New Posts  All Forums:Forum Nav:

HVAC Redo/Retrofit - Page 3

post #61 of 72
12/13/12 at 7:21pm
Quote:
Originally Posted by Mr.Tim View Post

That math is correct.

+1 I forgot to divide by 2 (or 4 depending on your tastes).
Quote:
Originally Posted by Mr.Tim View Post

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

+1 again.

The 75 cfm listed in the chart you referenced is what you might expect to see from a properly sized 6" duct supplied by your HVAC air handler. My understanding is that it is a rule of thumb and takes into account many factors such as frictional losses and velocity. An inline fan is going to list its maximum flow against very little restriction (that 160 listed is free air). It's a bit of an apples to oranges comparison.

I'm not sure where the 250 fpm number comes from, but I certainly agree that you don't necessarily need 250 fpm in the duct to get that at the grill. However, my gut feel is that if you establish a lower flow rate in the duct, and preferably a laminar flow at that, you will be less likely to have local turbulent regions at the register that are noisy. I think that's the issue that BIG and Morph1c had in the Black Cat Theater that caused them to upsize their ducts and plenum. Initially they were dumping a lot of fast moving air into a large box, but the transitions were too abrupt and it didn't have time to slow down. All that said, it's been a LONG time since I've had a fluid dynamics class, so I'm really just rambling.
Reply
Reply

AVS Top Picks

post #62 of 72
12/14/12 at 3:31am
Hart Cooley recommends 500fpm at the register for broadcast studios and "acoustically treated offices". There are no recommendations for anything less than that. That being said, Dennis has specifically stated 250fpm, which no doubt is based on his extensive experience in the area. I have no reason to doubt, and every reason to believe, 250fpm is what should be designed for at the face of the grill.

400fpm in flex duct is at the extreme low of industry standards, with 600 being used in a normal residential situation.

In the Black Cat there was 340cfm running through a 6" duct, to what appeared to be a 12x12 register (just guestimating on that one). That's over 1700fpm in the duct and 550fpm at the register. That's 3x the industry maximum fpm in a flex duct. This is supported by the fact they removed the grill and could still hear the problem. A 12" duct or 2-10" would likely have been acceptable at ~400fpm. Of course, when they redid it they took the opportunity to install duct liner, which solves a completely different issue. If the finished inside dimensions of their re-did joist bay after the insulshield were 8x12, they were still pushing over 500fpm in the joist bay and were happy with the results.

Tim
Reply
Reply
post #63 of 72
12/14/12 at 6:55am
Thread Starter 
Quote:
Originally Posted by Mr.Tim View Post

I would:
Quote:
-figure out how many cfm you have to move to achieve the desired ACH.
Okay. That number is between 75 and 80 cfm. It might be a bit less but I'll estimate high to stay conservative.
Quote:
-select a fan to meet that cfm goal
My fan spec says 160 cfm of free air. So, that number is probably high too. And I know I'll probably have some distance and bends between the fan and the discharge into the room. But if I stick with 160, again I'll be conservative. That's the standard inline duct fan. The 4" is rated at 65 cfm free air.
Quote:
-size the duct to the fan
Here is where I get confused again. If my fan gives me 160 cfm (I know it will actually be less - but follow me here) and since it is a 6" fan, I was going to use a 6" duct. 160 cfm / 0.2 ft^2 = 800 fpm. Does that just mean I need to compensate and provide a discharge into the room that is 160 cfm / 240 fpm = 0.67 ft^2, which is about 10" x 10"? That seems to make sense to me. Then, of course I would need to take into account that the area of the discharge (vent) would be reduced because of the slats/louvers. Isn't that number usually about a 20 percent reduction? So, that puts me at about a 12" x 12" vent. That makes sense to me.
Reply
Reply
post #64 of 72
12/14/12 at 7:10am
Mr. Tim, good info as always! I know that the 250 fpm number is generally referenced by Dennis, but I don't know where it comes from. I wonder if there is more to it than meets the eye. Similar to the 6 exchanges per hour business that doesn't just mean you need to move the air. I'm glad to have another documented reference to compare to, thanks.

Use a 6" duct for your fan, and If you've got the space I think a 12"x12" register would be a good choice.
Reply
Reply
post #65 of 72
12/14/12 at 7:18am
Quote:
Originally Posted by angryht View Post

Okay. That number is between 75 and 80 cfm. It might be a bit less but I'll estimate high to stay conservative.
My fan spec says 160 cfm of free air. So, that number is probably high too. And I know I'll probably have some distance and bends between the fan and the discharge into the room. But if I stick with 160, again I'll be conservative. That's the standard inline duct fan. The 4" is rated at 65 cfm free air.

