Adding duct work to addition

Old 09-18-02, 04:30 PM
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Adding duct work to addition

I've searched the adding ducts forum results and did not tap into any useful info except that HVAC is probably not an exact science because everyone complains about their heat and a/c. I went to a HomeDept "guru" who provided absolutely no help and the more he talked the more it was going to cost and I could never get him to advise me on the best way to go. I spent a lot of time at Barnes and Noble to see if any books were available and all I could find was "see a professional" or just tap into the main plenum. Surely there is someone or some publication that I can get that can help me do this work myself. With all due respect to the HVAC professionals, it can't be brain surgery, why is it so hard to get help and/or direction?
Old 09-18-02, 10:26 PM
lynn comstock
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If ducting were simple, most homes and offices would have uniform comfort.

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.
To help you about duct modifications of any type, please tell us more:

Where are the ducts located?
What are they made of?
Are they accessible?
Do you need help with tools?
Do you need help with procedures?
Do you need help with where to get parts?
Do you need help with design?
For Detail:
Look at
I thought it might answer at least some of your needs.

Also some general tips:,00.html,00.html

For design 3 ton example:

The trunk flex duct that I would choose for a 3-ton system is 18" diameter minimum. (This is 9x9x3.1415=254.5 square inches/144 sq in per sq ft=1.77 sq feet. The 3-ton needs at least 400 Cu ft per min per ton or 1200 cfm of airflow. 1200 cfm/1.77 sq ft=679 fpm velocity. A 20-inch trunk duct would drop the velocity and friction even more.

The sizing of the branch duct is similar. Proportion the air to the square footage of the areas served. Thus if the home had 1200 sq ft of conditioned space, figure 1 sq ft per sq ft of floor space. A bedroom with 160 sq ft of floor area would get 160 cfm. The branch duct would be sized to handle that much air at 500 to 600 fpm velocity. (If the home is 1600 sq ft, each sq ft needs 1200/1600 or. 75 sq ft per ton.)

A 7" diameter duct has an area of 3.5x3.5x3.1416 or 38.5 sq inches or .267 sq feet. At a velocity of 600 fpm the airflow through this duct will be .267x600=160 fpm.

These are rules of thumb. A real duct design is much more complex. However a real duct design is seldom done because it takes time, money and skill. This fact explains a lot of discomfort in home and offices.

The ductwork can divide like a tree. The trunk and branches terminology probably comes from that visual concept. Each trunk or sub-trunk has to handle the air flowing to all of branches that it serves.

Just figure the velocity of the air in the trunks and branches as you go. The duct area (in sq ft) times the velocity (in feet per minute) is airflow in cubic feet per minute. So if the area of the duct is 1 sq ft and the velocity is 600 fpm, the airflow will be 600 cfm.

The maximum recommended velocity in flex duct is 600 fpm...because this duct has double the friction of smooth sheet metal ductwork. That is why the duct size I recommend is larger than you commonly see. Most ducts are undersized and the airflow is under the factory recommended rate of 400 cfm per ton.

When you fall between sizes go up in size. Each 90-degree turn in flex duct adds friction equal to 15 feet of flex without turns. If any branch is very crooked, upsize it to compensate for the extra friction of the turns. (Sags are turns and they add up.) Seal your connections with mastic and plastic tie straps.)

Figure on using your high-speed fan setting for cooling. Recheck your fan specs. I expect the rated airflow to be achieved at .5 inches of water column external static pressure (ESP).

A CLEAN standard air filter adds .08" ESP. The grilles typically add .03" ESP. That means that without ductwork you have .14" ESP for the fan to overcome. (.03 return grille + .03 supply grille + .08 for a CLEAN filter.) If you have a furnace and coil, the factory CLEAN coil will have a friction of .20 to .30" ESP. (The factory coil specs will tell you the pressure loss at 1200 cfm for your new coil.) That leaves only about .06 to .16" ESP for the entire duct system if the rating was indeed with no filter. (Most furnaces are rated WITH filter and the available static pressure would be .08" greater than the .06 to .16" ESP that I just calculated.)

This is a simplifed guide to design. Is that enough help? This information and more is already on this website and can be accessed by searching. Good luck on the project.

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