Originally Posted by J_P_A
A couple things to consider. There have been several theaters designed around dead vents that exchanged air with adjacent rooms without connecting directly to the HVAC with varying degrees of success. The Black Cat theater comes to mind as a good reference as Morph1c did quite a bit of testing (here's the post I believe
). To make it work, you will need a higher air exchange rate than required from the HVAC because the conditioned air should be around 15-20 degrees cooler than the house air. So if your home is at 75 degrees, the air from the registers should be around 55 degrees. When using a dead vent, you are trying to make the 75 (or maybe less in a basement) degree air keep a room with several space heaters (the people) in it cool. Based on Morph1c's experience, it looks like you will need to be pretty close that 6-7 exchanges per hour, but there are so many variables that it would be tough to come up with a concrete number that WILL work.
Thanks so much for the link and the knowledge! Black Cat's post is a great resource for the testing. My take is that it shows that there needs to be a fan pushing the air back into the room, especially if there is such a powerful fan pulling out the air like Black Cat's Panasonic fan with 340 CFM - used by many others around here as well. You've got the Pany 440, right?
Below is a better diagram showing my room dimensions as well as the soffit:
When I do a little bit more accurate calculations I get about 785 CF. At 6 exchanges per hour, that gets me to 78.5 CFM required, right. (785 CF) * (6 exchanges/hr) / 60 min/hr) = 78.5 CFM.
I was only planning on using 6" flex duct and according to the chart on your build (fantastic resource by the way - thanks!) a 6" flex duct has a capacity of 75 CFM. I'm kind of thinking that with the couch and other items in the room I might actually be pretty close to the 750 CF (for the 75 CFM). I am also considering just using the 6" inline duct fans. They are rated at 160 CFM 'free air'. My thought was to have a total of 2 fans, one attached to a return (exhaust) to pull air out of the room and one attached to a supply. The space adjacent to the theater room (with the water heater and furnace) has a 6" duct that supplies outside air to that room. I added that to help 'feed' the furnace during the winter when I built the room and reduced the size of the furnace room. I thought if I could exhaust the theater room into that space and bring in some cool air from that space (say from along the floor of the adjacent room) I'd be good to go. I haven't built a hush box yet (I had sort of one before but it's gone now) but I was hoping to build one and just allow the air in the room go through it and out the exhaust along the back wall and out through the top of the equipment closet.
Originally Posted by J_P_A
That PVC pipe is intended to provide a massive coupling between the dead vent and the room. The idea being your room has massive DD+GG walls, and the dead vent has massive DD+GG walls, but the cavity between the two is a potential weak link. We try to use a heavy coupling between the two to minimize sound leakage out of the pipe. Now, between us ladies, I'm planning to try to extend my dead vent up to the back of my wall and leave a 1/4" gap or so that I will fill with acoustic caulk. Then I can just run my flex through the wall, or use a standard sheet metal coupling like you posted. For me, a piece of 8" schd. 40 PVC is pretty expensive for just the 12" piece that I need.
Hey, I just thought of an alternative to buying a 10' section of 6" diameter PVC and cutting it up. From time to time my work requires testing concrete and the molds they use are 6" diameter by 12" tall cylinders, cut the bottom out and attach the flex duct (as long as the outside diameter is about right). I bet I could get some of those. They look like this:
I'm not sure if they are PVC or a different poly. They would have them at local geotechnical/material testing firms. They even have a lip to hold it on the inside of the room. I'm still unsure how the register/vent gets attached to the theater room side.