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# HVAC Redo/Retrofit - Page 2

Got it. Both return and supply should be placed high. I was just looking at vents/registers the other day and I didn't see any with a 6" hole, they usually are rectangular. I won't get too caught up in that at this point. I am still curious if there is a certain type of register that is preferred and what size, based on such a small room. Do you typically try to match square inches of the flex duct to a percentage of the area of the register? Just curious.

Were the calculations (volume/exchange) that I showed in the ballpark (realizing mileage may vary, of course)? I'm guessing the length of duct would also come into play along with temperature differentials . . . etc. I'm just curious if I'm headed down the right path. I might just have to try it and see how comfortable it is. I think I will be running the flex duct along the back wall (eventually in a soffit). Actually, I was contemplating boxing it out and lining with linacoustic since I have very little head room. I will tie the projector into that (flex-duct or linacoustic lined soffit), having the air flow through the projector first of course.

I'll keep following your build. Thanks again for your input! I really appreciate it. I wish I could provide some of the 3D graphics that a lot of people seem to show, but I haven't got the hang of Sketch-up, yet.

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The size of the register, and the associated plenum behind it, are generally chosen to keep the flow rate below 250 fpm. For a given fan or duct size, you have an estimate of the cfm. Let's say you actually get 75 cfm from a 6" duct. The cross sectional area of that pipe is 0.785 ft^2. That means the air is flowing at 96 fpm (75 cfm / 0.785 ft^2). Similarly, if we want to keep the speed down and not restrict flow, then we need to put a register with a similar cross sectional area, so maybe a 12"x12" register. You can go to home depot and look at the register sizes they have and find one that works.

To add a little twist to things, let's say you put in one of those 300 cfm Panasonic fans. They have a 6" connection, so now you're moving 300 cfm through that same pipe. The result is your air is flowing at 380 fpm, which is much higher than the oft quoted 250 fpm goal. So, you'd have to have a larger grille and plenum to get the air slowed down before it enters your room. Otherwise, you'll hear the wind noise across the grille.
Excellent. Thanks again!
So, that magic 250 fpm is a maximum, right? As long as I am less than that, I shouldn't need to worry about hearing it. I had a 6" flex duct connected to the main trunk with a much smaller register/vent before and the thing whistled like a construction worker on his lunch break.
Yes. Less than 250 fpm is good.

I will add a caveat. Turbulent flow is unpredictable, so it is possible to have an average flow of less than 250 fpm with local values much higher. Sharp transitions or direction changes are generally the culprit. But that's one of those things that you address when they come up. The high end builds will use a bar diffusor to prevent that sort of thing.
Excellent insight, thanks again. I will make sure to take that into account and listen for any wind blowing through the system to make sure I minimize any flow restrictions.
A quick update. Last night I spent some time plugging the existing supply from the ceiling. I noticed about a half dozen nail tips coming through the floor of the room above, which had missed their intended target joists. I'd read that those can provide a path for sound to go into that other room so I cut them off. The only thing I had handy was a hacksaw. Had to remove the blade to get to the small space.

Not fun and time to get a dremel or grinder.

Oh yeah. I got some of those 6" concrete molds to possibly use as a duct between the room and a dead vent (ala Ted White's dead vent diagram). I checked the outside diameter with a piece of flex duct and voila - perfectly snug fit

Might be just what the doctor ordered. The sheet metal piece on the left is just a 6" coupler (crimped both sides).

Now I to need decide if I should just eliminate the can lights altogether.
Edited by angryht - 11/29/12 at 1:00pm
^Decision made, can lights will be removed and replaced with a few wall sconces. Not excited about taking down the ceiling but oh well. . .
I continued to remove more ceiling to be able to access the between the joist for electrical over the weekend. My progress is very slow because I encountered more nails sticking out (missed their intended joists) so I am using the hacksaw blade to remove them, again. I hope to be able to replace the can lights with some wall sconces placed on the side walls (possibly rear wall too). Here's a few pics showing my slow progress.

This pic shows the front of the room with the soffit. Directly above the soffit you can just see the joist/duct for the main trunk of the return. Is it typical to install some sort of block (ie MDF) to help prevent sound from getting into that? I'm just not sure insulation would do the trick.

This just shows the back of the room (projector is under the blanket).

