"Lymond" wrote in news:1155518731.910602.87760
@m73g2000cwd.googlegroups.com:

We have a one-story house, about 1750 sq ft. The whole house gets good
AC flow except one of the two small bedrooms. They are right next to
each other. Up in the attic, the cool bedroom has 6 inch flex ducting
from the main unit, while the warm room has only 5 inch flex ducting.

The difference in airflow (one seems like a fan is on, the other you
have to kind of concentrate to feel the cold air) is substantial and I
can't believe it's just from an inch difference in the ducting
diameter. I've checked the ducting for kinks and holes, but it seems
pretty solid. My only thought is that it's clogged or blocked
somewhere. The vent in the room is firmly painted to the ceiling and
while I may end up doing so, I'd rather not rip it off just so I can
see a foot into the 10 foot section of ducting.

I'm considering just buying some new flex ducting and replacing it. I
haven't checked cost, but doing it myself would undoubtedly be hundreds
of dollars cheaper than paying an AC technician. My concern is with
sealing it to the metal round ducts. I've heard metal duct tape is
good, but that there are also special sealants to use as well.

Thoughts on this fairly minor AC project?

Lymond



Not saying you don't have a problem but want to comment on your:

... I can't believe it's just from an inch difference in the ducting
diameter.

The difference in area between a 5" circle and a 6" circle is 44% more!

2
Area of a circle is Pi(radius)
Pi=3.14159

3.14x5x5=79 sq in.
3.14x6x6-113 sq in

(113-79)/79*100=44%



BTW, is the r-value the same on both pcs of ducting? I've seen cheap
black flex that is like R2/3/maybe 4. The last quality residential flex
I saw put in was insulated and foil covered (I think) but it had R6
stamped right on it. Could be a partial contributor.

the cool bedroom has 6 inch flex ducting
from the main unit, while the warm room has only 5 inch flex ducting.

5" duct from where? Not from the main duct? Has it been tapped from a
duct going to another room. That would explain it. Air volume from that
other room is being stolen and divided. You would not be getting a lot
less air volume than if that 5" was connected to the main trunk.


I've heard metal duct tape is good,...

Depends. I've seen cheap foil tape at WalMart that is some of the
stickiest tape I've ever used...but...you are using it on duct work which
means your local building code determines what is OK to use. For
instance, they might require duct tape to be UL-723 fire rated. Sure you
can put anything you want on it. Lets say you go to sell the place in 5
yrs and soem home inspector is trying to justify his services/fees by
finding something that is broke or doesn't meet code. You really don't
wanna go up there and pull off that sticky-ass WallyWorld tape.

All inspection depts have hours to take calls with questions even from
homeowners. Part of their job.


For the record, I'm not an HVAC guy.

As far as the math part, that I guarantee but have never proven Fermat's
Last Theorem of x**2 + y**2 = z**2. Left that one for Andrew Wiles :-)