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Help! Quieting a return grill register

pizza's picture

Hi. I recently bought a larger return grill to try and solve a sound issue with my existing air conditioning return grille. The one they installed with the air conditioning system made a lot of noise like a wind whistling out side the window in the winter. I think they sized their return grill too small so I bought a 12 x 12 grill today to see if the larger openings would solve this problem.

 

When I removed the return grill they installed with the system I discovered that some of the slot openings were actually blocked by the drywall (which was painted black) and I calculated that it reduced the throat area of the grill by 17 percent.

 

As close as I can determine and measure, I think the openings in the new 12x12 grill total up to around 118 square inches of total air way passage. The return ducting behind the grill is 12" diameter which calculates to around 113 square inches of opening or throat.

 

They originally  placed a grille that measures overall 9.75" x 17.75" over a hole in the wall that measures 9" x 13" . And its' very noisy when the air is being sucked back to the a/c unit. As soon as I remove the grill entirely it, of course, quiets down immensely.

 

So I figured if I could get a return grill that had the same throat as the ducting, the noise should quiet down. Does anyone think that if I open the hole in the wall to accept this new grill that the noise should then be reduced. It also sounds like a tuning fork at times when it hits its resonating frequency and its very annoying to try and watch TV with that going on. This register is in my living room.

 

Also I have another return grill up stairs, which doesn't make as much noise. Appreciate any suggestions. Thanks in advance. One last question, do you think the angle of the bent sheet metal fins in the grill is a large factor?

(post #112026, reply #1 of 10)

Before you start cutting the wall open, just place the new grill you bought over the existing hole with some kind of temporary fastening--even gobs of 'Fun-Tak' stuck on the back--then see if that solves your problem.


If it does not, it may be the fins. Try to find a finless model, the kind with a flat lattice-work profile like the top of a cherry pie. That will certainly improve air-flow, and the lattice bars being flat to the flow will not be likely to start vibrating like a blade of grass held between your thumbs....


 


 


Dinosaur


A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...




But it is not this day.


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #112026, reply #2 of 10)

I did that and it didn't improve it. It is also blocked by the wall at many of the slots. The noise was worse. Actually, after measuring the openings in the new grill, the total open area calculates out to 73 sq.inches, which is 9 sq. inches less than the old grill.


They oughta have a fact sheet somewhere that specifies the proper grill size based on the return duct's diameter and flow speed to prevent such noise issues. Or it should be written on the labeling on the return grill's packaging.


I've figured that I need at least a 14 x 20 inch grill which has about 130 (give or take) sq. inches of total slot opening area.

(post #112026, reply #3 of 10)

Okay, given what you just learned, we can advance the theory that the noise is caused by air being venturied into the restricted space under the grill opening where it overlaps the gyprock wall.


So in effect, what you need is a smaller grill, one that has a grill opening exactly the same size as the duct, and--this is important--that also has a collar flange that slips into the gyprock and then into the duct. That way, there could not be any air sucked into that narrow space between the grill and the gyprock around the edges of the duct opening.


 


 


Dinosaur


A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...




But it is not this day.


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #112026, reply #4 of 10)

Yup, that's what I figured. BUT I can't find one like that. To get such a grill into the hole in the gypsym would mean one that has a small footprint with large holes adding up to equal the duct size. As far as I can tell no such animal exists in such a small grill size. And if it did that wouldn't look so good with the large holes in the small grill up on my living room wall. My other option is to do a combination of larger grill size and getting them to lower the airflow (but the lower flow may mean a compromise onn the overall cooling performance). Jeez, it's never easy is it?

(post #112026, reply #5 of 10)

Based on your description, I'm not sure I'm visualizing this correctly now. Can you post a photo of the grill, and the hole & duct without the grill in place?


The type of grill I was referring to in the last post looks just like a regular stamped grill from the outside; its edges overlap the hole just like the one you took off. But on the back side of the grill is a sheet-metal collar that is tack welded to the back face of the register grill. This collar has to be the same size as the duct. The end of the collar is usually pleated so it actually slides into the duct about 1½".


 In essence, what this collar does is extend the duct itself right up to the back face of the register grill. So no air can start whistling as it gets sucked into and out of narrow crevices in the gyprock, or between it and something else....


If you can't find a commercially manufacturered register grill with an insert collar, buy a piece of galvanized duct the same size and shape as what's in your wall, trim it to the appropriate length plus 1½" (for inserting), braze it onto the back of a grill register you like, and pleat the end of the collar so it'll slip into the duct. Tape it all up nice and airtight between the register and the collar with metal duct tape, and install.


To test the theory before going to all that trouble, make a mock-up with heavy cardboard and duct tape using the register grill you have now.


 


 


Dinosaur


A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...




But it is not this day.


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #112026, reply #6 of 10)

I will post a picture tonight. I'm at work now. Thanks.

(post #112026, reply #7 of 10)

What is the noise level when the there is no grille on the opening?

(post #112026, reply #10 of 10)

Initially, following the central air installation, I measured the noise level with a Radio Shack sound meter (which unfortunately only measures down to 50 db). It was hovering at around 65-70 db, 1 foot away from the grill. After they came back and increased the duct size from the orig. 8" dia to 12" diameter,the sound level decreased to 50-55 db. They failed to replace the grill and kept the original one (which baffles me). Contractor claims he forgot (the same guy who claimed to have experience installing these systems in hundreds of my style of house). On the Hart and Cooley website (a manufacturer of return grills and registers), their engineering fact sheet says that 30 db should be the goal for residences. I need to borrow a meter soemwhere that can measure down to that level once they come back to fix this grill this week (hopefully for the last time).

(post #112026, reply #9 of 10)

Have a return duct right next to my computer, quieted that thing by a perceptive 20 dB, (measued as over 20 dB simply by comparing voltage levels on scope from a mirophone) by installing a "muffler". The muffler was simply a bunch of thin wall 100 psi 3/4 inch pvc tubing cut to 11 inches long (ran the calcs for length, dia, and holes on some acoustic equations from a textbook) and about 8ea  1/4 dia holes in the sides at 1-1/2 inch intervals approx. Put a bundle of them together to fit the register hole, did wonders.


Put onto the idea by an engineer who does jet enine noise reduction for a living.

(post #112026, reply #8 of 10)

Pizza, I have seen my HVAC guy bend the louvers with a pair of pliers to reduce whistling and vibration. Less resistance if the louver is more parralel to the air flow. Just a thought. Jay