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Re: Adding additional air ducts in unfinished basement

From: info@builderswebsource.com
Category: HVAC
Remote Name: 63.198.181.43
Date: 29 Sep 2001
Time: 12:06 AM

Comments

Your general method sounds correct. Be sure to use duct tape to secure all joints and prevent air leakage. Builders Websource prefers the use of flexible insulated aluminum ducting. The aluminum helps to keep fiberglass insulation from blowing through the ducts...and the metal is more durable. The only drawback of metal ducts is their higher materials cost and that they can get squashed if you're not careful...and once they're deformed, it's tricky to restore them to their original shape.

As a rule of thumb, each vent outlet should be fed by approximately a 6" round duct, depending on the size of the room. Larger rooms may require two 6" ducts or more. The rule of thumb is to work backwards from each vent location to determine the maximum diameter feeder needed. For example, a 6" round duct has an area of 28 square inches. Therefore, if you have two 6" ducts, you need 1 wye that has an 8" input and two 6" outputs. The reason for the 8" input is that the combined area of two 6" ducts is approximately 2*28, or 56 square inches. An 8" duct is about 50 square inches, so this is an acceptable match. Adding a third 6" duct to the 8" duct, would require a 10" inlet. That's because a 10" duct has approximately 78 square inches, or about the equivalent area of three 6" ducts. You can see that the pattern is roughly for each 6" duct you add, the supply trunk expands by 2" in diameter. This formula holds true typically for supply trunks in the 14-16" range, give or take.

Now, be sure that your return air duct is at least as big as your supply duct, otherwise the air flow will be unbalanced. Some installers use dampers inside the wyes in order to fine-tune the air balance for each room.

Determining how many air ducts you need in each room requires a BTU calculation based on the thermal loss of the structure and volume of air in each conditioned space. But assuming you follow the rule of thumb above, and assuming your heater can support the added capacity without damage to the furnace, then you'll be in the ball park.

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