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post #1 of 53 Old 02-19-2007, 01:39 PM - Thread Starter
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I recall reading somewhere that the shape of the HVAC register can have an great effect on the noise created by the airflow. My HVAC will run through the soffet then dicharge into the room. What shape or size of register(s) should I be looking for?

Thanks
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post #2 of 53 Old 02-19-2007, 08:09 PM
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what is the airflow? (CFM)
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post #3 of 53 Old 02-20-2007, 05:58 AM
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The larger the opening the lower the sound, it's that simple. Haven't you ever closed an air duct to the point it whistled?

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post #4 of 53 Old 02-20-2007, 09:27 AM
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http://www.nailor.com/digitalcatalog...WEB/CADBLD.pdf

You can look at the Bar registers.
One other point, the HVAC contractor will be determining CFM (cubic feet per minute). That's fine, let them have at it; however, YOU tell them that regardless of the CFM required per diffusor, you want no more than 250 FEET PER MINUTE velocity.

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post #5 of 53 Old 02-23-2007, 09:48 PM
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Quote:
Originally Posted by dpgoldberg View Post

I recall reading somewhere that the shape of the HVAC register can have an great effect on the noise created by the airflow. My HVAC will run through the soffit then discharge into the room. What shape or size of register(s) should I be looking for?

Thanks

This is sort of true. It really has more to do with the pressure drop created by the type of diffuser or grille. For example, the standard three-cone diffuser used in the commercial industry has been around for a long time and can be very quiet at a given duct velocity (as Dennis has mentioned 250 fpm is preferred and is what we have used for HVAC systems in lecture theaters - as well as duct liner, large radius elbows, acoustically lined air valves, and silencers). If you replaced a standard three-cone diffuser with a perforated face diffuser of the same size (which is often done for aesthetics), you would notice a difference in the noise level due to the increased pressure drop (more restrictive). Any diffuser or grille can be used depending on what suits your installation, but it must be sized and installed properly. The linear diffusers that Dennis has pointed to must use a plenum of adequate volume (factory plenum also shown) for proper air distribution and have a large enough supply duct to provide the low duct velocity. Often times the sheet metal contractor can fabricate these to suit site conditions as the factory plenums don't always fit. Duct liner will also help.

If you soffit is not large enough for a proper plenum you may want to avoid the linear slot diffusers. They sure look nice but can be quite noisy and are very unforgiving when installed improperly. The linear bar diffusers are a bit more flexible. Avoid tight spacing of the bars.

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post #6 of 53 Old 11-06-2008, 12:23 PM
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Bruce, et.al:

Sorry to resurrect tis old thread, but the right people may already be "listening"..

I'm finally getting back around to dealin gwith my own basement and theater space.

I've done the Manual J analysis, I am working on ducting design options via Manual D equivilant friction design. I would like to make some register and boot selections to continue that process.

I understand the basics of register selection, but I am hoping for some of the broad experience here to assist me in specifying the best *type* of registers for this job.

That said, I am looking for some advise on register selection for basements in general. For cooling, round ceiling diffusers seem like a good choice with their horizontal throws. But for heating, not so good. I would like to spec a register/diffuser that will give the best comfort for heating and cooling, while keeping noise down.

On of my biggest areas of concern is the multipurpose/theater room. It is about 16x30 and I had planned to spec 4 supply runs into that room to share it's ~330 CFM cooling requirement at about 83 CFM each. For the Home Theater seating area on one end of the room, I don't want air blowing in anyone's face, and I need the registers in that room to be as quiet as possible while still providing enough air entrainment.

Can any of you guys offer me some input on what type registers to look at using for ceiling mounted basement applications? I've asked on the HVAC forums, but those guys are pretty touchy about anythig that even smells like DIY. Mine won't be DIY, I haven't the time or inclination to do the HVAC myself, BUT, I will be designing the duct system. I've seen plenty done horribly wrong, I can ensure that dosen't happen.

Just need someone with some experience to hold my hand a little with register selection. I've already noted Dennis' max velocity recommendation.

Thanks very much for the help.

