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Old 07-19-2012, 11:13 AM   #1
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Sizing return air supply


I'm currently finishing up a plumbing project in my house and happened to learn a little more about the HVAC supply and returns in my house, after openning up parts of walls and ceilings.

My upstairs is hard to cool and heat.
Just a got a new unit 2.5 tons, up form 2 tons. Its a small 2 story townhouse.

What i learned about my HVAC is they use the wall and floor cavities in some places as the air and return duct. The one return duct i have the most access to is the return from the master bedroom. What i notice is that it comes down the second floor wall( the vent is located at the top of the wall), then turns 90 uses the floor cavity(maybe 10ft run) then back into a first floor, and then down again into the basement.

Wondering if going from a wall cavity to a larger floor cavity(twice its size) then back to a wall cavity would limit the air return?

i think my unit has enough push to get up to the second floor, i think the return is not enough.

There is returns in all 3 bedrooms, and a large return on the first floor.

Would running a duct in the floor/ceiling cavity, trying to better match the wall cavity size make a better flow?

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Old 07-19-2012, 12:24 PM   #2
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Sizing return air supply


In general, the bigger the return the better off you are. I don't know that it would make a big difference in the upstairs vs downstairs temperature. Generally speaking the only way to control that is to either (a) install two systems or (b) install a zoning system. Depending on how your ducts are currently run, you may find the cost of doing either to be quite expensive.

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Old 07-19-2012, 01:12 PM   #3
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Sizing return air supply


The problem is that its small on both ends, with the big section the middle.
Wondering if that would cause some type of lose in flow.

wall cavity is 8'h 2.5"x16". both ends.
floor/ceiling is 10'L 8"x16".
If its equal on both ends, its okay.

Was thinking it would be better to connect the two smaller sections with a duct of equal sq. inches.

Zoning and such, to expensive, just wondering if i could help out the return a little, while i have the ceiling open(for the plumbing i did)
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Old 07-19-2012, 02:19 PM   #4
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Sizing return air supply


Air is like anything else you move through a pipe - it's only as big as the smallest section. So if I have a 16" return that reduces down to 6", and then bck UP to 16" ... I've got a 6" return.

So your intent is to DECREASE the size of the 8" x 16" joist space to align more closely with the 2.5" x 16" wall cavity? If so, that's a waste of time, effort and money. It would not do anything at all to help your return situation. As stated above, you're effectively dealing with a 2.5 x 16 return at this point - regardless that it passes through a larger space.

Last edited by scottmcd9999; 07-19-2012 at 02:23 PM.
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Old 07-25-2012, 01:11 PM   #5
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Sizing return air supply


Your problem. particularly during cooling, is typical in a 2 story house. Your furnace/AC supply fan should be running continuously on "low/mid low" speed at all times then kick in to HI speed when the thermostat calls for cooling i.e., your thermostat temperature rises above the desired "set point" temperature and asks for more cooling. The continuous circulation should produce even temperatures throughout. Cold air is harder to move than warm air so, this is not usually required during heating mode but could also be beneficial if necessary. Your thermostat may already have a "cool" "on" setting that automatically runs the fan in this manner. If not then it can be rewired to do so. If the TT will not currently do this then, if you have a manual/off/on switch on the furnace fan you can switch it to run continuously and leave the TT on "auto" to prove if the continuous fan operation imprves the air temps throughout. Do not put TT in "fan only" setting then cooling will not come on. Other alternative is to directly wire the fan to run continuously, foe a day or two, leave the TT on auto and see if that works. Usually does. This is assuming the system isn't ancient and is equipped with a multi speede fan motor. Most have been for about 20 years now. AIR CIRCULATION IS THE KEY If this doesn't work then fan/duct capacities should be checked. MAKE SURE FAN AND FILTERS/COILS ARE ALL CLEAN!!
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