive cleat" was significantly less stiff which is understandable since it has 2 (at most) layers vs the 3 plus layers for ess cleat). I figure if I need more stiffness I could even interdigitate two pieces of ess cleat to get an even more ridgid band.

I will let you know how this works...

Reply to
blueman

Actually, I think the banging problem became much worse because I recently closed up an "inadvertent" basement return. Specifically, the bottom end of the plenum was open but sitting on the rough basement floor. I had previously used metal tape to try to seal it off but recently did it right with a sheet metal cap. So, in a sense you are right.

But my understanding is that it is not a good idea to do that for several reasons:

  1. It draws in the cooler (unheated) basement air which needs to be heated and ultimately results in either heating the basement or drawing in more cold air from outside since the air has to be replaced from somewhere.
  2. The negative pressure could draw in more radon (and we have that here in granite-laden New England). This is a concern particularly since our basement floor is 150 years old so it is not fully intact and we also have a crawl space with only a loose fitting vapor barrier covering the raw earth.
  3. If there are any open combustion chambers in furnaces or water heaters, the negative pressure may draw combustion gasses and CO into the basement and from there through the return into the house.
  4. More generally, you will draw in fumes and other potentially undesirable odors, gasses, etc. from the basement.
Reply to
blueman

Im where houses are 200 yrs old, but AC is the only reason you actualy need returns on every floor, sure its probably more even heat with returns but I think his are undersized, even a small 2" vent in the basement would help and circulate heat better, but if he has AC he need a pro to evaluate his ductwork sizing, whith my new unit thats exactly what the installer did and found my returns to small, we added alot more and it help balance temps.

Reply to
ransley

  1. 2. 3. 3. sure if you want a cold unused basement,
Reply to
ransley

If it's about as stiff as 1/8" flat steel, you're wasting your time. Even 1/16"x1/2" angle aluminum will provide way more stiffening than

1/8" flat steel.

1/2" ain't gonna kill ya. Just round off the corners with an angle grinder or belt sander and screw it on, either in an X pattern across the panel that's bowing, or horizontally across the middle of the panel, making sure to overlap the ends.

Reply to
mkirsch1

Doesn't sound like you'd want to use his basement even if it was warm.

Reply to
mkirsch1

One piece of angle stock placed diagonally across the middle where it is oil canning will fix it.

Reply to
jamesgangnc

You don't necessarily need to stiffen the duct. If you can get inside you can wedge a piece of wood between middle of the sides that 'oil can' (pushing them apart).

You can put blocks of wood on the center of the sides that 'oil can' and tie a rope around the duct to push the blocks in. (With a little thought you could the same thing much better.)

Reply to
bud--

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