Return plenum "bangs" (contracts/expands) when blower starts/stops

blueman

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!!!!

Jack

Oil canning

is what the condition is called. Usually a braod expanse of sheet metal will have X-shaped breaks in it to prevent this. Go to your local hardware store and get a couple pieces of angle iron. 1/2' should be sufficient. Light stock is okay. All you are trying to do is stiffen the plenum. Self tapping screws into the plenum and you have braced the metal and it should be ok.

geno54

Banging

This "banging" is from the expansion and contraction of the return duct. Undersized duct work for the airflow you have and/or a restriction will cause this. Check your return path from the return grill on in for any restrictions, ie filter plugged or too restrictive. . You may also have some problems on the supply side that could cause this. If needed a good sheet metal company can correct this. Installing angle metal may partially stop the noise but is not stopping the problem.

blueman

Banging

Issue probably is a combination of not enough return supply and constriction in the *mess* of flex duct that some hack put in combined with the lack of the "X" in the return plenum on that furnace.



I know that I need to replace the ductwork with good, insulated rigid duct. When doing that I want to have all the heating calculations redone to make sure returns & supply are appropriate. Also, it pays to synchronize that work with adding AC and a major renovation we are planning. Based on estimates and input from HVAC contractors, it doesn't pay to do this piecemeal.



So, it may be a couple of years before we are ready to get it all redone and meanwhile I just want to make it as efficient as possible with the current layout. And in particular, I need a good solution for the banging...

blueman

Oil canning...

Thanks for supplying the right name.

As I mentioned the "X" is missing from the return plenum on that furnace (but not on the other identical one that was installed at the same time by the previous owner).



Do you think ess or drive cleat stock would be stiff enough?



Also, do you really mean "angle iron" because wouldn't that stick out and be somewhat awkward? It would seem that  one or 2 pieces of flat iron banding (of sufficient thickness) wrapped around 3 sides and anchored with self-tapping screws should be plenty stiff.

Wayne_16

Stop the oil canning

Another option to stop the oil canning is to use a spacer located inside the problem duct. Cut a piece of spacer pipe 8" long,( the width of the duct) conduit pipe, pvc, copper etc. cut threaded rod 9.5" long or longer.  Drill a hole in the duct just big enough for the threaded rod to fit through.  Most of the time, drill this hole in the duct side that is the hardest to get to.  Drilll a matching hole in the opposite duct side to slide the spacer pipe through place the treaded rod through the duct work to the back side start a regular nut on the rod.  Slide the spacer through the larger hole over the threaded rod.  Last piece needed is a 2" square patch with a hole in the middle.  Slide the patch over the threaded rod, install a nut and tighten.  Lastly secure the patch to the duct work with some self tapping screws. 

The duct work is secured from oil canning, and still looks nice from the outside.  This may take a little bit of time to fabricate and install, but will work.  Most of the time, this spacer is installed prior to installing the duct work when needed.  Commercial duct has requirements for spacers once the duct reaches a specific size or larger.

Minnesota Wayne

blueman

All fixe - my solution was aluminum angle on the plenum interior...

After being unsatisfied with the idea of protruding angle iron and realizing that flat stock would not be stiff enough, I came up with the obvious idea of applying aluminum angle to the *inside* of the plenum.



I simple removed one of the duct returns entering the middle of the plenum and then attached two pieces of horizontal aluminum angle (1.5"x1.5"x1/8") one above the center duct and one below. The aluminum angle was easily secured by driving in sheet metal screws from the outside.



Solutions works like a charm and is basically invisible save for the heads of the sheet metal screws.



Note: the positioning of the return ducts did not allow me to use an "X" pattern so I settled for two horizontal bands at the points of maximum deflection.



Thanks all for the ideas and pointers that pushed me in the right direction!