Question on Ventilation and Air Pressure

I have an exhaust 6" inline fan 409cfm running at half speed with some 4" (size of holes in wall) duct, 6" duct, and 8" (size of duct silencer) duct. The intake fan is a 6" 150cfm centrifugal running also at half speed with mostly 4" duct. I've been experiencing frequent ear popping. Is this because of the difference in air pressure caused by unequal intake/exhaust cfm's, or from using 4" duct with 6" fans? Also, many people just use passive intake with no fan, but doesn't this create suction on the door, making it difficult to open?

Okay, so you have: Intake 75cfm Exhaust 205cfm Difference -130cfm So your room is at negative pressure. The general advice on this is to have positive pressure in the room. This means that upon opening the door air comes out, meaning any dirt or debrit from outside is pushed away. If you have negative pressure, this is drawn into the room instead. Assuming you have built your room well then it should be 99.99% airtight, so having such a large input/output ratio is un-neccesary, and working the fans too hard. If you can, try your intake at full speed (150cfm), and your exhaust so that is slightly lower. The closer they are, the less of a pressure change you will feel when you close the door.

The cfm numbers I posted are at 0" and 0.1" static pressure for the 409cfm and 150cfm fans, respectively. Static pressures are higher than that, due to used of 4" duct and length of duct runs, bends, etc. The way I have it right now, there is a slight but quite noticeable suction on the door. I could try running the 150cfm fan at full speed for slight positive pressure, but it would be too loud (it's a bathroom fan). Would modifying the setup by enlarging the holes in the wall to 6" and using 6" duct help? I figure the circular area of the duct would be approximately doubled, so wouldn't that allow me to run it at full speed with the same noise that I get now at half speed with 4" duct?

The cfm numbers I posted are at 0" and 0.1" static pressure for the 409cfm and 150cfm fans, respectively. Static pressures are higher than that, due to used of 4" duct and length of duct runs, bends, etc.

OK, but static pressure applies to the entire system, from intake register to exhaust register, not for individual segments, so both fans "see" the same static pressure. Therefore, the fans should be identical, not different, and should be run at the same speed, not different, regardless of how the duct is sized.

Would modifying the setup by enlarging the holes in the wall to 6" and using 6" duct help?

That would reduce air velocity and static pressure, which in turn would reduce the noise generated by air movement inside the ducts, but that won't fix the imbalance in fan volumes.

I figure the circular area of the duct would be approximately doubled, so wouldn't that allow me to run it at full speed with the same noise that I get now at half speed with 4" duct?

Duct size does not affect FAN noise. It only affects air-motion noise, which is the noise created by the air itself moving through the ducts, not the noise created by the fans. Those are two different aspects of HVAC noise. What you should do is first figure out how much air you need to be moving, in terms of "room changes per hour", based on room volume / occupancy, convert that to CFM, then figure out what size ducts you need to be able to move that volume of air at air velocity less than 300 fpm. Then you figure what the static pressure will be in those ducts, and based on that you look for fans that can provide the necessary volume of air (CFM) when faced with that static pressure load. - Stuart -

I just finished enlarging the holes in the wall and duct box from 4 to 6" and am now running 6" duct only to the exhaust fan. Intake is now passive for the time being, due to a reduction in the suction on the door. I have a very annoying problem now though that I didn't have with the 4" system. There is now a faint low frequency rumble noise inside the box when the door is closed. I suspect it might be due to the presence of the midsection, a double drywall panel with green glue to divide the intake/exhaust sides. This is my duct box, with intake on the left side and exhaust on the right side. The box is made of double 5/8" drywall with green glue on all sides except the one facing the wall of the booth. The 2 sections of metal duct that go through the wall have 2 1/16" layers of sound dampening pipe wrap. Inside each side of the vent box is insulated flex duct with fiberglass insulation batt tucked in the empty spaces. http://imgur.com/a/ituFO I find that if I pull the duct box out a few inches from the wall of the booth, the rumbling is gone. The rumbling might also be due to the midsection. The midsection I had before, had 2 open gaps on the top/bottom where you see the two small pieces of drywall in the last pic. It also only had one layer of drywall. Does anyone have an idea on how to get rid of the rumbling?

