I was having low air supply from 6 registers causing hot/cold zones around my house. Found the 4" supply ducts were not only undersized for their respective rooms but were leaking, badly dented , 1 was completely disconencted and 2 of them were split to heat an upstairs and a downstairs room. Another register was simply a grill splapped over a hole cut in the side of the plenum...this register was porbably had the best flow.
I replaced 3 of the 4" supply ducts with 5", ran a 6" split into 2x 4" for the 1 of the up/downstairs rooms and added 1 new 5" for the other. Completley removed 2 of the 4" ducts from 1 room and capped them off at the plenum. That room used to have 3x 4" and 1x 6" supplies, it now has a 6" and a new 5" duct. I plugged off the plenum hole register and ran a proper 5" supply. Stuck dampers in all the lines.
Now the house is evenly heated without any cold spots. Airflow is pretty good/balanced at all registers. Should I add additional return air for the increase in duct size? How can I tell if I'm starving my furnace? Thanks for the help.
sort of a redneck ballpark way would be to remove the blower door and see if there is any drastic increase in airflow also hold the blower door in front of the opening and slowly lean it towards the opening as if you were going to put it back on if it really sucks it out of your hands strongly when your a couple of inches away you need more return, of course the proper way would be to measure static pressure
Tabb, It all depends on your furnace and existing setup. Take a look at this website. It has a very good discussion of duct design. If you read it carefully you'll probably have a good idea of whether your returns are adequate. http://www.descoenergy.com/duct_system-02.htm