HVAC Makes Loud Sucking Noise...

[Jack Hammer]

...My furnace's intake makes a very loud sucking noise when it turns on. I measured it at 63db at my listening position. It really makes watching movies difficult at times because the audio gets muffled. Due to the layout of my small place, there is nothing I can really move around to avoid it. How can I knock it down a few db?

Thanks

Jack

 

[Nick250]

...My furnace's intake makes a very loud sucking noise when it turns on.
Jack

I have a bit of dyslexia so when I read the above sentence for the first time my mind processed it as "My fiance's intake makes a very loud sucking noise when it turns on" and I am thinking what the heck?? LOL.

Jack, I wish I had an answer for you. The only thing I can think of is to turn the furnace off, though I suspect that may not be a viable option.

Nick

 

[Jack Hammer]

I have a bit of dyslexia so when I read the above sentence for the first time my mind processed it as "My fiance's intake makes a very loud sucking noise when it turns on" and I am thinking what the heck?? LOL.

Yeah, maybe I should have previewed this one before I posted it, one could have a little fun with the wording.

Jack, I wish I had an answer for you. The only thing I can think of is to turn the furnace off, though I suspect that may not be a viable option.

Nick

That is how I dealt with it before, but now it is starting to get cold out and I usually forget to turn it back on. I'm just curious if there is something I can put over it to muffle the sound without blocking the air flow into the unit. Are there specialized covers? Would some sort of sound deadener across from it help block the sound?

Thanks

Jack

 

[farscaper]

I'm guessing you have one intake for the whole system and it's right against the furnace?

 

[Jack Hammer]

I'm guessing you have one intake for the whole system and it's right against the furnace?

Only one intake. The furnace is in another room outside my apartment (300sq ft). There is about a 2 foot extension through the cinder block (then drywalled) wall from my place to the furnace in the other room. The extension is a rectangular metal box that is 22"x14", which leads to a 30"x22" opening in the drywall. Sorry, I dont know the names of parts and I'm trying as best I can to describe it. This is all located in an entry hall that leads into the rest of the place. Adding a door is not possible due to the screwy layout of the place - lots of changing widths and not very uniform.

Jack

 

[MDS]

It sounds similar to the placement of my hvac unit in my house. It is basically enclosed in a wall between the kitchen and garage with the kitchen wall having the grates you can open to replace the filters and the garage wall having a door to open to service the unit.

I have the same noise issue you describe (but more like 70 dB!) and I believe the problem is that the turbulent air created by the blower spinning so fast is trapped in that enclosure and radiates sound through the walls.

I think the only solution is to line the interior of that enclosure with sound deadening material and it probably must be heat resistant and fire retardant (which means expensive). I had talked to a dealer of such panels and they do sell panels that would meet the fire code to be safe. An AC repair guy that came here to replace the igniter for the furnace said that Home Depot actually sells insulation that could be used as well. It's on my list of things to do eventually, but for now I just deal with it and turn up the volume when it kicks on.

 

[majorloser]

To decrease the noise, you need to decrease the air velocity. This can be done by adding more intakes to the main duct, increasing the size of the duct, increasing the size of the intake register or replace/remove the intake register (grill).

I'd try running the unit with the grill off. If that helps we can work from there. Don't want to waste any money if you're just renting. It might just be the restriction you noted with the change from 22"x14" to 30"x22". That's a huge difference in ducts.

Could also be something as simple as the filter is restricting flow.

 

[Jack Hammer]

To decrease the noise, you need to decrease the air velocity. This can be done by adding more intakes to the main duct, increasing the size of the duct, increasing the size of the intake register or replace/remove the intake register (grill).

I'd try running the unit with the grill off. If that helps we can work from there. Don't want to waste any money if you're just renting. It might just be the restriction you noted with the change from 22"x14" to 30"x22". That's a huge difference in ducts.

Could also be something as simple as the filter is restricting flow.

Currently there is no grill. I rent, but my friend owns the building, so there may be a little leeway there. The smaller dimension is the actual duct, the larger dimension is the size of the opening in the cinder block and drywall. The duct ends at the edge of the cinder block closest to my place and then there is about a 4" open space between that and the drywall.

Removing the filter makes no noticeable difference in sound. Would adding an additional return(?) in another part of the apartment help? If I ask my friend, he may let me try that.

Here is a basic, not to scale, drawing I made of my place.
View attachment 3672

 

[Savant]

Jack,

Have a read through this link:

http://www.acoustics101.com/hvac.asp

I put the page together back when I was with Auralex. There are quite a few sources to consider when addressing HVAC noise. Some are relatively easy fixes; others not so much.

