Gas and Oil Home Heating Furnaces - Extend Duct to New Addition

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09-22-01, 03:54 AM
I'm adding a new 400 sq-ft room on to my house. I currently have sheet metal ducts in the attic on each side of the house that are connected together by a 10" flexible duct. Would it be a problem tapping into the 10" flexible duct with a Y duct fitting and extending it out to two registers in the new addition. The run would be less than 20'. Should I use 10" flexible duct to the new addition or go down to a smaller duct, like 8". I believe the current heater and cooler will be able to handle the extra load, it easily heats/cools the existing space. Thanks!

lynn comstock
09-24-01, 09:51 PM
Where is the source of the air? The best solution is to go to the source to steal the air for the addition.

If the air available in the connection duct is barely enough to do the job downstream of your proposed takeoff, the air that you steal will not be enough for the addition or the space downstream (This is a common error.)

As for size, err on to large side because it easy to choke it down but nearly impossible to boost it.