DIYbanter - View Single Post - Furnace return plenum "bangs" when blower motor starts/stops

blueman

"hr(bob) " writes:

On Jan 21, 12:30Â*pm, blueman wrote:
jamesgangnc writes:
On Jan 21, 1:42Â*am, blueman wrote:
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"?

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

THANKS!!!!

Go to lowes and get a small piece of angled iron or aluminum. Â*Can be
1/2" x 1/2". Cut to fit diagonally, drill holes, and put some screws
into the sheet metal.

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).- Hide quoted text -

- Show quoted text -

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.

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...