If I install a system and put a return in every room, should the ductsize for the return match the supply for all rooms?
depends upon how leaky the door is -- it would be best to have a larger return pipe since someone may put in a close fitting door, as I did in my master bdrm for Shelter- In- Place against hazardous vapors. -- I have a large crack under the closet door with a return from the closet into hallway.
one wants to make it easy for the cooler air to get away!
In college we were instructed to size the returns at 20% bigger than the supplies.
At least no smaller than the supply.
thats odd i thought the rule is 80% of supply.
Not a guru here, but this is my take... Return is almost always sized at a lower static pressure than the supply. So to get the same CFM back to the unit that you're puttin in that room, you need a larger duct.
But.... I'd say if you can get one at least the same size in every room, with a couple main returns that are good size you'd be more than happy.
i built a home with returns in every room except bath rooms and kitchen area and sized the same as supplies then added 2 main reurns the the home. i have fantastic results home is evenly heated and cooled. so yes it's well worth the effort to do so.
no returns in some rooms?
I am seeing smaller homes (2,000 sq. ft.) built here in FL where one large return is put nearthe center of the home and instead of returns in the bedrooms, etc, vent's are cut through the walls into the common areas.
Having never seen this done before, I am curious as to how this can be as effective. Theoretically, if the path is unrestricted and the thru-wall vents are of sufficient size - these should work - until someone plants a large flat piece of furniture in front of one!
Evidently this meets local codes. Is this a common design in other areas?
Reason for those vents
It is my understanding Florida building code requires no more than 3 Pascals difference in pressure between one part of the house and another. Other states tend to have no such requirement. In a house such as mine built 1989 in Texas, they gave no return path except air flowing under the door (with some houses over a ton of AC airflow trying to pass under a door when a bedroom door was closed). The results are pressure differences that exceed FL building code by a lot, perhaps a factor of two. My doors used to slam shut when the air handler started blowing. In theory this can lead to enough unwanted air infiltration and exfiltration to cause moisture problems and even mold growth -- though this danger may be more theoretical than real.
Cutting an inch or more of space under the door is one way of providing a return path, though many consider that unsightly. And if the homeowner puts in thick carpet, that door gap will need to be cut again.
Thru-the-wall vents are another fair choice, here is a treatise by a vendor which describes the problem and benefits:
One *must* seal this opening so that it does not create a leak into the interior of walls. You pointed out another drawback, the homeowner can kill the benefit by placing something *too* close to the wall and covering the vent. However a couple inches clearance will allow it to work, in my estimation.
An old fashioned transom over the door would be a great solution, except that went out of style along with fedoras and running boards <g>.
Naturally putting a return in every bedroom is a third way to get good airflow and avoid over- and under-pressurization when the door is closed. The higher price solution but you do get what you pay for.
The "V" part of HVAC has turned out to be one of the more subtle and interesting parts of this subject, in my opinion.
Hope this helps -- P.Student
[Edited by perpetual_student on 03-12-2005 at 10:15 AM]