"BrowserGuy" wrote in message
...

* 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



This part about the garage scares me, totally against code in most areas to
heat your garage with the house's furnace.
Now if this is remodeled space that you can no longer get a car into, then
no problem.

If it is still a garage, rip out all the duct work that attaches to the
house's furance, and add a second furance to heat just the garage. The two
systems need to be completely independant of each other, and no connection
what so ever for air to flow from the garage to the home.
Greg