Best, Scott
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post #7 of 53 Old 11-06-2008, 07:35 PM
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I can tell you that based on my experience, the most simple diffuser design (while not the prettiest) often works best. Case in point, if you go to movie theaters, you will notice that a lot of them use standard 3 or 4 cone diffusers and often square as they are installed in a t-bar ceiling. However, part of what helps them is the high ceilings. The biggest culprit of noise (aside from balancing dampers) is often the neck of the diffuser which is usually the last form of turbulence and hence noise.

As far as heating versus cooling, I wouldn't worry too much about the selection of diffusers the type of diffusers. What I would recommend is installing return grilles in the wall with a common plenum. A high grille that would be open in cooling mode and a low grille that would be open in heating mode. hen one grill eis open the other is closed. Trying to direct air without any significant velocity is difficult and then you start to introduce noise. Placing diffuser too close to each other causes "dumping" of air downward approximately in between the two diffusers (this is dependent on their throw patterns; velocity at specific distances from the diffuser).

The best diffuser would probably be a round 3-cone diffuser from EH Price or Titus. I have found that the fancier the diffuser, the more noise you end up with. The larger neck sizes help to reduce the air velocity prior to the air being diffused. If however you are looking to achieve a specific design style then you may want to choose another style of diffuser to suit.

I hope that is what you were looking for. Not too technical but I have found that the manufacturers numbers don't always provide the end result you expect.

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post #8 of 53 Old 11-07-2008, 07:03 AM
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Thanks Bruce. Yes, that's the jist of what I was looking for. I had some HVAC guys recommend just using a (high end/aluminum Price) rectangular register with 2-way adjustable vanes. Basically have 2 registers wash the one long exterior wall I have (~32', 2 windows, 1 SG door)) with air in the "down the wall" configuration. I only have the one exterior facing wall that's above grade, figured that might be a good idea. Then on the non-exterior side place some (off center) round ceiling cones which could throw across the ceiling and toward the closer wall. That way I wouldn't have 2 output streams colliding in the middle casuing a downwash onto the HT seats. Maybe .

I'm pretty restricted on return locations. I have an open truss ceiling structure and considered using it as a open plenum. But that provides a huge path for flanking noise. The other more preferabl eoption was for a low return in a periscope configuration or topped into the truss plenum area. Line with acoustic material to help flanking some. But except for the open ceiling return registers, which I could put anywhere; I'm very limited on a practical wall register location that won't add too much to my pressure drops.
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post #9 of 53 Old 11-07-2008, 07:11 AM
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What's good to look for in duct liner? The branch runs in this room will be flex as there is no other way to get solid trunks into the trusses. I had figured some duct board trunks but I haven't found anyone around here skilled in proper duct board construction. I've never see what the liner materials look like. Is it just duct board? Do they make it for round?
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post #10 of 53 Old 11-07-2008, 09:06 AM
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Quote:
Originally Posted by GetGray View Post

What's good to look for in duct liner? The branch runs in this room will be flex as there is no other way to get solid trunks into the trusses. I had figured some duct board trunks but I haven't found anyone around here skilled in proper duct board construction. I've never see what the liner materials look like. Is it just duct board? Do they make it for round?

Liner material is usually a fiber-faced fiberglass. There is also a closed cell foam based liner that is used by people paranoid of fiberglass strands being released into the airstream but this is typically only for high velocity systems. The foam, while easier to work with, has poorer acoustic absorption qualities.

The liner is typically tacked into place with weld pins that have a "fender washer" type of head for greater surface area.

Fiberglass liners not typically available for round (I have not seen any). Round duct applications usually get silencers or some have foam liners.

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post #11 of 53 Old 11-07-2008, 09:17 AM
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Quote:
Originally Posted by GetGray View Post

Thanks Bruce. Yes, that's the jist of what I was looking for. I had some HVAC guys recommend just using a (high end/aluminum Price) rectangular register with 2-way adjustable vanes. Basically have 2 registers wash the one long exterior wall I have (~32', 2 windows, 1 SG door)) with air in the "down the wall" configuration. I only have the one exterior facing wall that's above grade, figured that might be a good idea. Then on the non-exterior side place some (off center) round ceiling cones which could throw across the ceiling and toward the closer wall. That way I wouldn't have 2 output streams colliding in the middle causing a down-wash onto the HT seats. Maybe .

