I have added a room to my home and the A/C tech connected a new 7" flex duct
to the plenum. Off of that he SAID(??) he connected a 7" wye with 6"
reducers. And then off of that he connected a 5" duct dropping into a 64 sq
ft. room and a 5" duct dropping into a 77 sq. ft. room This does not add up
for me if a 7" carries about 170 CFM and 2, 5" ducts carrying about 70 CFM
each. Thats 140 CFM. What happens to the other 30???
Please let me know if this is correct duct design.
Flow limited by smallest point just like a water hose -- flow will
balance to an equilibrium point. The flow ratings of a open-ended
duct are based on just that -- add a restriction and the pressure will
rise and the flow go down.
If you have adequate but not too much in the new room and still
adequate but not too little in the remainng house, it's good enough...
What "extra" 30 cfm? There's a fan from the furnace of some capacity
-- larger duct implies lower pressure/velocity. Larger duct size is
typical for branch runs to minimize drag loss and possible wind noise
but don't want a huge outlet into a small space.
The only real problem as I noted previously I would foresee would be
the new duct is sufficiently large as to allow for an excessive
diversion from the rest of the house into the new area. You'll soon
find out that if the rest of the house isn't getting sufficient
cooling. A real pro could measure flows and balance but unless you
have a problem in practice is undoubtedly overkill.
If you want your system to work properly, size it according to ACCA
Manual D. (Short version below)
Take a measurement of all your fittings and layout on your duct system,
then on the back of most ductolator is “an equivalent length of all the
fittings,” to use for figuring the total length of the system.
If using an older gas furnace you have .50 external static available.
With newer furnaces look at blower performance chart. Minus the A-Coil,
filter, etc.,, and all the other external component pressure drops to
your furnace blower performance rating.
Use the formula: Friction = leftover available static X’s 100, divided
by total equivalent length, and that is the friction rate for sizing the
Depending on a lot of existing, at this point unknown factors, there
could be some CFM delivery problem shortfalls in many if not all of the
rooms. The ductwork and/or the blower may need upgrading to deliver the
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