Here is where I get confused again. If my fan gives me 160 cfm (I know it will actually be less - but follow me here) and since it is a 6" fan, I was going to use a 6" duct. 160 cfm / 0.2 ft^2 = 800 fpm. Does that just mean I need to compensate and provide a discharge into the room that is 160 cfm / 240 fpm = 0.67 ft^2, which is about 10" x 10"? That seems to make sense to me. Then, of course I would need to take into account that the area of the discharge (vent) would be reduced because of the slats/louvers. Isn't that number usually about a 20 percent reduction? So, that puts me at about a 12" x 12" vent. That makes sense to me.

I'm not sure what hey mean by free air. The fan will likely put out the 160cfm since the static pressure will be so low when the ducts are sized properly.

You want to size the duct properly, so you may end up using a taper reducer to connect to the fan. I mean, there's a 6" inlet on your fan and there's a 6" inlet on the 340cfm too, they can't both be sized properly. For a theater application neither of them are sized properly. For a bathroom.. maybe..

You are correct that the fpm through a 6" duct will be >800fpm. Too much. You are going to want to enlarge it to at least 8", with a 9" or 10" being preferred, but sometimes impossible to fit within the building constraints.

When sizing the register you want to calculate using the free area of the grill. I used Hart Cooley registers throughout my house because the engineering data is there. My local supplier had quality-looking stuff but I couldn't find data. For your application I would recommend the RH45. You could also use the RHF, which accommodates a filter, but you have to upsize it due to the filter.

Looking at the data you can see it only goes down to 400fpm, which is 100fpm lower than what they recommend in the quietest of situations. We're shooting for 250fpm, so you can just interpolate.. A 12x12 will handle 250cfm at 400fpm. which in rough numbers is 200fpm at 125cfm. If you want to get really technical, use the Ak, which as magical and mysterious as it seems is simply the free air of the register in square feet biggrin.gif

Tim
Reply
Reply
post #66 of 72
12/14/12 at 7:36am
Quote:
Originally Posted by J_P_A View Post

Mr. Tim, good info as always! I know that the 250 fpm number is generally referenced by Dennis, but I don't know where it comes from. I wonder if there is more to it than meets the eye. Similar to the 6 exchanges per hour business that doesn't just mean you need to move the air. I'm glad to have another documented reference to compare to, thanks.
Use a 6" duct for your fan, and If you've got the space I think a 12"x12" register would be a good choice.

Thanks man. I had to learn how to do it myself because I couldn't find a local professional who could figure it out. I don't think it's that difficult once somebody explains it.

The problem is that the HVAC industry is generally very protective of their techniques and job-leanred knowledge. Whereas a plumber or a carpenter is usually happy to tell somebody how to do something right, I personally have found that is rare with HVAC techs.

Being clear that while I may not agree with that approach, I do respect it.

Tim
Reply
Reply
post #67 of 72
12/14/12 at 7:57am
Thread Starter 
^ I guess that means I need to get a pretty big boot (from duct to vent), right? I think the biggest I saw was 6" by 14". I guess it's either that or build a plenum lined with linacoustic, which I would prefer but darn that Christmas budget!
Reply
Reply
post #68 of 72
12/14/12 at 8:02am
You probably want a side-outlet diffuser box and appropriate taper reducer.
Reply
Reply
post #69 of 72
12/14/12 at 2:37pm
Thread Starter 
Great info, guys. Thank you so much for the input!
Reply
Reply
post #70 of 72
12/28/12 at 7:19am
Thread Starter 
A brief update (and some photos) to my 'to do' list from a few posts back:
Quote:
Originally Posted by angryht View Post