I guess I would need to know a model number of the register they were recommending. It almost seems from the description that the HVAC contractor was recommending a linear bar diffuser. Again, they need a properly constructed plenum (which takes up some real estate) in order function without excessive noise. Otherwise this sounds like it will work well (once you get your returns sorted out).

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post #12 of 53 Old 11-07-2008, 11:26 AM
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No just a normal rectangle. The trusses run perpindicular to this wall so the bar difusers aren't a good fit. Unless I try to fit them into a soffit.

I think the others were the model 500 series.
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post #13 of 53 Old 11-08-2008, 11:52 AM
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Agree with everything Bruce said. Just remember the basic principles associated with reducing HVAC noise. You will either be hearing the sound from the electric fan in the furnace/air handler traveling through the duct work, or you will be hearing air turbulence created at the register and within the duct. Usually both. No register will magically fix either.

Work the problem from as many fronts as possible given the constraints of your home. Locate the first register as far from your hvac unit as possible. Run the fan as slowly as possible. Introduce 90 degree turns in duct work to block fan noise traveling through the duct. Avoid long straight duct runs which allow sound to travel freely. Fiberglass duct liner is most helpful in absorbing fan and air noise. Minimize air turbulence with lower air velocity; extra large duct and slower fan speed. Select registers with minimum vanes and extraneous parts to cause turbulence. The more open the register, the better. Do not allow dampers at the registers, place then in the duct further upstream. I'll mention again the value of duct liner. If you cannot afford or do not have the space in all of the duct, line 6 or 8 feet before each register. Avoid unlined round pipe. Lastly, as Bruce alluded to, distance from the registers to you listening position does matter. Locate your registers as far from your main seating as possible.

Good luck. Hvac noise is always difficult.
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post #14 of 53 Old 11-09-2008, 04:55 AM
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I spec'd Hart Cooley registers for my house. They don't offer specs down to 250FPM, but they do go down to 300FPM.

Looking at the chart for a A612 2-way register, you could use a 12x8 register that would produce 300FPM at 95 CFM. Without going through the math, you can interpolate that you will get less than 300FPM at 83CFM.

If you really want to do the math, the Ak is .320

The Hart Cooley website has all the register specs.

Tim
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post #15 of 53 Old 11-09-2008, 06:36 AM
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Hart & Cooley also has specific noise ratings (Noise Criterion) numbers for their registers and diffusers at various flow rates. If you shoot for NC<20, you will be in great shape!

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post #16 of 53 Old 11-09-2008, 06:44 AM
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Thanks guys. Much appreciated.

Tim, where are your registers located? Ceiling pointed down, or sidewalls?

Thanks,
Scott
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post #17 of 53 Old 11-09-2008, 06:50 AM
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Quote:
Originally Posted by steve3366 View Post

I'll mention again the value of duct liner. If you cannot afford or do not have the space in all of the duct, line 6 or 8 feet before each register. Avoid unlined round pipe.

I'm trying to get the branch runs up into the trusswork (running perpindicular to them). So lined rigid duct won't work there. I *may* be able to run them in soffits, I'll have to study the sizes where teh CFM is greatest. But assuming flex, how do you feel about the "acoustic" flex in lieu of lined duct? I would have sizable lengths of this flex from boot to the main trunk. >25' mostly straignt runs in 2 cases, Probably about 10' with bends on the other 2. This acoustic flex (if I can get it) appears to have a fabric inner liner instead of a plastic film.
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post #18 of 53 Old 11-09-2008, 07:21 AM
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Quote:
Originally Posted by GetGray View Post

Thanks guys. Much appreciated.

Tim, where are your registers located? Ceiling pointed down, or sidewalls?

Thanks,
Scott

Both.. sidewalls in the basement and first floor (with 2 exceptions) and ceiling on the second floor.

Of course, haven't had the money to complete the system.. baby steps..

Some models of the Hart Cooley registers can be used in either sidewall or ceiling. I used a lot of 8x4's because of the low design velocity of the system and the throw problem like you mentioned. The returns were obviously bigger.

I see where they make reference to the nc criteria Terry mentioned.. but I don't see the actual data for the grills. Maybe it's only available on commercial diffusors?

