Noises (singing and rumbling) at return grille

Old 01-28-10, 09:40 AM
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Noises (singing and rumbling) at return grille

Background info:

We recently had a Goodman Rep out to correct some problems we were having with our dual fuel HVAC system. Our system was installed three years ago. We had all new duct work put in back then, together with these Goodman components:

Indoor coil is Model CHPF3642C6CA; serial 0907129414
Furnace is Model GMV950704CXA; serial 0601773716
Condenser is Model GSH130421; serial 0605212296

A General Model AC-3 air cleaner and activated carbon air purifier is also part of the system, on the furnace side of the system.

The return duct from the hall back to the system in the crawlspace is 18” flex duct. The main supply duct is 16” hard pipe. There are 10 4x12 and two 6x12 supply registers in the floors. The supply ducts to the registers are 6" flex, except for the two larger registers, which have 8" flex ducts.

Our house is a single story, brick rancher (1964) built over a crawlspace. We are in northern Alabama.

The Noises

While here, the Rep checked and corrected the DIP switch settings on the main circuit board in the gas furnace.

He changed the settings to: 1-on, 2-on, 3-off, 4-off, 5-on, 6-on, 7-on, 8-off.

The main effect of these changes that I notice is that the blower is now pushing/pulling a lot more air through the system, especially in heat pump heat mode. This is causing two kinds of noise at the return in the hall: the grille ‘sings’ and there is a rumbling sound in the plenum behind the grille.

The Grille

The grille is of the stamped, cheap variety. I turned the grille upside down. That reduced the singing a little, but it’s still audible in nearby rooms. The grille is an odd size (22” by 26”) so I haven’t been able to find a replacement, ‘high volume’ grille.

  • Any suggestions about what do do about the grille?
  • Any vendors I might try?

The Choke Point

The rumbling sound is coming from a “choke point” in the return plenum, where the plenum goes through the floor. This is the only old part of the duct system. The opening through the floor is rectangular (about 20" x 15").

There is obvious potential for turbulence and resonance here since air entering the grill has to change direction and encounter the rectangular opening before it can get down into the larger, box-shaped part of the plenum under the floor - and finally enter the 18" flex return to the system on the opposite side of the box.

The original installer probably should have enlarged the opening through the floor. Instead, when we complained about the noises, he set the blower to a lower setting.

I’m not sure whether I should try to get the choke point enlarged.

  • Are the rumbling and the choke point likely to cause maintenance or reliability problems with the system?
  • Are they likely to be causing any reduction in the system's performance?
Old 01-28-10, 07:20 PM
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Remove the grille & measure the cut-out. Some manufacturers measure their gilles by cut out size, some by over all size, & others somewhere in between. Filter grilles are measured by the size filter they take.
If possible, enlarge the cut out to fit a larger grille.

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