1. Professional Member
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we use 8 inch tall supply duct. my boss tells me the
rule of thumb for calculating the number of round 6 inch
takeoffs for that width duct is to double the number of
takeoffs and add 2. e.g. for 5 takeoffs, twice 5 is ten,
add 2 which yields 12, so 12 by 8 duct is ok. Any more
takeoffs than 5 would yield a result of 12 by 8 duct being undersized.
What should the minimum distance be between the last takeoff and the duct endcap?

thoughts?

Rules of thumb seem weak to me, I know there is more to it than that,
from a little independent study. suggestions on a real good source to learn more? duct sizing, velocity, static pressure, balancing, etc.

(But this leads to another gripe. Implicitly we must be
able to rapidly do a perfect install yet remain more
ignorant than the boss or we apparently want his job.)

Of course for all you owners reading this I know we are
in a cut-throat, dog eat dog world. We aint working on
Taj Mahals but rather variations-on-a-theme tract houses
and I am grateful for the steady work. So he has to be doing
something right, the company owner, that is. And I know the builders
we work for are only interested in two things, speed and low price.

2. Purchase a copy of ACCA Manuals D & T.
Check out http://www.nationalcomfortinstitute.com,these will help get you going.

Good Luck

3. Let's take that Rule of Thumb for a spin around the block.

Based on .1" static, a norm for residential ducting, a 6" round duct carries 135 CFM of air in a 100' of friction run.

Since a 12 X 8 duct carries 500 cfm of air, this would make the RoT method a little light. BUT! If we are figuring on each 6" round to have to supply only 100 cfm of air, then it does work.

Now, lets see what happens with more air under the same condition;

10 6" runs should be a 22" duct, which carries 1,050 cfm under the same conditions. So, at least for these two numbers at that specific amount of friction it does work out.

Still best to use manual D on new construction design.

4. Professional Member
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thanks, gents, will comply. whoo hoo, ready for the big move from pathetic to nearly acceptable!!!!!
that's good news I don't have to hunt a new job working
for a feller without any thumbs. (who apparently in the past has cut
corners on safety)

5. buy a ductulator for \$5.

6. Regular Guest
Join Date
Feb 2005
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## Rules of thumb (ROTs)

The ROT for the 8" trunk is probably a culmination of years of experience shared between HVAC techs.

While I wouldn't advise using this ROT for design, it is a handy "tool" when trouble shooting or doing a quick budget figure.

I have heard many ROTs over the years. If one takes them for what they are, some ROTs are helpful. Of course some are completely wrong or outdated- (1 ton/ 500 sqft).

Don't automatically discount a ROT, but don't blindly accept it as fact either.

Good luck...

BTW- If you buy a ductalator please read the instructions. There is more to it than .1" for supply and .08" for return.

[Edited by ruud-man on 06-20-2005 at 02:49 PM]

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