Tim
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post #19 of 53 Old 11-09-2008, 07:36 AM
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Nothing wrong with acoustic flex duct. We often do not allow contractors to use more than about 6 feet prior to any diffuser as they can sometimes get lazy and allow for too much bend and hence restricted airflow. If installed properly they work fine. If you intend to put bends in try to stick with 90s versus 'hairpin' loops. However, if you account for the reduced airflow by oversizing the flex duct or with increased number of branches, that will be fine too.

I like that you can get fabric-lined versus plastic-lined flex duct. That will definitely yield better absorptive characteristics. Not knowing what is upstream of the straight runs, when we design branches off of a plenum, there are usually about three bends prior to the diffuser. 90s will help to reflect noise back towards the source versus transmitting down a straight length of duct. Just something to consider.

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post #20 of 53 Old 11-09-2008, 08:03 AM
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Thanks guys. Bruce, my "critical path" (highest PD) as I plan it right now, will be a straignt run, ~26-27 ft of the acoustic flex (if I can get it) pulled mostly straight, with the boot, and a wide radius 45 and 90 (turn up into trusses, then perpincidular to trunk) from a round trunk at the source end. If I can find anyone around here to build a ductboard trunk in this one room section I will, but so far I don't trust any of them to do it right. Duct board is unpopular around here for whatever reason, not knowing how to do it right is my expectation.

For the other branches, I'll have PD to spare so adding some bends shouldn't be a problem, but I'll run the PD numbers for them to be sure I stay well below the worst run. I'm going to us a VS blower, and I want to keep the PD as low in it's range as possible so it stays more quiet, too. AC coil eats about 0.1"wc, I plan to make the returns wide open, can have too little PD there as I underrstand it.

I need to introduce some makup air so I need a good filter to clear the allergens for the fam, as we all have them. Blower willl do it's required CFM from 0.1 to 0.5". I'm still working on the media filter choice, but they appear have high loaded PD's even at 600CFM and merv11.

Thanks, Scott
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post #21 of 53 Old 11-13-2008, 01:32 PM
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A follow-up question. Dennis' comments about the bar diffusers and the conversation above made me look. As it turns out, I think I could completely encapsulate my room in drywall, then build soffits (soffits planned anyway). Soffits woudl be about 12-13" tall, 3' wide. I have plenty of room in there for an ell and one of the factory made plenums for the bar registers.

I would have 4 of the bar registers in the room. They would have to be in the soffit. As they throw horizontally, and over the "lip" of the soffit, I'm not sure how that woudl perform, good or bad.

As for fit, it would appear I have plenty of room for the factory plenums, except for one place where the main trunk would run beside the factory plenum. I could attach this one plenum directly to that duct with a "T" right into the plenums fitting on it's back. I didnt' see any plenums with fittings on their ends.

2 questiosn:

1. Would 4 such bar diffusers be suitabl efor air circulation in the room?
2. How big of a no-no is it to hang that diffuser plenum directly off the feder duct with a T?

Thanks! Scott
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post #22 of 53 Old 11-13-2008, 04:09 PM
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You could use the soffits as a plenum. They would need to be lined with duct liner.

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post #23 of 53 Old 11-13-2008, 04:26 PM
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Quote:
Originally Posted by Dennis Erskine View Post

You could use the soffits as a plenum. They would need to be lined with duct liner.

That's what I did. The HVAC ducts penetrate the ceiling, but terminate there. The HVAC contractor had made a fancy custom sheet metal ductwork to go from the ceiling to the register. Then one of the carpenters came up with an idea to add a couple of internal partitions in the soffit (at each supply and return) to create a plenum. I then lined each "box" with duct liner. In the end, simpler and cheaper.

EDIT: Need to clarify that I have a separate ceiling penetration for each supply (4) and return (2). Now that I think about it, would it have been better to have one penetration each and use the soffit to rout the air? This would have eliminated 4 penetrations in my "fish bowl". Oh well. Can't change it now and the sound seems to be contained pretty well.
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post #24 of 53 Old 11-13-2008, 04:39 PM
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Quote:
Originally Posted by Dennis Erskine View Post

You could use the soffits as a plenum. They would need to be lined with duct liner.