I've also had to cut some drywall from the walls so I can access for the wiring but you can't really see that in the pics. All of the can lights will be removed.
The supply ducts in the trusses are shown here:

They make quite a racket when someone walks on the floor above. It seems like since they are sheet metal, they make a noise when the floor flexes. I would think they would be easily replaced with the cardboard type duct work I've seen at Menards/HD. Then I could put a piece of linacoustic in there as well.
Something like this:

So far I've only found one reference to Thermopan here. Is there anyone who has used this with success. Seems like a simple no-brainer!
Edited by angryht - 12/4/12 at 6:16am
Here is my 'to do' list. It's just a rough sort of thinking out loud list at this point:

1. Replace sheet metal joist headers with 'thermopan' I-Joist Header (see previous post). There are 2 that are accessible. Installation of the new will include lining with Linacoustic.
2. Complete installation of new wall sconces. I think I will go with these: http://www.lowes.com/pd_294377-43501-FB08-034_4294715753__?productId=3031021&Ns=p_product_avg_rating|1&pl=1&currentURL=%3FNs%3Dp_product_avg_rating%7C1&facetInfo= because they are small and affordable. There will be a total of 4 (2 on each side wall).
3. Reinstall insulation (R19 in the ceiling) and 13 in the walls.
4. Replace ceiling and wall drywall that was removed. This may include a new piece at the conduit penetration for the projector.
5. Install new flex duct along the back wall (in ceiling/wall corner) and determine exit configuration. This will likely be routed straight out of the equipment closet. Possible soffit and hushbox for the duct that will run along the back wall.
6. install new duct for supply from the right side wall (right side near top of wall). Verify runs to adjacent room.
7. Determine if I will build a dead vent or other means of keeping sound controlled for exhaust and supply.
8. Provide better access to water shut off in back left corner and water heater. Currently they are just a pieces of plywood screwed into the opening. Perhaps an MDF access hinged door.

Again, just thinking out and trying to get my 'stuff' together. This started as a HVAC redo but it has become more of an extreme makeover.
What are the dimensions of the ducts that are making the noise?
Quote:
Originally Posted by J_P_A

What are the dimensions of the ducts that are making the noise?
They are the headers for the return. They are standard in-joist headers between the joists. You can see them in the photo if you zoom in on it. Sorry, I probably didn't make the photo large enough. They make sort of that popping sound when someone walk over the floor above.
Took down the face of the soffit and here are the pics.

Any thoughts on removing the sheet metal joist header and replacing it with a thermopan lined with Linacoustic? Any input sure would be appreciated.
I know the sound you are talking about. The floor/ducts in my old house did the same thing. I believe you are correct that it's the metal flexing, so I don't see any issue with replacing the metal duct with a ductboard of some type. You just need to make sure you maintain approximately the same internal cross sectional area.
Thanks again, J_P_A. That noise drives me nuts and initially I thought it was just subfloor and joist creek but the more I heard it, the more I heard a metal popping sound right at that location. My house was built in the eighties so it's fairly old.

I'll keep the overall final area in mind so I don't restrict flow to the main supply duct.

Did you do anything to address it at your old house? I was also wondering about replacing the actual duct in addition to just the joist headers but is probably a bit beyond my comfort level at this point.
The Thermopan site calls it "oil canning" which describes it well.
Quote:
Originally Posted by angryht

..........

Yeah..... We moved out
That's certainly an effective way to deal with a situation!

I'll need to dig deeper into that website. That describes what I think I need just about perfectly.
Is 3M 77 the appropriate adhesive for Linacoustic in ducts?
Update: I stopped at Menards today and picked up a couple of the thermopan joist headers. I took out the sheet metal headers and just slid in the thermopans to see if the 'oil canning' noise would be eliminated. By the way, oil canning describes the noise to a tee - just like tin-man in Wiz of Oz! I didn't put in any Linacoustic . . . .yet anyway.

I am thrilled to report that the noise is completely gone! It doesn't take much to make me happy but man I'm lovin it. I'm still unsure about the adhesive and, of course, treating any cut and exposed edges with sealant. Not sure I can find those items around here. Thoughts?
A couple of pics showing the installed thermopan and the joist duct. Hey. . . . what about putting Linacoustic on the outside of a duct? Doesn't that make sense if you are trying to prevent sound getting into the duct? Maybe I'm over simplifying. Maybe it's better to fill the space between the drywall and the duct with the pnk fluffy stuff??