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.
- Done. Well worth it. No more oil can noise! Thanks J_P_A for your help with that.
Quote:
[*] 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).
- Done. Ended up doing some rewiring and it turned out very good.
Quote:
[*] Reinstall insulation (R19 in the ceiling) and 13 in the walls.
- Done
Quote:
[*] Replace ceiling and wall drywall that was removed. This may include a new piece at the conduit penetration for the projector.
- Done. Did not redo the drywall piece at penetration but I'll try to seal around the penetration better. It's a bit patchy looking but I'm thinking I'll cover it with a combo of dark paint and treatments. All of the joints were sealed with acoustic sealant
Quote:
[*] 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.
- Still need to figure this out.
Quote:
[*] install new duct for supply from the right side wall (right side near top of wall). Verify runs to adjacent room.
- Done. Actually, I routed from the center of the room in the front, over the existing main ducts for supply and return, around the corner to the right and into the adjacent room. I plan on using it for supply from that room (with the furnace) and if I need to tap into the actual forced air system, I still have that option. My plan is just to exchange air with the adjacent room with an exhaust fan out of the theater room and one into the room.
Quote:
[*] Determine if I will build a dead vent or other means of keeping sound controlled for exhaust and supply.
- Right now I just have flex duct running to a duct fan which is behind the furnace. I really can't hear it when the door is closed so for now I'll probably leave it. I have thought about just putting it in a box lined with Linacoustic.
Quote:
[*] 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.
- Still looking for ideas on these. For the water access, I'd like to try something like a piece of MDF that just slides into place. This is because the couch sits right next to the access panel and just lifting a wood panel would be the best solution. In other words I can't swing out a door (hinges). Currently, I need to move the couch out of the way in order to access the screws to remove the panel. If there was a door that could just slide into place, that would be much better. I would think some simple L-shaped pieces on the sides of a piece of wood would ( smile.gif double words are funny) do the trick. Any ideas or examples of this would be appreciated.
[/quote]

Now the pictures:







I'm lovin' these sconces! Still a lot to do but I'm making progress.
Reply
Reply
post #71 of 72
12/28/12 at 7:31am
Thread Starter 
Here are a couple of photos of the hole in the ceiling (penetration) to the adjacent room - supply via duct fan.





I plan on building out the soffit (just to the other side of the penetration), installing a few baffles to break up the path of sound, lining it with linacoustic then installing a vent on the bottom of the soffit. Sorry about the dark photos.
Reply
Reply
post #72 of 72
1/17/13 at 6:48am
Thread Starter 
Here's an update. I completed what I will deem the cheapest hushbox I could come up with. It's a plastic walmart bin flipped upside down and mounted to the ceiling, lined with some strategically placed linacoustic. There is a flexduct that discharges the hot air from the projector and is sucked out via inline duct fan from the adjacent room. Here are the pics:

This is the bin mounted to the ceiling with the holes cut out for the air. I used a 6" diameter plastic cylinder mold (mentioned earlier) to connect the flexduct. It's the white cylinder sticking out of the side. The projector is not attached to the mount, which is just a 'monkey man' type diy. So, the top of the bin (actually the bottom but top as it's mounted) has a hole for the penetration into the 'hushbox'.


Here is another shot of the same but a slightly different angle.


Here is the box with the flexduct attached. It's just typical 6" diameter flex duct (not acoustic). The projector has been mounted inside the bin and the flexduct actually goes behind the 'hushbox' after it bends around from the outlet. You can see the duct goes to the penetration above the equipment closet. It actually has a small hole where it goes through the equipment closet to help pull some of the heat out of that area as well.



Here is the 6" cylinder (actually a concrete test mold) I discussed earlier in this thread. I used a few of these for my penetrations with the bottom cut off.



Fan:


Then I just put the linacoustic back on the back wall and covered the duct with some black fabric so that it disappears.



There is linacoustic (1" thick) inside the box. I put 2 layers on the bottom of the 'hushbox' (again, upside down bin, it is actually the lid). Here it is in action.


This was very inexpensive (and it kind of looks that way - at least when the lights are on). But it is effective. The input (hole in the back) is right next to the projector input filter/fans and the discharge on the side is right next to the projector fan discharge. I did the paper trick to check that there was good air flow - pulling air outside of the box. The paper trick is just me taking a piece of paper and placing it over the hole in the 'hushbox' to make sure it sticks - and it does quite well. And, it appears at least, that the air is not being short circuited from the other input holes. Just thought I'd share. I may try to wrap some fabric around the box to hid the fact that it's a plastic bin. The fan a 6" diameter inline duct fan, Suncourt, and discharges into the adjacent room which has the furnace and water heater. I just plugged it in to a wireless remote extension. I have not done anything, yet, to automate it with a thermostat. For now I just make sure the fan remains on well beyond the cool down time for the projector after shut off.

It's very effective at removing the hot air from the room and I have another inline fan to circulate air back into the room via the new penetration mentioned in the previous post. For now I have just put a register (vent) over the hole.

I'm always very inspired by all the nice wood hushboxes I see being installed but I just don't have the skill or budget for something like that (wish I did and maybe someday). This'll do for now!
Reply
Reply
New Posts  All Forums:Forum Nav:
  Return Home

AVS Top Picks