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yam 08-21-2007 04:29 PM

Air return through vented doors
My AC return is inside my home theater room. The home theater room has a vented bifold door connecting to the entryway, which opens to a few other rooms through open doorways, ending in a kitchen which connects to the home theater room through a solid bifold door. So there are two doors to the movie room, each connecting to the opposite ends of a loop of rooms with no other doors to block the air. The theater/entryway door is vented, and the theater/kitchen door is solid.

Should both doors be vented? I think there are two issues. First, how much vented doorway area does it take to feed my 2.5 ton AC unit. The vented doorway has slats with thin gaps between them so Iím sure it is much more restrictive than the stamped metal return grill. Second, would my AC operate more efficiently if it had two return paths (ie air flows through the loop of rooms in either direction) rather than just one path (ie, air blown into the kitchen has to go all the way around the house for its return path).

The movie room was originally a formal dining room, with an open doorway to the entryway (this is where I installed a vented bifold door). The solid bifold door to the kitchen was already installed sometime before I purchased the house.

yudamann 08-21-2007 05:42 PM

Air passing through grilles or slats should be less than 400 ft/min velocity so that noise is not an issue. Calculate yours by taking the air volume [cubic feet/min.] and dividing by the sq. feet of opening [using 144 sq. in.= 1 sq. ft.]. Just measure the openings of the slats and include the free opening below the doors and convert to sq. ft. If the velocity is greater than 400 fpm, you will need more opening by either cutting off the door bottom or adding another return grille [usually add 25% restriction for the metal part of a grille face area]. So a grille of 12"x12" measurement will be 144 sq. in. or 1 sq ft. times 0.75 [100% less 25%] equals a free area for this grille of 0.75 sq. ft.

yam 08-21-2007 06:13 PM

Thanks Yuda. I'm familiar with the 25% restriction for a stamped grill, but what percentage should I use for the slats on a vented bifold door? It is much more restrictive than a grill.

Unfortunately the air handler is pretty loud, even though I've got plenty of return grill area. I think it's just a loud unit. It's a 1996 Amana unit. Someone told me that Amana units of that vintage are just plain loud no matter what you do.

Big Bob 08-21-2007 06:38 PM

my first read on your post had me ask why can't they just leave the doors open. So the real problem is a noisy air handler.

Try finding some good size scrap FG duct board (available in the dumpster outside most Heat & Air contractors) and line your air handler closet. This will help deaden the noise. ( Don't forget to wear long sleeves and gloves)

yudamann 08-21-2007 08:18 PM

The 25% factor is an estimate since it is hard to tell the actual metal area that blocks flow on a register. For your louvers, measure the approximate [1/4"?, 3/8"?] slat opening [convert in. to ft.] x width x quantity. No factors needed. Add the undercut of door[s] and any other openings that air can pass. Re loud air handler, try lining the AHU closet w/ carpet scraps stapled to the drywall to dampen the noise. Make sure that the noise is not register noise at the closet; if it is, increase the size of this return by adding another.

yam 08-21-2007 08:41 PM

Thanks guys. I guess there are actually two problems - quieting down the noisy air handler and also making sure I've got plenty of airflow through the doors so I don't starve the return. This is the main unit for the house so it needs to run efficiently whether the doors are open or closed. They're almost always closed - during the day to keep the dogs out, and in the evenings when I'm watching movies.

I'll try adding ductboard or carpet to the closet for noise. Do you mean in the actual closet, or in the enclosed return area between the return grill and the air handler intake?

shawn T 08-21-2007 09:19 PM

I know that per the ICC codebook the surface area for wood vent surface loss should be estimated at greater than 25 percent, I think its more like 40 percent. If your door is louvered and not just a vent cut into it you should not have any problem at all. If you had a gas furnace in there you would need to duct out of the utility room. Too late now, but when installed furnace mounts or vibration isolation pads might of helped.

Big Bob 08-22-2007 11:37 AM

other options may be to install through wall return vents or if access permits run return duct to your return plenum.

You can test your air circulation patterns with smoke.

yes, line the closet. AND If vibration is likely cause, check to see if lines have enough play so that you can lever unit up and add rubber pad, (strips of an old mouse pad may be just the thing). (Be careful not to break any refrigerant lines or connections) As Shawn T advised unit placement is the best time to install.

good luck

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