But I need to have individual dampers on each diffuser to balance the system. If the soffit was the plenum and the Bar diffusers were built into the soffit, how do you balance?

Also, I wouldn't have much control over the duct velocity. They are pretty long runs. I'd be concerned abut condensation in them if they don't have some velocity. I planned to take you advise and keep the face velocity < 250fpm, but internally in the supply at around 400 fpm.

On the other hand, if you feel it's OK to have the bar diffuser effectively "built into" the air trunk (soffit in this case), then I'd expect having one hanging right off of a trunk line with a T wouldn't be much worse for noise.

Thanks,
Scott
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post #25 of 53 Old 11-14-2008, 05:20 AM
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Where code allows, it works fine and the technique of blocking the soffit internally to create supply and return plenum works well. As to velocity...the bigger noise problem is velocity at the diffusor, not within the duct itself.

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post #26 of 53 Old 11-30-2008, 09:51 AM
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Quote:
Originally Posted by Talented Amateur View Post

Nothing wrong with acoustic flex duct.
I like that you can get fabric-lined versus plastic-lined flex duct. That will definitely yield better absorptive characteristics. Not knowing what is upstream of the straight runs, when we design branches off of a plenum, there are usually about three bends prior to the diffuser. 90s will help to reflect noise back towards the source versus transmitting down a straight length of duct. Just something to consider.

Starting new theater in exsisting room. I need information on where to start with HVAC considerations. I am certain current ducting and flow not adequate.
Room = 12' x 22' 6" x 10' 4" high (with existing standard 2x4's and one layer of drywall installed. No windows. Currently one 6" duct above where equipt closet will be, at rear of theater. Two doors, one flush with rear and one 6' from front. Rear door will be enclosed to create the equipment (except projector) closet outside of stated space with it's own ventilation into a large storage room to reduce btu load.
Considerations: This room is in lower level and all walls (4") and ceiling (6") are reinforced poured concrete. Initial plan is to have small stage, four rows (3x3 & 1x4) seating. My concerns are size requirement for additional ducting? And how to best distribute throughout room taking into consideration concrete walls/ceiling. Suggestions are requested.
Thank-you
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post #27 of 53 Old 11-30-2008, 03:50 PM
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To get the manual-J loads for the room, a fairly easy to use calculator is avail from hvac-calc.com. About $50 for a residential use copy. Enter the physical charasteristics of the structure. Don't enter a ceiling if conditioned space is above. Enter the number of people you will have in the room (looks like you have a lot). Add a misc load for any AV equipment (stated wattage x 3.4 to get BTUH IIRC). The program will give you the heat and cooling loads for the room, along with CFM requirements. Take the CFM and split into the # of supply registers you need. Keep register face velocity below 250fpm per D.Erskine. I sized my ductwork to keep supply (acoustic flex) velocity at 4-450fpm.

HTH, Scott
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post #28 of 53 Old 01-27-2009, 03:08 PM
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Ok guys, I'm picking registers and diffusers. Here's my space: two supplies at the front (5"), and one supply at the back (8").




I was thinking about using two of these for supplies mounted on the underside of my soffit. They come in six inch duct sizes:

http://www.price-hvac.com/catalog/C_...Request=RCDE_1




And one of these at the back of the room in the soffit:
http://www.price-hvac.com/catalog/D_...geRequest=80_1




Any of the HVAC guys have issues with this? I am concerned about the return being off to one side of the room, but I don't think it can be helped...unless I run a lot of ducting inside the soffits and a lot of the space will be taken up by electrical and can lights.
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post #29 of 53 Old 01-27-2009, 03:48 PM
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The round returns are meant to "throw" the air horizontally. If it's in a soffit it will throw it against a wall. May be better with some other ceiling type diffuser.

HTH
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post #30 of 53 Old 01-27-2009, 04:06 PM
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The diffuser you chose is designed more for center of the room location. I would recommend the hart and cooley 821 light commerical diffuser. It is four way directional and has wide vains. Also is availble in may sizes. As far as the return placement, where you have it is perfect placement. You want to flip the air over.

psn: nameless_hero81
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