Noise from returns

NJ, Designer

I'm looking for some new ideas how to make my returns quieter, I generally put a return filter grill in every room in a house (except kitchen bath etc.) I size them for no more than 300 FPM, and they still seem to make noise even when there is some flex on the branch and the machine is hung with vibration pads and there are at least two 90 bends before the return box. I mean they are sucking in the air, I do not over size and I know and follow manual D in my sleep..... any ideas please...

Thanks all,

It is not that noisy after all but the rich boys don't want to hear nothing, And I can't blame them.....

don_9

You might have to try a grille that has bigger blade spacing along with a pleated filter.

don_9

Hey now i did not ask for a heart face with my post.

Techman

Open up the return filter grill and listen for the noise ,then remove the filter and listen . What static press are the returns sized for? Try this non-professional trick. Put a dollar bill very close to the return grill, does the dollar bill get sucked onto the grill quickly ,or gently? Did you check out the return grill info,like CFM, velocity, etc?

Spence

What is your ESP?

NJ, Designer

I typically use .7 IWC, for my ESP, and thats why I can't use Most manufactures PSC motors. and even some ECM motors

The static pressure of the returns are typically around .08 Iwc

Spence

Not your design static. What are you getting at your system?

Techman

In my neck of the duct world we use .08 at the highest and .05 more and more.

don_9

Here as well.we do .80 for supply n .05 for return.in many cases we lose out on some bids to the boys that design everything at .10.

Don_197

The standard up here for non Manual D types is .1 and .05

Spence

You can't "assign" static amounts arbitrarily to duct systems! The static design is different in every job, because it is based on the blower performance and the resistance of the internal components and ducts!

don_9

Why sure you can.you can go into any home we design with those number n you do not hear any noise period,and the airflow is traveling six feet across that area and on it way back to the return.depend on where you like to fall on your fan curve.we design around med speed so we can make adjustment high or low if need be.arbiltarily lets me sleep at night.lol

Spence

I agree it probably works to a degree. However, you can't ignore pressure drops within the system that reduce the blower performance and resultant CFM. A glance at your duct calculator tells you that.

You're only a few steps away from making a good system a great system.

don_9

It is all good my brother.we all do what one is comfortable with.I have a house now that has a shr 0.92 so do i stay with the 20 degree temp delta t at 400 cfm per done or do i go with a 17 degree with 550 cfm a ton? I agree all home are different however with most home you are not going to do any better then .05 return n .08 supply.that the best you are going to get in most home today that are being built.remember most builder dont design around hvac they are all about taking all the room they can get n then they want you to make it fit.

Techman

How about this.On a given system, there are 5 items affecting TSP (Total Static Pressure). Return duct#1, filter#2, evap coil#3, then blower, then heat coil/exchanger#4 and supply duct system#5. Now, a blower motor has two functions, (proper physics aside here) , pulling and pushing. Now, you can do ducts that use any ratio of Return to Supply you want. But, having a "system" that has a 25% R and only a 75% S ratio as compared to 5% on the R gives me 95% available on the Supply. With smaller returns the velocity goes up. At what velocity does the condensate get pulled off of the evap coil?. So, having 95% of the motors "workability" available for the supply gives me a few options. I think.

don_9

That it in a nut shell techman what velocity is one willin to live with in the distribution system n at the grille.but one does not design for velocity at the supply grille we go for throw.now at the filterback grille it is recommended at 300 fpm.that help keep the noise down n lets the filter become a filter.again for me my posted friction numbers works for me bc,when designing i always know when doing the math my dpl are always the same in most cases and i try to keep my tel of my distribution system at so many feet.that why i can arbitrarily run with my numbers.not to mention i like working the wedge as we call it.lol

Spence

How is it possible that you can arrive at .1 and .05 on every job?

NJ, Designer

Spence, You got me there I never actually measured the static pressure of the system, I don't even own the proper tools to do so, however I wish I did... anyway what is the point of this question?

I was wondering if these vibration collars really work at all, or is there a good duct linear that won't cacth all the dust, that I can use in the return plenum to get things a little quieter?

Spence

If you have noise, you probably have a velocity issue or a high static, and my guess is the latter. We need to measure your ESP under operation to determine where the problem lies. Contact me directly and I"ll mail you the tools you need.

don_9

I would put money on it being a velocity issue myself.if it were a high static issue he would have noise at the supply grille as well.noise return grille issue are a indicator of one not selecting the correct grille or pleum to maintain the recommend 300 fpm.kindda hard to meet that 300 fpm in a joice space that only has 14 inches between them.if i may ask what size is this grille n what size duct is connected to it?

Spence

The recommended 300 FPM is for the filter, not the RA system. If it were so, then if, for example, your duct is 20 x 10, the CFM would be 417.
I hope no one is using joists for returns, as they only move 150 CFM if you're lucky.

unclejohn

NJ;
What kind of noise are you hearing. Mechanical or air?

NJ, Designer

Sorry all for my delayed response I am thankfully very busy, I do think it is air noise, I do own a testo large vane velometer, and the velocity is a little above the 300 FPM it was designed for, (the total air flow is above my design by about 10%. I can't lower the speed it would be way to little air...) but even the returns that are under 300 FPM still make some more noise than I would like.

