ransley writes:

On Jan 21, 12:42Â*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!!!!

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

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.

3. More generally, you will draw in fumes and other potentially
undesirable odors, gasses, etc. from the basement.