Noise from returns

Spence

Don: Even though you may be reading the duct calculator (@ 100') correctly, you're saying that an x by x duct delivers x CFM at x IWC. You have to have a blower that can do that, so just assuming the duct carries a certain amount of air isn't accurate, because you haven't figured in the blower air flow, component PDs, and blower ESP. Look at your duct calculator; it tells you this.

don_9

No what i have said all along that blower on any given unit came at.50 esp in the old days.also the pd were most always the same.so if you knew going in that if you kept your tel around the same your friction rate came out to about the same.i think it was in the early eighty in the rses duct design class our instructor said to us that .1 for supply n . 08 for return would work find in most home with450 feet of tel.you must agree that most asp fall any where from .20 to 035.then it all about tel n velocity after that.again .08 to 07 for supply n .05 for return work fine for us in most cases.we seem to get to 90 % of our rated capacity out of our system we put in.

Techman

Hi don , I went to the RSES duct class in Feb 85' up in Boston

don_9

I kindda assume from reading your threads you were an old fart.lol just kidding.yep rses has gotten lots of my money over the years.

Spence

Don: Now that you've given me the whole picture, I understand what you and T-man are doing, and I appreciate that. I've never liked average methods (like that 400 CFM thing), especially in these times with big evaporators that can easily eat .25 IWC or more. Add an air cleaner or MERV 8 at .15 or higher, plus your terminal units, the blower is now at maybe .70 IWC to have enough left for ductwork. And that won't be much ductwork at all, especially since builders seem to forget about us.

don_9

I hear ya spence.add to the list issue with txv,freon leaks,blower motors issue,etc,etc it make you want to throw in the towel at times.people ask me all the time what the best equipment? I tell them they are all junk and your only saving grace is me.lol

Spence

That's because you're right.

Techman

How about a little reading material? Duct related ,of course! RSES, of course.
1- Low Velocity Air Duct Design -SAM #630-21
2- Duct System Design Consideration-Part#1-SAM#630-148
3- " """"""""""""""""""""""""""""""" Part#2-SAM#630-149
4- The Equal Friction Method of Duct Sizing-SAM#630-15
5- Duct System Design- SAM#630-18
Come on now!!! What do you have to lose?? NOTHING!!!
Come on now!!! What do you have to GAIN?? LOTSandLOTS!!!
Maybe a better ,more rounded education?

Spence

Don: I'm still confused about one thing. I see from where you are coming with the equal friction method for duct design, yet I'm still in the dark about the blower because you don't mention it. It seems to me that because changes in blower pressure would cause a change in the duct pressure, the friction rate would have to change.

don_9

I did mention asp like .20 to .035 seem to be our working pressure in our distribution system..20 being the best to .035 being the worse.And yes the pressure would change with fan adjustment that why you try to design around med speed so you can make adjustment high or low if need be.Just remember to always read the foot note when picking esp.like wet or dry coil, standard filter or none at all.Not reading those will get you in trouble down the line.

Spence

10:4; thanks!

Techman

Also,most "packaged " AHU have the evap coil PD already figured into the SP rating and "add on"evap coils the PD has to be figured in and added . I thought ,a system is calculated by the BTU/cfm requirements and a blower is picked to be as close to the target CFM as possible, a little more/less is OK, and the duct is sized to deliver the btu/cfm/velocity as needed, with ALL of the PD devices figured in. Then months later a HEPA filter is asked to be added,but, only if there is enough "umph " left in the blower/duct , if not ,then the customer does not get the HEPA filter.Simply because the "system" can not handle it.
There is a reason why blower SP are figured from .5 to.8 and med to hi speed blower ratings are picked.

NJ, Designer

Thanks All,

Any ideas for quieter returns?... wasn't that the question....

My take on this all is there are some rules of thumb with correction factors that do a fine job. however modern technology (I.E. computers) allows the younger generation to do things much more precisely with out as much guessing, thanks to manual D. Now the hardest question I always have is all the time I spend doing manuals J S D and T, to the T, is it really worth the cost?.......

Did any of you that look at the blower ESP chart ever wonder why they all stop at .7-1 IWC,

I think the answer is EER and SEER, so now we are stuck with the meager .7-1 IWC in the forever increasing large homes with no room for duct work, well I thank them all cause at least I have bread to eat....... stay comfortable all.............

