return duct size

[Friesen]

I am installing a duct work for a ducted aircondition (cold air only).
I am new at this work.

The supplier of the air handler equipment suggested that the return need only be 60% (measured by cross-section) of the supply, but the instruction manuel, which is very brief, says that the duct of supply and return shall not be size smaller than opening on air-handler.

can somebody answer my question or have I omitted pertinant information.

John

[Hube]

I always size the return to be at least 10 % more in area than the the supply. Example; 12 x 18 supply.....12x20 R/A Hube

[imeduc]

You dont say what you have there So you need 400cfm per ton thats like a 3ton you need 1200 cfm. For the duct 70 to 80 sq"
per ton all the time for a good job. So for a 3 ton unit go for 210 to 240 sq" for out and return and it will work very good for you.

ED[B)]

My mistakes dont define me they inform me.

[Friesen]

thanks for responding
I am installing a barometric by pass damper to compensate for air volume when the zone dampers close. I am planning to divide the airconditioned area in six zone one zone which is 33% of area and the warmest will always be open. Does putting in a barometric bypass damper into the return duct change the way return duct size is calculated? l

quote:Originally posted by Hube

I always size the return to be at least 10 % more in area than the the supply. Example; 12 x 18 supply.....12x20 R/A Hube

[imeduc]

You dont say just what you have there. how big how long is the main duct. With out a print I would not say. I find on big jobs it dont pay to go to zone dampers . We dont even like for 10 tons, two 5ton on same duct and 2 stage tstat works a lot better. I have done homes where we have put in 8 heat cool units for zones. Have done large jobs on one 350 ton with 2 of them 22 ton and all the rest 10 and 5 and 3 ton units. Zones are to much work to keep up and like you ask with the bypass just a waste

My .02 cents ED

My mistakes dont define me they inform me.