Scotch:

B Trying to remedy a rube goldberg duct work system that results in, ...,
B less than optimal air flow to the far end of a run. What I can determine
B by eye, tape, and feeling around up inside the duct looks like this:
B
B * GMPN080-4 furnace/AC, plenum & duct work originally serving 1288 sq ft
B * All original branches used top take offs & 6" flex pipe - all work fine
B * Two long runs into garage/workshop area were added later - 576 sq ft
B * Each run to 3 4x12 registers spaced above windows & along outside walls
B
B The concerns I have with one particular run are all the twists and turns an

B I know thats killing the air flow. I make it about 65 feet but I know each
B change in direction makes it seem like more. How much more I don't know...
B
B * Top take off to 18 ft of 7" flex pipe between floor joists
B * Then to a metal 90 and 5 ft of 7" flex pipe across the bottom of the jois
s
B * Then to a metal 7" round to 7" oval straight boot
B * Then to 2 vertical 45s 7" ovals to get back up inside the joists
B * Then to a 90 and 10 feet up inside the wall cavity with 7" oval
B * Then to another 90 to get horizontal again
B * Then to a 7" oval to 7" straight boot
B * Then 24 ft (or so) straight 7" metal pipe with 2 of the registers
B * Then a 90 to 8 more feet of straight pipe with 1 of the registers
B
B I can't see any other way to have done the ducts given the constraints of
B joists / walls / register locations. I can see 3 different things to try:
B
B 1) get rid of the flex duct and go all metal (reduce resistance ?)
B 2) install an in duct aux fan in basement and/or out in work shop
B 3) boost the main fan speed
B
B #1 appears easiest/cheapest thing to try but will it help ?
B
B #2 might be exactly what is needed but I really have no idea.
B
B #3 is beyond me. I have no manual & no idea how to check it but I've read
B that this unit has 3 selectable speeds. I'd really hate to pay a pro $100
B or whatever to come out and flip a dip switch. If anybody has the book on
B this unit...

I would suggest going with a version of Option #2 first: might be
easiest and cheapest of the three. Put a fan at the outlet (in your
workshop) to pull the air from the furnace/air conditioner. The
blower is currently pushing, so a little help pulling may do the
trick. If you have a suitable fan you could test, else a vent-mounted
fan should be obtained for around $20.

I have a bit of an airflow problem here in the Computer Room, located
in the upstairs of a 1«-story house. Gets a little warm in here in
the summer and sometimes a little cool in the winter. Tested using a
4" muffin fan (electronics surplus) which helped but eventually died
after several years. Was at the hardware store and found the
'official' vent fan: thermostat control, magnetically attaches to the
vent cover, bigger blades (think it was $19.99). Seems to work even
better; quieter than the muffin fan.

Note: you might want to support or suspend the vent fan. Either
because the vent cover has been painted a few times or the type of
metal but the magnets on the vent fan didn't hold like they should.

-
¯ barry.martinþATþthesafebbs.zeppole.com ®

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