Techman

Hi NJD, I think this was a real good conversation about supply AND return ducts. I wondered about this quiet a few moons ago and I was directed to RSES which has a SAM piece titled "Low Velocity Air Duct Design" Section 56. It divides air distribution systems into 3 pressure categories Lo, Med, & Hi press. These 3 press categories happen to match the 3 fans classes, 1, 2,&3.
Lo press =Fan class #1= up to 3 3/4 " wg.
Med press=Fan class #2= 3 3/4" up to 6 3/4" wg.
Hi press = fan class #3 =6 3/4 " up to 12 1/4" wg.
So I ask , what is a class 1 fan?.
Back in the 70's, Carrier had a Customer Relation System Sizing Sales Program. At the customers house the salesman did his Load Calcs , called Carrier and Carrier did a system sizing on the phone in minutes. And Mr.Customer you need a 3t system. Computers were used then, those computers were not what is available to todays generation.
Main trunk velocities are not the same as branch velocities.

don_9

Yes nj here is your answer.a pair of pliers.

Spence

Point one: noise is pressure related; hence the discussion about measuring your system under operation. For example, if either of your supply or return systems are .30 or higher after taking out the accessories, that is your early warning system. Very easy to do and I will help all that I can.

Point two: most residential blowers are wheezed out at .90. The national average is .82; stupid high. And the blower uses twice the energy or more to produce that. Factories think .50 should be enough to do most situations (look at the performance chart), yet we're often pushing .75 to have enough juice to push through the accessories and have something left for a duct system. The more we educate on starting at the blower, the better the chance to drop the national average to something reasonable.

Our customers deserve it.

Techman

Spence, we ARE getting closer to a common understanding of ducts, yea ha! Weather or not we realized it awhile back!!!!!!. Carrier says they want .25 "wc left over for the duct system after all losses are determined. So, checking to see if the evap coil is figured in/out of the mfrg charts is very important.Carrier also limits the house sq.ft. to 2000'. I wonder how "tight houses" fit into that limit of 2000sf?
Just a quick Q here. Two identical houses ,down to the BTU, 4t, house #1 has a short return of 50TEL, what is left over for the supply? House #2 has 150'TEL return ,what is left over for the supply duct system?

Techman

Another point,with the returns using a wall bay,the "Surface Roughness" is very,very high as compared to a sheetmetal return in a wall bay.

Spence

Using stud spaces as returns averages 150 CFM if you're lucky.

Spence

Regarding SA and RA percentages, I don't use them, because every house is different just like FR and CFM are never the same. My TEL would be the total of the longest supply and return (in equivalency, not LF). The FR would then be ASP (from the blower less accessories) x 100 / TEL from above.

Spence

A huge impact made by tight houses is the SHR used to determine your target CFM, because many load programs, and the manufacturers too, base their performance on a .70 SHR. Great for the consumer comparing units, but a huge error for those contractors who use the AHRI Guide to size equipment. I haven't seen a .70 SHR or a 20 degree delta T in years in my area.

Techman

I'll be back.Printer screwup.

Spence

Aaaaanold, my Android won't open your attachment. I'll look at it from a desktop as soon as I can.

Techman

Lets try this again.

Techman

This chart puts those RA vs SA % into a clearer perspective for all to see and use.That last sentence means a lot.

Spence

So, what you're telling me is: you run a load to determine your CFM requirement for the space. Then you find a blower that will deliver that at what pressure. After subtracting your accessory components, terminals, etc., and you have .18 left, you use .09 for multiple RAs, .12 SA and .06 RA for several area returns, and .13 SA and .05 RA for a SCR. Yes?

Techman

Well now, I'm just passing along what Carrier says. So are you telling me that Carrier is wrong ? Reading isn't believing? Where did I go wrong? I guess I should have contacted you 40 years ago and saved myself a lot of studying. Things that make you go "humph", oh well!

Spence

The duct doesn't give you CFM, the blower does! You can't say an "x" sized duct produces "x" CFM at "x" pressure because it doesn't!

Carrier is not wrong. What they are saying is that you use their chart AFTER you know what available pressure you have. See their line "total available pressure?"

Look at your duct calculator. Use any pressure you want and any CFM you want. What size 100' duct is it? Now, imagine there is a coil in that duct that soaks up .25 IWC. Subtract that from your earlier pressure and look at the same CFM. What size 100' duct is it now?

