Supply trunk line

Old 08-06-02, 09:50 AM
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Question Supply trunk line

This weekend I am replacing my supply trunk line under
mY house. I have heard that it should be "stepped down"
every 10 feet. I have 40 feet of supply. In the first 10
feet or so 6 runs will come off of it.I have a total of
11 runs. I will be starting out at 24x8. Should I step
that down to 20x8 for ten feet and then 16x8 for the
remainder of the trunk line, or go from 24x8 for 10feet
then 20x8 for 10 feet then 16x8 for 10 feet and then
12x8 for 10 feet, or go from 24x8 for 10 feet and then
16x8 for the rest? After the initial 10 feet I will have
another 5 runs. One in each bedroom(3), and bathroom(2).
It is a 2.5 ton heat pump with a 3 ton air handler.
Old 08-06-02, 07:48 PM
hvac4u's Avatar
Join Date: Aug 2001
Location: NW atlanta
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you need

a ductulator. this device tells you air capacity of given size duct at standard static pressure. every 10 feet means nothing. after a few take offs, the duct downsizes accordingly to maintain pressure. if you cannot find a ductulator online, repost, and i will be more specific. (this also gives me time to dig mine out!)
Old 08-09-02, 04:02 PM
lynn comstock
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Most of the air comes out in the first 10 feet.

Make a single reduction at that point. It will work fine. (The trunk duct concept is called an extended plenum.)

3 tons of air is 1200 cfm (cubic feet per minute) and the velocity in the 24x8 duct (inside clear dimensions) will be about 900 fpm (feet per minute). That is good. 3 bedrooms and 2 baths is probably about 500 sq ft of area. In lieu of a room-by-room heat calculation distribute the air proportionately to the floor area.

In your case (1200 cfm and 1251 sq ft) the air to each room will be about 1 cfm per sq ft. If the duct after the reduction serves 500 cfm, size it to handle the air at the start at 900 fpm velocity. Reduce the size with a transition to funnel the air gently into the smaller trunk.

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