The two most likely sources are blower (air handler) noise and turbulent air noise. The former will have more LF content ("rumble," "roar"); the latter will have more HF content ("hiss," "whistle," "whoosh"). If you have a sound level meter, you should be able to get a (very) general idea of frequency content by comparing the A-weighted and C-weighted readings. If they're roughly the same, high frequency noise is (more or less) dominating. If they're quite different (dBA less than dBC), low frequency noise is (more or less) dominant. Note that you should use a windscreen if you're measuring sound close to the intake.

Insulative approaches are very effective for HF problems. More (longer) duct - also insulated - is a good approach for the LF stuff, among others.

All of which you may have very little control over, unfortunately...

 

[farscaper]

Did the noise increase recently or has it stayed the same?
Is it fan/motor noise (mechanical) or air velocity noise? (as majorloser has mentioned)

If its air velocity noise, then check to see what fan speed the furnace is set at. Most (if not all) of the direct drive furnace fans are multi-speed. The wiring diagram on the back or the fan compartment cover will tell you which color wire is what speed. If the fan/motor are belt-drive, then its trickier.

If your dealing with mechanical noise, thats a whole other ball of wax. Savant's article will help.

 

[Jack Hammer]

Jack,

Have a read through this link:

http://www.acoustics101.com/hvac.asp

I put the page together back when I was with Auralex. There are quite a few sources to consider when addressing HVAC noise. Some are relatively easy fixes; others not so much.

The two most likely sources are blower (air handler) noise and turbulent air noise. The former will have more LF content ("rumble," "roar"); the latter will have more HF content ("hiss," "whistle," "whoosh"). If you have a sound level meter, you should be able to get a (very) general idea of frequency content by comparing the A-weighted and C-weighted readings. If they're roughly the same, high frequency noise is (more or less) dominating. If they're quite different (dBA less than dBC), low frequency noise is (more or less) dominant. Note that you should use a windscreen if you're measuring sound close to the intake.

Insulative approaches are very effective for HF problems. More (longer) duct - also insulated - is a good approach for the LF stuff, among others.

All of which you may have very little control over, unfortunately...

Thanks for the link, it seems helpful. The noise is more of a 'whoosh.' I measured the difference between A & C weighting, near the intake, and got 73db with C and 70db with A. So I guess it's a LF problem. I'm not sure if I can extend the length of the duct work, as the unit is just on the other side of the wall. I'll look into insulation of some sort for it. Is this something I can get at Home Depot?

Did the noise increase recently or has it stayed the same?
Is it fan/motor noise (mechanical) or air velocity noise? (as majorloser has mentioned)

It's an air velocity noise issue. It has always been there, but I don't use the unit in the summertime, only when it is cold out. Last year I just dealt with it. With the upgrades to my system, it really bothers me now. I got spoiled this summer without having to listen to it and forgot just how irritating that sound actually was.

 

[Savant]

Thanks for the link, it seems helpful. The noise is more of a 'whoosh.' I measured the difference between A & C weighting, near the intake, and got 73db with C and 70db with A. So I guess it's a LF problem. I'm not sure if I can extend the length of the duct work, as the unit is just on the other side of the wall. I'll look into insulation of some sort for it. Is this something I can get at Home Depot?

Even insulating the duct will provide minimal improvement. And it will decrease the cross-sectional area of the duct, thereby increasing flow velocity. So you could wind up with net-0 improvement with such a short run of duct.

It's an air velocity noise issue. It has always been there, but I don't use the unit in the summertime, only when it is cold out. Last year I just dealt with it. With the upgrades to my system, it really bothers me now. I got spoiled this summer without having to listen to it and forgot just how irritating that sound actually was.

If you're fairly certain the noise is flow-induced and not ductborne noise from the unit, you can consider building a "duct" on the wall, over the existing intake. The best approach would be a "box" big enough to decrease the flow velocity. The box innards should effectively be a "snaking" path for the air, first down from the intake and then back up with an opening approximately where the original intake is on the wall. I would suggest 2x cross-sectional area - e.g., if the intake is 2 ft², make the cross-sectional area for the airpath in the box 4 ft². This should reduce the flow velocity by a factor of 2. The path for the air should be lined with insulation, being careful to account for the thickness of the insulation in the cross-sectional area calcs. The new opening should also be as close to 2x the cross-section opening of the original intake. ("Box" drawing attached - not even remotely to scale. For reference, most cold-air returns - the "intake" - are about 6" high by X inches wide. To double the area, make the inside cross-section of the box 6" deep - =~12" total box depth total when you "snake" it around - and double the width. When it's all built out, you should have a "box" that is no more instrusive than a column soffit. If you're handy, you might even be able to figure out a way to affix it to the wall in a temporary manner, though it will have to be an airtight seal at the wall.)