I use manual D to size the trunks and branches, My high limit on the trunk and branches is 700 FPM (I'm thinking this is the problem) the filter return grills are sized with the engineering data from Hart and Cooley, and I size it for max 300 FPM, So trying to keep uniform I use mostly 12 x 12 filter grills which give me up to 180 Cfm per filter return.

unclejohn

What size flex are you using at the F/G. You need at least 7" - 8" would be better.

don_9

The 700 fpm in trunk is fine.i would add a pleated filter,check my tsp n make a fan adjustment.Being you cant go down go up n balance it out with the filter.Next go around step it up to a 16x16,the 300 fpm at the grille is a recommendation only.So design around the noise next time.My hat off to you, you are doing a fine job.

NJ, Designer

don, the pleated filters have a large pressure drop, 10 x 10 is the cheapest size, can't get much air through each one of them, I'm putting a return in each room a average home I do has 14 rooms, without the kitchen and baths, 16 x 16 won't fit in a wall framed 16" on center, so 14 x14 may be the answer, aesthetically I like 12 x12 better.

Spence

Hold the phone! A filter in each RA duct? There is no way your blower can handle that!

Techman

Good point Spence, but I'm not sure. Why wouldn't the the "one" return with the longest TEL be used in the formula and all of the other returns ignored?
The TOTAL TEL of the supply and return trunks added together is limited to 400'. So, if the return is 300'EL then you have only 100' EL for the supply or the other way around.
NJ, the wall returns are going thru a regular wall joist of 4" x 16" , thats not much air ,about 125cfm at .05"wc . You use .7" when sizing? I was taught to try to use a blower in the 0.5" -0.8" range and med to hi blower tap.

Spence

You have the right thoughts, but you're doing it the commercial way. In residential work, we find a blower first, and the ducts are last. This way you don't ignore the difference between friction rate and pressure drop, which appears to be common in our industry.

The issue behind TEL is that if your blower can provide the correct CFM at the farthest equivalent outlet, then all the rest can be adjusted by dampers if needed.

For example, your load requires 958 CFM. You find a blower that provides 1006 CFM on medium speed (way to go, T-man!!!!), and that is at .70 including a filter. If this is new work (we'll talk retrofit later) and you have allowed .06 PD for your terminals and your coil is .25, then you have .39 left for ducts. Lets assume your TEL is 375' for this example. Since your duct calculator is in 100 feet of friction rate, what you have left now for ducts is .104.

I did this on purpose to show how WRONG "designing ducts to .1" is, because it only works IN THIS SITUATION. Change any number, and the friction rate changes, because the component PDs change!

Let's say you're adding an air cleaner and the PD is .25. The net PD is .17, because we said above the blower ESP at .70 included a filter, and the factory uses .08. Look how the numbers change. How easy it is to see that assuming an x by x duct gives x CFM in all cases can't be right.

unclejohn

There is no problem with a return in every room but why not put one filter at the unit. Changing 12 to 14 filter every 3 months is a lot.

unclejohn

And furthermore if you are running these return ducts through a wall you most likely are using 3.5 x12.5 duct which puts your static around .15 and at 180 cfm you are at 750 FPM

Techman

You agree that an AC system that has 100' of TEL for the supply is sized at .10"wc , and a very short return is sized at .05 ? Carrier and RSES says yes!

Spence

How is that possible when you don't know what the blower is?

Spence

How does a 3.5 x 12.5 give you a friction rate of .15 IWC and 180 CFM?

Techman

Geez, I thought this one was DIRECTLY off of the Ductalator. Or, "The air fricton chart is nothing more or less than a graph on which the co-ordinates are the friction loss in inches of water for each 100' of equivalent length of duct, and the cubic feet of air per minute carried thru the duct" This is ACCA , out of their "Understanding the Friction Chart" ,Technical Topics . Lets not talk about 95' or 105' , just 100' . Carrier and RSES have a "Correction Factor" formula for that. Then at a RSES "Duct Design" class they printed this "For residences, .10 inches of water column is recommended for system design pressure.". Again, don't forget the " Correction Factor" for those all too common duct TEL of 101' or 301'.

Spence

The blower. You're not addressing the blower. It isn't the duct that gives you FR. It's the blower.

Techman

OK Spence, I adjusted the belt driven blower to deliver 400cfm at .10 static w/ 100' of straight round duct as per Carrier, RSES, Trane, ACCA, and, the blower tries to overcome the friction loss of the duct , of the air rubbing on the duct surface, and the air turbulence which is ALWAYS there.

Techman

Does the return grill on the wall and the 90* angle of the wall cause an "air turbulence"situation like in a no radius ell. Turbulence = noise.

don_9

You guys are getting to deep for me.lets get back to velocity for a second.if your recommended velocity in a trunk duct is for example 900 fpm n of course we know we need 400 cfm per ton bc we want that 20 degree delta depending on shr of course.now remember many years ago the .1 for supply n .08 return work for many of many years.could we still get our cfm across the coil n hx.we sure couldbut the velocity was like a 1000 fpm.then came along heatpump so to keep the air coming out of the vent for most folks felt cold.what are we to do.decrease velocity of course.take your duct caclulator n say you are looking for 1000 cfm do it at .1 n note your velocity.then go down the line n do it again at .08.note your velocity once again.then again at .7.then do it once again with the reccomended velocity of 600 fpm at 1000 cfm and what does it show for a friction rate? Please note as you go along the duct sizes n keep in mind as you do the home you go in to.let me know if you feel the need to step out of the box.lol

Techman

lol, don. I think you are describing a "static regain" type of duct system. I think i've been talking an "equal friction" type, I hope so, anyway.

don_9

There you go again using them big words again.i only know equal-friction method.so i am outta here.lol