Looking at the images and reading your description, I'm not at all sure that I understand what you did there, or why you did it. You seem to be saying that you built a silencer box, but then instead of putting baffles inside it, you ran a piece of flex duct through it? If so, then I don't understand the purpose of the box at all! Why not just use the flex duct by itself? Or alternatively, I don't understand the purpose of the flex duct: Why not just build a normal silencer box, and have the large added benefit of the impedance mismatch at the entry and exit points? Did you bend the flex duct around inside that box? If so, it is probably kinked and therefore creating turbulence and noise due to the velocity and pressure changes, as well as the direction changes. Or maybe I'm just not understanding your description correctly.... Maybe you could post photos of how that box is built, or perhaps add the details to the 3D model you posted, so we can see exactly how it is built, and how to fix it. - Stuart -

I have updated the album with pics of the duct box. http://imgur.com/a/ituFO I tried doing 2 180-degree bends in the duct on the exhaust (far) side and it seems to have helped quiet down the rumble. Fan noise outside the booth has increased a bit though. Before, the exhaust duct was the same as on the intake (near) side. The duct box was made based on a design I bought online years ago. It hasn't worked out and I've had to make many changes to it through the years. I would've gone with silencer boxes instead, but didn't know about them then and would prefer not to build them now. Also, the holes are already in the wall so I have to work with that.

The solution is simple: Cut the flexduct at the entry point and exit point, take it out of the box, put in a couple of baffles, and glue duct liner all around the interior. This is the general idea of how silencer boxes should be built: viewtopic.php?f=2&t=1929&start=74 viewtopic.php?t=8425&start=2 viewtopic.php?f=1&t=11542&start=5 viewtopic.php?f=2&t=9761&start=0 viewtopic.php?f=2&t=11485&start=98 viewtopic.php?f=2&t=11508&start=157 viewtopic.php?f=2&t=13821 viewtopic.php?f=1&t=15378&start=44 viewtopic.php?f=2&t=18202&start=16 The flex duct is not doing anything at all positive for you, and as you have already discovered, it is doing bad things. If you make this into a proper silencer box, then the sudden dramatic change in cross section at the entry and exit creates a large acoustic impedance mismatch, meaning that sound cancels itself out to a certain extent as it tries to enter and exit the box. The air flow also slows down greatly as it enters, then speeds up as it exits, and move through the box at a much lower speed, creating far less turbulence and noise. The baffles force the air to change direction drastically a few times, wile blocking the sound, and the duct liner helps to keep things quite as well as damping any natural resonance inside the box. So you don't have to replace your "duct liner box": you only need to modify it so that it is a proper silencer box. - Stuart -

I'm not sure I have enough cross-sectional area in each half of the duct box to make it into a dual silencer box. The inside area I have to work with right now (on each side) is about 35" H x 16" W x 8-3/4" D. Also, what do I use to attach the baffles to the box? Can I use drywall with green glue for the baffles? And where do I find duct liner, can I order it online? Home Depot doesn't carry it.

I'm not sure I have enough cross-sectional area in each half of the duct box to make it into a dual silencer box.

Just taking out the flex duct will more than double the cross sectional area. :) Do the math: The cross sectional area of your 6" duct is about 28 square inches (in2). You say your box 8-3/4" deep, and 16" wide. So you have 8.75 x 16 = 140 square inches of cross section in there, vs. 28 for the flex duct. :shock: You will lose 1" all around from the duct liner (it is 1" thick), leaving you with a an interior cross section that measures about 6-3/4" by 14" = 94.5 square inches. That's well over THREE TIMES the cross section of your flex duct, meaning that the air velocity will drop to less than ONE THIRD of the current speed. And since noise is exponentially related to speed, you get a LOT more than 66% reduction in noise created by the air flow itself, due the lower speed.... If you just did that alone, without any baffles, you'd get a pretty good reduction in noise. But with 35" of space vertically, you have space for two baffles, leaving about 9-1/2" x 6-3/4" of cross-sectional area 64 square inches in between baffles. That's still well over twice the area of the flex duct, and those baffles will help greatly to improve your isolation even more.