Of course, before you do any of this, you should be absolutely certain that the noise is primarly coming through the duct/opening and not through the wall. LF noise from the unit that is breaking through the wall will still be there after you build the "box."

 

[Jack Hammer]

Again, Thank You...

...I'll experiment over the weekend and see what I can come up with, between making a box and/or picking up some insulation. I spoke with my friend who owns the building and he said he will look at adding another return (intake, whatever it's called ) in another part of the apartment for me. He said it's ok if I want to put insulation on the duct work, "it's your money, go for it." (he'll install and pay for another return, if he can do it.) I may be able to fit a box in the space between the wall and the cinder block, or at least a portion of it.

I really appreciate the help. I may have a few more questions this weekend after I pickup stuff at Home Depot. I'm also going to look for those vent covers (diffusers) mentioned in the article.

Jack

 

[Jack Hammer]

If you're fairly certain the noise is flow-induced and not ductborne noise from the unit, you can consider building a "duct" on the wall, over the existing intake. The best approach would be a "box" big enough to decrease the flow velocity. The box innards should effectively be a "snaking" path for the air, first down from the intake and then back up with an opening approximately where the original intake is on the wall. I would suggest 2x cross-sectional area - e.g., if the intake is 2 ft², make the cross-sectional area for the airpath in the box 4 ft². This should reduce the flow velocity by a factor of 2. The path for the air should be lined with insulation, being careful to account for the thickness of the insulation in the cross-sectional area calcs. The new opening should also be as close to 2x the cross-section opening of the original intake. ("Box" drawing attached - not even remotely to scale. For reference, most cold-air returns - the "intake" - are about 6" high by X inches wide. To double the area, make the inside cross-section of the box 6" deep - =~12" total box depth total when you "snake" it around - and double the width. When it's all built out, you should have a "box" that is no more instrusive than a column soffit. If you're handy, you might even be able to figure out a way to affix it to the wall in a temporary manner, though it will have to be an airtight seal at the wall.)

I just did an experiment to see if this will be worthwhile for me. It was a little different from your box design, but it's just to see if it's going to make some difference or not before I buy a bunch of materials and spend hours for an unknown result. I put an unfolded cardboard storage box lid over 2/3s of the exposed duct in the wall. I put another piece over the opposite 2/3s of hole in the drywall (about 6" space between the two lids), and took before and after SPL readings from about 3' away and at the listening position.

All I can say is...

... WOW!!!

What a difference with just thin cardboard. At the listening position there is a 2db drop with C weighting and 4db w/A. At 3' from the opening there is a 3db drop w/C & a 10db drop w/A. I'm going to pick up some 1/4" plywood and insulation and make this into the wall, pending OK from my buddy, (BTW, he is going to also add the additional return). It really helped alot. Hopefully I'll be able to have it all done this weekend.

 

[JohnA]

I have/had the same issue with my home. The air intake is right in the entry which is open to the living room. I added acoustical tiles to the inside of the air intake, I also built a closet next to the air intake using the bottom portion of it as part of the duct, and I lined the space with even more acoustical tiles. But you can't build a closet, but what about getting some heavy material to "close off" the hallway?

 

[Jack Hammer]

...but what about getting some heavy material to "close off" the hallway?

Due to the layout of the place, that's not really an option.

Here's what I ended doing. I bought some 5/8" acoustic ceiling tiles at Home Depot for $1.72 each. I lined the inside of the intake duct (14"x23"x20") with some tiles. This really didn't make any noticeable difference, but it only took about 5 minutes to do.

Then I took another tile and covered about half of the opening (14"x23") on the duct itself. This made a much more noticeable difference.

Lastly, I took a full 2'x2' panel and covered the opening on the wall opposite the side of the duct I covered, and let them overlap by about 4-6". That made a huge difference. There is about 4-5" depth between the panel on the duct and the panel on the drywall (the duct was recessed into the cinder block).

I bought a bunch of wood and other stuff and was going to build the box that Savant had recommended, but when I started measuring, I realized the the layout that already existed allowed me to make something similar utilizing what was already there. The duct's opening (14x23) led to a cavity in the wall that is 30"x22." Essentially I halved this, so there is still a 15"x 22" exposed area in the wall for air to flow into. This should allow for plenty of airflow so as not to make the fan work hard.

Soundwise, I can still hear the fan, but it not overbearing. In fact what I hear now is the vent on the ceiling by the kitchen sink. It makes an 'air rushing' out noise that I am now able to hear - I'll take that over the fan. I'm looking for one of those noise rated registers listed in that article to help minimize that. Home depot and Menards don't carry any.

Final result is a 7db C weighted drop at the listening position, now 56db. And the A weighted response doesn't register on my radio shack spl meter.

Again, A HEFTY Thank You to all for helping me with this.

Jack