The home is about 3500 sq/ft and is getting a 5 ton system installed in the attic. The home was built with radiant heat and no air conditioning so a full duct system from the attic is being installed.
Due to the design of the roof system and the current weight load on it I was advised by an engineer not to modify the ceiling joists. So I have requested the installers use return plenums that fit within the existing 16" on center joist spacing.
The original design plan called for 3 x 14x25 filter return air boxes. Two of them would be placed right next to each other on the main landing upstairs. Air from most of the first level flows up that direction so it made sense to me. The original design plan stipulated that these two return boxes would be connected directly to the return plenum as they were to be located directly under it.
Now the design plan has changed due to the installer finding some wiring in that area. However the air handler has already been installed.. The installers would like to move those return boxes over a few feet and run 14" flex a short distance to the plenum.
In summary, the proposal is the following:
Return 1: 14x25 box with 14" flex approx. 30ft to return plenum
Return 2: 14x25 box with 14" flex approx. 5ft to return plenum
Return 3: 14x25 box with 14" flex approx. 5ft to return plenum
I am being asked to agree to this design change and would appreciate urgent advice. My concern is that although the math makes sense for the total area of returns, it doesn't add up for the 14" flex delivering the air from the returns to the plenum.
The installer assured me that because 2 of the 14" flex runs are so short it wont be a problem, but I would like a second or third opinion. The references I find online are saying that 14" flex will only flow 500cfm to the return, but I cannot figure out how a short duct length of approx. 5 ft affects that 500cfm assumption..
Due to the design of the roof system and the current weight load on it I was advised by an engineer not to modify the ceiling joists. So I have requested the installers use return plenums that fit within the existing 16" on center joist spacing.
The original design plan called for 3 x 14x25 filter return air boxes. Two of them would be placed right next to each other on the main landing upstairs. Air from most of the first level flows up that direction so it made sense to me. The original design plan stipulated that these two return boxes would be connected directly to the return plenum as they were to be located directly under it.
Now the design plan has changed due to the installer finding some wiring in that area. However the air handler has already been installed.. The installers would like to move those return boxes over a few feet and run 14" flex a short distance to the plenum.
In summary, the proposal is the following:
Return 1: 14x25 box with 14" flex approx. 30ft to return plenum
Return 2: 14x25 box with 14" flex approx. 5ft to return plenum
Return 3: 14x25 box with 14" flex approx. 5ft to return plenum
I am being asked to agree to this design change and would appreciate urgent advice. My concern is that although the math makes sense for the total area of returns, it doesn't add up for the 14" flex delivering the air from the returns to the plenum.
The installer assured me that because 2 of the 14" flex runs are so short it wont be a problem, but I would like a second or third opinion. The references I find online are saying that 14" flex will only flow 500cfm to the return, but I cannot figure out how a short duct length of approx. 5 ft affects that 500cfm assumption..
source https://hvac-talk.com/vbb/showthread.php?2226917-Concerned-about-change-to-return-air-system-for-a-5-ton-unit&goto=newpost
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