When we say to "oversize" the register or grill, is there a rule of thumb for how much to "oversize"? Would the "oversize" be based on the duct size, air handler cfm or fpm?
As an example if the register was 5"x10" (based on it's cfm value) would we possibly move up to the very next register size that also delivers the same cfm? Possibly go up 2 sizes that deliver the same cfm?
Thanx!
"Oversizing" Registers/Grills
Originally posted at johnlsayers.com, topic 9136.
Anyone??
Cami, you are a very confused young man........ first ff - there would be no value oversizing a register. You oversize duct - which automatically makes the register larger( you need to cover the duct - and a 10x10 register will not cover a 12x12 duct). Duct size has everything to do with the speed of the air (FPM) as well as the back pressure to the air handler unit....... So you size the duct based on the desired CFM at the desired static and the desired velocity (FPM). It tkes all 3 together. The balancing of the system (using balancing dampers) is what adjusts the CFM to meet the design parameters - and the rest (static pressure and velocity ) fall into place automatically........ some peoplelooking for a cheapway out will increase duct size right at the wall/ceiling - in an atempt to slow down the velocity 0 and then for that they need a larger grill than the ductwork feeding the room would suggest - but never the grille itself.When we say to "oversize" the register or grill, is there a rule of thumb for how much to "oversize"? Would the "oversize" be based on the duct size, air handler cfm or fpm?
What you describe above is not possible. First off - a larger grille is always dsigned to have a capability of deleving more air than a smaller one- so there is not (anywhere in existence) a particular manufacturer who manufactures a 6x10 grille that in designed to deliever the same air as their 5x10 grille does. RodAs an example if the register was 5"x10" (based on it's cfm value) would we possibly move up to the very next register size that also delivers the same cfm? Possibly go up 2 sizes that deliver the same cfm?
When I first read the "very confused young man" part in the last post I :lol:
But then I thought, "wait a minute"... :?
So, my thoughts are as follows...
Ok Rod, if you don't mind, "un-confuse" me on this register/grill issue...that is as far as how I "wouldn't" come to my conclusion based on info I gathered. After you read all of this, is it possible a "novice" would come to my conclusion? Yep, this is somewhat long, but I would appreciate you reading it so I can voice my opinion as to "HOW" I came to writing this post. Before you do that, the thread below pretty much sums up my situation concerning my Studio HVAC...
http://www.johnlsayers.com/phpBB2/viewt ... ght=#62620
(BTW Rod, if you get time, I'd really appreciate a response to that last set of unanswered questions in my last post)
Below is sort of a "series" of how I came to writing the "oversizing" post in the first place.
Rod, your book is EXCELLENT for those installing a new designed system from scratch, but as stated in the thread above, there are those of us that have existing systems where the performance of those systems are set. At that point, it doesn't matter what your heat or cooling needs are, "yer only gonna get what the dang thang will put out" :?
Your comment here substantiated those sentiments...(at least er' to "my" undertanding)
I never got a reply for my last post in the above thread, so I decided to obtain HVAC info via the net and locally. As I did this I tried to remember 2 things... 1.) p.111 in "the book" concerning HVAC...The best you can do in that case - if it's a dedicated system for the basement - would be to maximize your flow - and live with a fan when it gets unbearable.
2.) And a quote from Steve concerning most HVAC contractors..."This is probably one of the most overlooked items when it comes to home studios".
In trying to keep this brief, all I'll say is I went to the following sites (along with others) to ask questions concerning HVAC in general, for my personal Studio HVAC needs, and for a post I plan on putting on here for novices like me concerning HVAC. http://www.hvac-talk.com/vbb/showthread ... ost1524880 http://www.hvac-talk.com/vbb/showthread.php?t=118634 http://www.hvacmechanic.com/forums/ress ... /24985.htm http://en.allexperts.com/q/Heating-Air- ... stions.htm There are other personal emails I got for questions I submitted to other people/places, but the above links are accessable to you via the net. Some of the questions are repeated on different web sites, but that was only due to me trying to get a consensus of answers pertaining to a particular question. There are varying opinions to some of the questions I've asked concerning HVAC. That leads me to some responses I got concerning registers and grills. Here is a question I posted on HVAC Talk and the response from a professional member in October of 2006... question:you'll find that virtually NO one in the building trades (unless you get VERY lucky) will have ANY idea of the concepts that make a room quiet (and still breathable) - so you need to learn enough about these to educate your contractor/laborers BEFORE they screw up your dream.
reply:What I need to know is where is the best position for the air return vents to be located in each room? Near the floor or ceiling? Presently, both of my supplies are mounted in the ceiling.
question:You'll want a NC(Noise Criteria) of < 25,suitable for acute pickup of all sounds,I'd think. See http://www.Hart&Cooley.com Look for commercial grilles ,size by cfms(air quanity),and NC. You'll want to increase the return size ,before getting to the grille to slow the air down.If it's metal duct it may still be noisey.
reply:Should I oversize the SUPPLY duct before it reaches the register/grill? Should I even oversize the Supply register/grill?