Also, what do I use to attach the baffles to the box?

Make the baffles from plywood, OSB, MDF or something like that. Screw and glue them in place, then put in the duct liner.

And where do I find duct liner, can I order it online?

And HVAC supply house will carry it. Google it for your area. You might find this useful: http://www.hvacagent.com/Suppliers.aspx?ssState=34 - Stuart -

I made a few models of a baffle box that I'd like to run by you before I start modding the duct box. The baffles are double 3/4" plywood with green glue, measuring (not counting the 1" duct liner) 8-3/4" x 10-1/2". http://imgur.com/a/WCxmM I have some questions about my design. 1. The cross section goes from 6-3/4 x 8-2/3" to 6-3/4 x 3-3/8". Would the 3-3/8" be too small a space? Should I scale down the baffle width to enlarge the space? 2. Due to the 3 studs sticking out in the middle, I'll have to make the cut outs on the duct liner as shown on the right (exhaust) side. Do I cover up all the studs with liner or can I leave gaps as shown? 3. I still don't really see how to attach the baffles to the box. Do I just use liquid nails to glue them on and then caulk up the joints? 4. Any ideas on attaching the top baffle on the right side where there's a stud in the middle? Maybe divide the baffle into 2 pieces, one 7-1/4" deep up to the stud and one 1-1/2" deep to fill in the gap, then caulk the joint? 5. Do all the dimensions look good in the 2nd pic? 6. Do I use duct liner adhesive to glue on the liner or will liquid nails do? Also, I don't think the hvac supply houses sell small amounts of liner to homeowners. I called up one place and they said they only sell them by the 100' roll. I only needed 13'. Thanks for your help

I made a few models of a baffle box ...

That looks fine, but please upload the images right here, to the forum, instead of linking to them in a different location. It's actually one of the forum rules, and for a good reason: Hosting sites disappear over time, or change their links, or structure, or the links expire, or any number of other problems that cause the images to no longer be available. So when someone comes along here in a year or two, and wants to see how you fixed your problem, they won't be able to do that because the links won't work any more. So please just upload those images right here, into your thread.

1. The cross section goes from 6-3/4 x 8-2/3" to 6-3/4 x 3-3/8". Would the 3-3/8" be too small a space? Should I scale down the baffle width to enlarge the space?

You could scale it a bit, so the change is not so sudden. Try to ensure that the cross sectional

area

at each point along the path stays greater than the cross sectional area of the duct, and remains relatively the same along the entire journey. Of course, you don't need to go crazy about that! Just keep it reasonable. So make your baffles a bit shorter, to increase the cross sectional area, but also don't make them so short that they no longer block the passage of sound from entry to exit, or that they no longer cause the air to make a full turn.

2. Due to the 3 studs sticking out in the middle, I'll have to make the cut outs on the duct liner as shown on the right (exhaust) side. Do I cover up all the studs with liner or can I leave gaps as shown?

What you show there is fine.

3. I still don't really see how to attach the baffles to the box. Do I just use liquid nails to glue them on and then caulk up the joints?

Screw through from the outside of the box, and also glue them in place on the inside.

4. Any ideas on attaching the top baffle on the right side where there's a stud in the middle?

maybe I'm not understanding the question, but I don't see any stud causing problems there! Maybe you can do a close-up image of what the problem is?

6. Do I use duct liner adhesive to glue on the liner or will liquid nails do?

Use only what the manufacturer of the duct liner recommends.

Also, I don't think the hvac supply houses sell small amounts of liner to homeowners. I called up one place and they said they only sell them by the 100' roll. I only needed 13'.

You might be able to find off-cuts at building sites. Maybe ask the distributor if they know of any large projects that might have offcuts. Or buy the entire roll, then sell the excess on eBay, or even right here on the forum: I'm sure there will be other studio builders around who need it! - Stuart -