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?
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.