My whole point repeatedly is the best duct design method in the world is only as good as the blower you put with it, accounting for pressure drops! In all of our conversations, and I enjoy them, you don't mention the blower. Only the ducts. And I keep plugging away at the fact that even your duct calculator shows you that in residential work, the duct design comes AFTER the blower design!

Spence

Two questions:
1. What happened to NJ designer?
2. Why is my face green? I look like an olive.

NJ, Designer

Spence, I'm right here listening carefully, waiting for your call.

Techman

Yo Olive! Popeye here! Have you glanced at or even read those 5 RSES duct related ServiceApplicationManuals? Its only about 30-35 pages TOTAL????!!!??? I also like this "sparing match", its interesting. I think you, besides me, must have been a problem child,also.. LOL! Spence, when we meet I get the 1st and 2nd round!

Spence

I did glance at them with the intention of digesting them in a more quiet moment; hopefully by this weekend (whatever that is).

If you get 1 and 2, I'll get 3 through 10. Then we'll both be "popeye-d."

don_9

Hmmmm are you guys beating a dead horse? Lol
Nj mention he a big manual d fan so with that said we now back to the return grille.The fact that he uses a return in every room alone should drop the pd on the return side of the system.so my thinking again is it not high static but,too small of a return pleum and grille.I do believe i have mention that in the beginning of the thread.As Spence mention that you have to test to know if your math was on the money or not.The filter should not exceed 20% of rated fan static.Again my money is on a grille that is too small n the blade are to close together.Nj those stamp fbg stink.Break out the manometer n start measuring pd on start up.After all how can one dail in the refrigerant side of the system with out knowing airflow?

Spence

I know you're merely joking, Don, about beating a dead horse. Nationally, 57% of our residences have inadequate ductwork, so our charge as environmental specialists is to make that awful number go away. Hopefully there is a growing band of brothers and sisters who want to pick up a big stick.

This topic stream began with a call to NJ Designer to measure his operating ESP, and I offered to help. Unfortunately, when he found out who I am, he shot himself.

don_9

Lol,well who are you? Maybe i will shoot myself too.I hear you n i take my hat off to
you for trying to put out there the right way to design a distribution system.However you can lead a horse to water but you cant make them drink.You can educate the customer,the builder n even the guys in this field all you want but when it come to the bottom line one out of ten will choose the cheapest route period.Example blower door testing a great tool to have and should be the beginning with an hvac design but yet when i purcase these tool many years ago i may have gotten four job out of it.Now they sit there collecting dust.Again it about what one percieve as value.

Spence

Excellent points. I like to ask customers if they have just a few moments to allow me to show them that value of which you speak. Many times it doesn't cost more. If it does and I have a tough one that I know in my heart I can fix, I let them hold the difference until the point is proved. I've yet to get stiffed.

Good guys like you will never win the race to the bottom. But I have this to say in comparison: when unemployment was 11% in my area, I still saw luxury cars with 30-day tags and huge houses being hammered together. Perhaps we don't lose jobs because of price, but lose them because we didn't show that value.

NJ, Designer

Well, I don't know if I have seven life's but I am back alive, (hey don't ever try shooting yourself it's awful....)

Don, I quote you here "The filter should not exceed 20% of rated fan static" Please explain, I try to keep the airflow across the filter bellow 300 fpm that should have a PD of .03 IWC.

Also I use filter grills in each room are you saying there is a big difference between the filter grills on the fin spacing, I am aware of that but never did anything about it, I got to cut some corners somewhere, I spend so much on supply registers and grills that I thought at least on the return I can use "cheaper" filter grills in each room!!!!!!!!!!!!!

don_9

To find the 20 % rule you would take your rated static pressure say .50 and then times that by 20%= 0.1 that means you need to find a filter that has a rated pressure drop of under .1 at your desire cfm.

Back to the stamp filter back grille. Anything under a 18x18 it has been my experience make noise.even if you did every thing right on duct design.Try using a fbg with 3/4 blade spacing i think you and your customer would be happy with the results.Yes my friend it is hard to offer quality on a shoe string budget.I feel your pain.heck i am clapping for you just knowing you got the job with a return in every room.

NJ, Designer

"Try using a "fbg" with 3/4 blade spacing " What's FBG?

RobG

filter back grille