(BTW...here is the link for those questions/responses... http://www.hvac-talk.com/vbb/showthread.php?t=118634) Between this comment from a professional and other info I've read about "oversizing your registers and grills," THAT's what lead me to do this post. Why you ask? Because...even though the question about register and grills and oversizing them seems to be a "general" one concerning HVAC that any HVAC contractor could answer, I have a "superinsualted structure" with a more critical need for quiet air supply & return. Thus, I felt it should be mentioned and explored on my good ol' John L Sayers web site! In one of my HVAC post, I mentioned about increasing the duct size of my branched off duct from my trunk, but Rod, you stated that it would make things worse unless I used a damper to control the supply air. I then decided to keep my duct size the same. So, add it all up... From info I had read about "oversizing"; the professional's comment and the fact that my duct size was going to stay the same (both supply and return), guess what I decided to do??? YEP, this is it!...Supply griles are sized by cfm required,pressure loss,and "throw"(how far the air will travel after leaving the grille).You want the throw to be minimum of 75% of the distance to the oposite wall,and maximum of the distance to the wall. Larger will reduce the presure loss and any noise,but the throw will be reduced as well. So yes larger ,but not too large,try the hart and cooley site,for the details on grilles that are similar to wht you have ,or buy that brand. Probelm is, you probably don't know, the actual cfms(cubic feet of air per minute). 7" likely 120 to 155 cfms 8" likely 145 to 200 cfms
EXCEPT...EXCEPT...I was going to enlarge the the ductwork to match the register/grill JUST before the opening. By doing THAT, I would have to match the cfm my unit is producing at the wall/ceiling opening (along with the static pressure) with the "new" register/grill size. Oh, and tell me if this is correct...I cannot get info from the installer of my HVAC unit. Therefore, I don't know what my CFM/FPM/Static pressure is that my system has. The manufacturer also couldn't tell me. My manual states at "x" static pressure, the unit will produce "x" cfm at "x" fan speed (only got 2 fan speeds). This means I have to hire a contractor to come in and measure my cfm and static pressure based on my trunk (plenum) size etc etc. By using a pitot guage, a hood or other measuring device he can tell me the needed values. Now, is that right? Concerning duct size, volume and other items in this area, I DO understand most of the concepts involved. For one thing, I know we should try to achieve a large volume of air at a slower speed to have a quiter system but once again, "my" system is already in place. As I stated in another post, I can change my duct size from my supply trunk, but I CANNOT change my return size. As far as the register size, my understanding is the register needs to line up with the duct size and the cfm produced from the duct. When I mentioned about "moving up to the next register size" I meant increasing the register size in relation to the increased duct size near the register opening. From what you wrote, that's not possible, so I GUESS what you're saying is if my system is producing 250cfm with static pressure figured in; my duct size from my trunk is 8", I want to increase the last 2 ft to 10" duct, I couldn't find a register/grill that's configured for a 10" duct to line up with whatever my cfm is (250cfm or slightly lower) that's blowing out of my 10" duct. (BTW, my uh, idea was to increase the duct size/register size about 2 ft before the opening. For example, I was going to increase the duct size from 8" to 10" 2 ft before the register/grill) *EDITED* 7/26/07 Was going through my notes for HVAC in one of my books and I ran across this...some peoplelooking for a cheapway out will increase duct size right at the wall/ceiling - in an atempt to slow down the velocity 0 and then for that they need a larger grill than the ductwork feeding the room would suggest - but never the grille itself.