When a look at my second photo above, it sure looks like some Linacoustic would fit nicely in that joist-space. If I calculate the area it is approximately 15"x9" = 135 square inches. If I were to line the entire 4 sides with 1" linacoustic it would reduce the area to 13" x 7" = 91 square inches. That would be far too much of reduction in area (about 33 %). But if i just put a 1" piece in the bottom of the space, the reduction is only about 7% (14"X9" = 126 square inches). That seems reasonable especially considering there is another joist space on the other side (right side) that feeds the same trunk. If the one piece is placed in the bottom of the truss space then the only exposed edge would towards the camera and I would make sure that is not a cut edge and no spray adhesive would be required. Does that make sense or am I spinning my wheels?
Hello?
A couple of updates. Progress is slow. I cleaned up some of the dust within the joist cavity using the shop vac. I also placed one piece of 1" thick Linacoustic on the bottom of the joist cavity (as shown on the picture) just to help a little with some sound absorption and only minimally reducing the cross sectional area. I also placed one piece of 1" linacoustic on side of the main duct that was accessible (running perpendicular and below the joist cavity shown in the photo. And one more piece of linacoustic on the end (header) of the main duct (left side of the photo). I installed the Thermopan headers and I did line those with Linacoustic. So, overall I put some strategic duct liner to help with some of the sound but not enough to restrict the air flow. I did not take any photos with the linacoustic installed, the references to the photos in the previous post are just to explain where they are located.

I picked up 4 of the sconces mentioned earlier from Lowes. They are cheap but I kind of like them:

and I'm redoing some wiring for those. I made the holes for the remodel electrical fixture boxes. Actually, one is a new construction because I removed more drywall. Once I get those wired and hung I should be able to continue with the HVAC. Again, my plan is to run an exhaust out into the adjacent room and a supply fan from the adjacent room separately and try to take advantage of the cool air instead of tapping into the main ducts of the HVAC. I'm not sure if I'll build dead vents until I see how much noise goes into the adjacent room. I've also found some acoustic duct supplier near my house at a fairly reasonable price (less than \$2.50/LF)
Oh yeah, I also stopped at Menards this weekend and was surprised to see Green Glue was being sold. I didn't realize they had been bought by Certain Teed and carried by Menards: http://www.menards.com/main/building-materials/drywall/accessories/green-glue-noiseproofing-compound-setting/p-1725463-c-5655.htm
Wiring questions/confirmation: I need to wire the sconces and I have power running into the first fixture. Previously I wired the first fixture directly to the switch and just used 2-conductor cable between. This time the power source is further away, so I was going to run a 2-conductor cable from the power source to the first fixture, then 3-conductor cables between each fixture and to the last fixture box to the switch box. Like this:

^Well, that wiring worked like a charm.

Now, who's gonna fix all the drywall I had to remove to get access to all the wiring?
Back to some HVAC theory. I think I need some help with this:
Quote:
Originally Posted by J_P_A

The size of the register, and the associated plenum behind it, are generally chosen to keep the flow rate below 250 fpm. For a given fan or duct size, you have an estimate of the cfm. Let's say you actually get 75 cfm from a 6" duct. The cross sectional area of that pipe is 0.785 ft^2. That means the air is flowing at 96 fpm (75 cfm / 0.785 ft^2).

Shouldn't the cross sectional area for the 6" duct be more like of 0.196 ft^2? That would make the velocity closer to 382 fpm (75 cfm / 0.196 ft^2).

I'm a little confused. If I verify the air exchange rate of 6 based on my room volume, I get close to the 75 cfm. But when I size the duct based on the fan, which is rated at 160 cfm for free air, I get a different number all together (160 cfm / 0.196 ft^2 = 457 cfm). But supposedly I can only expect 75 cfm from a 6" diameter duct: http://www.avsforum.com/t/1339540/the-plains-theater#post_20530565. It would seem to me that first you determine the air exchange rate, then you pick a fan. Then verify the fan will not exceed the 240 cfm based on the size of the ducts. If my fan is 160 cfm and my target velocity is 240 (less than 250), then 160 cfm / 240 fpm = 0.67 ft^2 and that would require a 1 ft diameter duct (or a 10" by 10" square). Is that correct?
Quote:
Originally Posted by angryht

Back to some HVAC theory. I think I need some help with this:
Shouldn't the cross sectional area for the 6" duct be more like of 0.196 ft^2?

That math is correct.

I would:
-figure out how many cfm you have to move to achieve the desired ACH.
-select a fan to meet that cfm goal
-size the duct to the fan

I see the 250fpm number used a lot. I know the face velocity at the grill should not exceed 250fpm.. I don't know that necessarily equates to 250fpm in the duct.

Tim
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