Duct Sizing

[65Stang]

I am trying to get some ductwork figured out for my Furnace that I am installing in the garage rafters. It's a 60,000 btu furnace, American Standard Freedom 80, I believe manufactured in 2000 or 2002. I do not have any manuals for it nor could I find any online with the model number.

The supply/return openings are approximately 13 1/2" by 20". I had planned coming off these ends with a square to circular duct transition. I will most likely fab this myself, but what I am trying to figure out is the diameter of the ductwork that I should use. HD and similar only carries 12" diameter, but I have been told that 14" is required. The return run will only be about 5 or less in length. The supply will be about 5' to the first main vent, then will have a couple supply runs branching off, but not a lot. Any thoughts appreciated...

 

[redsky49]

Duct work is sized based on the air flow of the unit, not the heat input rating. Rather than guessing, look for a data plate on either the outside or inside of the unit. Somewhere you will find a CFM rating. That will determine the duct size.

If you cannot find any fan information, I can make an educated estimate for you and size the duct accordingly.

As always, offered only as opinion

 

[65Stang]

So what are the ratings/CFM for 12" and 14" duct?

 

[redsky49]

For a typical residential application, 12" dia. about 600 cfm.
For 14" dia., about 900 cfm.

These are standard selections based on an air velocity of 750-850 fpm, and a friction loss of .075" static pressure per 100' of duct (industry standard criteria). This will work fine for ductwork well upstream from the air outlets (grilles or diffusers). For connection(s) to your furnace, you will be safe to use these numbers, especially if you are talking about a short run at this size.

For duct(s) closer to the air outlets, an air velocity closer to 400 fpm will ensure quieter operation. While this might not seem important for a garage application, it is difficult to reduce the noise of an existing system after the fact. Better to get it right the first time. I have never had anyone complain that the mechanical system was too quiet. Additionally, proper sizing of ductwork will ensure that supply air delivery will not be restricted due to undersized ducts.

I would recommend that you sound line the ductwork, both supply and return, through at least the first duct elbow. A few stick pins and some sound liner won't break the budget, and once lined, the duct will not require any additional insulation. Providing a 90 degree duct elbow, especially if lined, somewhere downstream will also do a lot to attenuate the fan noise traveling down the duct.

Many installations will provide a box or plenum at the furnace discharge and then **** tap into the plenum with a round supply duct and extending the ductwork to the conditioned space. While this sometimes works, it is not recommended. The preferred arrangement would be to come off the unit full size of the opening (13x20) and with a gradual transition, bring this to the required round duct size over the space of 2-3 ft. That is, going from a rectangular to round duct over the space of a few feet. This will again encourage the best possible airflow.

Just remember that air is a fluid and doesn't like to make quick changes in directions or other obstructions to the flow. Keep that in mind when laying out the duct work.

Hope this isn't too much information. Good luck with your project.

As always, offered only as opinion