"BrowserGuy" wrote in message
...
Trying to remedy a rube goldberg duct work system that results in, ...,
less than optimal air flow to the far end of a run. What I can determine
by eye, tape, and feeling around up inside the duct looks like this:

* GMPN080-4 furnace/AC, plenum & duct work originally serving 1288 sq ft
* All original branches used top take offs & 6" flex pipe - all work fine
* Two long runs into garage/workshop area were added later - 576 sq ft
* Each run to 3 4x12 registers spaced above windows & along outside walls

The concerns I have with one particular run are all the twists and turns
and
I know thats killing the air flow. I make it about 65 feet but I know each
change in direction makes it seem like more. How much more I don't
know...

* Top take off to 18 ft of 7" flex pipe between floor joists
* Then to a metal 90 and 5 ft of 7" flex pipe across the bottom of the
joists
* Then to a metal 7" round to 7" oval straight boot
* Then to 2 vertical 45s 7" ovals to get back up inside the joists
* Then to a 90 and 10 feet up inside the wall cavity with 7" oval
* Then to another 90 to get horizontal again
* Then to a 7" oval to 7" straight boot
* Then 24 ft (or so) straight 7" metal pipe with 2 of the registers
* Then a 90 to 8 more feet of straight pipe with 1 of the registers

I can't see any other way to have done the ducts given the constraints of
joists / walls / register locations. I can see 3 different things to try:

1) get rid of the flex duct and go all metal (reduce resistance ?)
2) install an in duct aux fan in basement and/or out in work shop
3) boost the main fan speed

#1 appears easiest/cheapest thing to try but will it help ?

#2 might be exactly what is needed but I really have no idea.

#3 is beyond me. I have no manual & no idea how to check it but I've read
that this unit has 3 selectable speeds. I'd really hate to pay a pro $100
or whatever to come out and flip a dip switch. If anybody has the book on
this unit...

Any advice from a pro/handyman/DIYer welcome.
--
Scotch.

I am sorry, but there are not easy fixes and without a proper total
evaluation and correction, you will only be adding to the Rube Goldberg
system.

To start with you need to do a Manual "D" to get all the data you need
to figure all this stuff out. It does the math to get it right.

I also suggest that you will need a experienced professional. This is
an art form that needs the years of experience of finding ways of addressing
those construction situations that seem to defy doing the job right. Any
good professional will do the Manual "D" and I would not be surprised if
they wanted to do other "Manuals" to assure your HVAC equipment was up to
the task.


--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math