Air Handler-Optimum Size of RETURN Plenum


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Old 12-30-22, 10:38 PM
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Air Handler-Optimum Size of RETURN Plenum

Installed in 2006 as replacement for 1989 system:
Carrier Heat Pump: Model: 25HBR336; 3 ton
Carrier Fan Coil/Air Handler: Model: FK4DNF003 000AAAA; 5 KW Heat

Can anyone advise me as to how I can calculate the optimum size of the RETURN Plenum located immediately below the Air Handler?

I've search online and browsed a number of books, but I can't find any pertinent data on determining the optimum size of the Return Plenum box. It seems to me that this size may be important...

The ORIGINAL Return Plenum was composed of Duct board. Depth of this box (as viewed from the front) matched the depth of the AH base; Width was equal to the distance from the Left Hand side of the AH to the RH side of the Return Duct (descending from the ceiling); and Height was the distance from the concrete slab floor to the bottom of the AH and Return Duct. Two rectangular holes were cut in the top for the AH intake, and the Return Duct. The Return Plenums in my previous dwellings were about half the size of this one. IMO, the size of this Plenum was apparently calculated for ease of, and speed of installation...

I'd like to replace the duct board box with a smaller steel Plenum. Steel is preferred due to previous leakage issues, and even more because of the heavily dust-encrusted and delaminated duct board that I removed. Another semi-issue is that the copper tubing and PVC drain line passed through the duct board box in the original installation. Even with the metal box I'd prefer to avoid possible sweating or leakage issues into the Plenum. Coolant leakage has never been detected, but given the 34 year age of the copper tubing I probably should replace it anyway.

The attached diagram depicts the dims of the currently installed equipment (gray), the original plenum is outlined in red, the NEW Duct Transition is yellow, and my proposed Return Plenum is green. I can keep the original dims of the box if I replace and relocate the copper tubing and condensation drain pipe, but this would double the cost of the steel for the plenum box. The volume of the original Supply Plenum is 11.7 cu. ft. and the volume of my proposed Plenum replacement (Return Duct Transition volume is not included) is 6.35 cu. ft.

 

Last edited by taneal1953; 12-30-22 at 11:25 PM. Reason: formatting; replace diagram
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Old 01-23-23, 09:47 PM
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74 views and 0 replies. Huh. Well thanks to those who took the time to read my post...
 
 

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