The suggestion was never disputed one way or the other. Here's the link for the entire thread... http://recording.org/ftopic-15952-days0-orderasc-0.html *END EDIT* Rod, could I challange your knowledge of Studio construction? No way. Maybe doing my registers/grills like this is right, maybe it's wrong, but it's certainly NOT through my efforts to NOT find out the correct way. If all this is wrong, that's fine, I won't do it. But, I won't know until I ASK. All I CAN do is ask questions whether they're here or elsewhere. From my reading on this site, "MY OPINION" is that HVAC is the most COMPLICATED issue in studio construction. As you stated in your book, "there are many different factors that come into play with HVAC systems" thus, I know you can't answer every HVAC question in your book or on here. I know NO ONE here is at my beck-and-call every time I have a question. So when I research HVAC and take into account the info I get may be on a "residential information" level, I try to confirm it on here in the STUDIO forum. Ok, that's enough of all that... Rod, I appreciate all your help man plain and simple. And I sure hope you'll stick around and help me till' I'm done. "Confused" you say? Well, I guess that's one way it could be put, although in this particual situation on a "public" forum I felt some other adjective or wording period would be more appropriate. Do I think you meant anything by phrasing it that way? I would say no. Would "I" have said that of someone in a public forum? Naaaaahhhhhhhhhhh...In any case, I'm thick-skinned enough to absorb the way you phrased how you feel "I am" about this topic. I Don't agree with it, but I simply block the punch and move on... Just wanted to let my thoughts be known...If possible increase the size of the duct right before it comes out of the wall. If you have an 8" duct bringing the air to the room put a 12" duct for two to four feet right at the duct into the room. The duct should be at a right angle to the vent. This slows down the rush of air and makes it quieter. If you line the larger part of the duct this helps a little more. One thing that might help is suspending a fabric covered 703 panel in front of the duct to block or redirect the airflow a little. You know, between you or the mic and the duct. Seperate feeds to and from the rooms are very important :p:
Camistan,
I will not try to intervene for Rod. He's certainly more experienced than I am, but I'll try to help you anyway. I've had a bit of experience in the A/C field so I'll at least lend a little insight.
I am going to understand that noise is your problem, not heating/cooling capacity, so all that you really aim to change is the noise from the air flowing from the register. I'm also not going to get into the tech details and just stick to some practical suggestions.
First, Rod is somewhat correct, most registers are "standard" sizes. That means that for the most part getting an oversized register to fit your duct may not be possible. However, you still have choices.
1. Have a register custom made at a sheet metal shop. Actually you'll buy a register/grill of the size you want and have the "boot", the actual transition for the duct to the "grill" custom made. It's not overly expensive, at least not here.
2. You can leave the grill off. It will not distribute air as well, but it will make less noise and it will not hurt the system.
3. You can get a bigger register, whatever size you think will reduce the air noise (which basically comes from the air flowing past the fins of the grill) and get a reducer to fit between the new register and the duct.
4. Depending on your ductwork, how much space you have to work with, etc. You can get a larger register and a duct of a reasonable length (3-6 ft) and to attach to it. Then attach the larger duct to the smaller one. If it's flex duct, it's really easy. The longer length of larger duct will slow the air as it leaves the smaller duct and the larger register will quieten the noise even more.Get someone to show you how or e-mail me directly. My address is in my profile. I'm not on the forum a lot.
5. Again, if you have the space, you can "Y" off of your duct and split the airflow into two places. Cut another register in your wall/ceiling/floor and you get better distributed air and less noise.
6. Get technical: Your furnace/air handler has an output CFM rating. Get in touch with the manufacturer. Unless it's an older unit, they should have that information. Have the model and serial number for the furnace/air handler when you call. Also, get in touch with someone that does ductwork regularly and designs A/C systems. They can tell you how much volume and velocity will work for you.
Good luck,
Scott
Thanks Scott, great tips!
Your #4 is pretty much the direction I was heading. I'll also consider your other suggestions to. Right now, I do have an opportunity to make whatever changes to the branched duct from my trunk (for my supply air).
As far as the manufacturer, I did call them. All they pretty much told me was the same info in the owners manual. One thing they said that made since was they couldn't tell me the cfm's I was running because they had no way to know the plenum size, length from the air handler, duct material etc. to determine the static pressure. They said the people who installed my unit should have "tailor fitted" my duct to match my system and determine the static pressure which in turn would tell me my cfm/fpm.
I did do a poor job in selecting the air handler unit and even the installers. At that time (in 2005) I did have some sufficient info to have a better system than I do now, but in the end it simply came down to $$$. Man-O-man, if I could only go back and know what I know now... EVEN if I had the same amount of $$$ to spend...!!!
Right now I'm in the process of installing my ERV before I have a HVAC contractor to come out & read my system to determine my cfm/fpm. If all goes as planned, the HVAC contractor will be here next week.
Anyway, it will all work out fine...I firmly believe that! :wink:
Hey, thanx for the suggestions and interest! I will take you up on the emailing. I need ALL the help I can get!