Question Re: New Furnace and Duct Sizing

DSimo · 06-15-2008, 06:03 PM

I am replacing a 50+ year old Natural Gas forced air furnace with a new 90+ efficiency "Hamilton" single stage forced air furnace model # WUN-100-4C. Both old and new furnaces are 100K BTU.

The new furnace outlet is the same size as the old, (19 3/4w x 20d) as is the plenum. The installation manual for the new furnace specifies a minimum return total size of 360 sq. in. and a minimum supply of 280 sq. in.

There are currently 5 ea. 6" round ducts from the plenum and 5 ea. 6" round ducts used for return air.
5 ea. 6" round ducts = 141 sq. in. Will I have any issues with this duct arrangement even though the old furnace used this duct arrangement for many years? If so, what can I do to resolve the issue/s?

This system will used for 3 ton central AC also.

javan · 06-16-2008, 07:34 AM

What is the size of the supply outlet of the furnace, and what is the size of your supply duct where it would connect to the furnace? That is the real question here.

I assume that this is a constant volume system, one zone?

DSimo · 06-16-2008, 10:24 PM

The outlet of the old and new furnace is roughly 19 1/2 x 20 inches. This is also the plenum size. Also, yes, this is a single zone system.

javan · 06-17-2008, 08:24 AM

I don't see where you would have any issues.