Adding to Heat Ducts

[Linda]

We recently added on to our house and have a question
on the heat ducts. We have forced air heat with duct work running in the crawl space. The duct is 8″x24″ and approx.20’long. There are about 8-6″round ducts going to each room, branching off the main. We added a new duct, 8″x20″x16’long off the main about 4′ from the end. We tapped into the side of the main and go straight out to the addition where we run 4- 6″ round runs to rooms in the addition. We also tapped into the cold air return and added 1-6″ & 2-8″ returns. Our furnace is a 3 yrs old Bryant 80 and should heat 2000 sq ft. Our house is now 1600 sq ft. The question is, why doesn’t the new heat ducts have much air flow? Is it where we tapped into the main or do we need a blower
added to the duct? Any suggestions would be appreciated.

[Jason]

Greeting Linda,

I’m not the pro on this but I am currious.

It seems to me, not remembering the formular to find out how many square inches there are in a circle, that you have more square inches of exhaust the you do for supply!

I believe the only numbers that are relevant would be the size of the duct from the furnace because that is all the volume you can have, and the amount the volume of all the 12- 6″ ducts combined.

You have an 8″ X 24″ supply or 192 square inches of opening that feeds 12- 6″ openings. (I don’t know the number of square inch’s but, someone will)

Much like connecting a 3″ hose to a 1/2 ” spicket, you will only get as much volume as the smallest connection.

Do you have any dampers installed in this system so you can control the air flow and cut it back where it is not needed?

It’s just a thought, and I am probably wrong but, like I said, I’m currious.

Good luck,
Jason

[Steve]

I have a 1986 95% efficiency Hotpoint furnace and want to improve the efficiency by trapping as much heat as I can in the Ducts. I’ve wrapped the initial Duct coming out of the Furnace with reflectix but when the furnace is on, it is still too hot to touch. Is there a special Duct that belongs next to the furnace so the outside feels cool? I feel this will improve the efficiency by not radiating the heat into the garage through the duct.( and then I can wrap it )

[Author]

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