ok... if your tesp is 1"... you do not know how undersized your ducts are because 1" is the maximum the blower can produce...
at a certain pressure, a pipe can only flow so much air.
all systems need a certain flow of air to transfer the heat to where you want it.
if you have ducts that will only flow 1/2 of the required air the equipment needs, the blower will spin faster and faster until it's efficiency is maxed out, and it will stall. the equipment will not have sufficient air to move the heat and either the furnace limit will pop, or the air conditioning compressor will flood with liquid...
it's got to be the CORRECT amount of air for the equipment to work right.
supply ducts are not one long duct with holes in it.
a blower should blow into a large plenum, and small ducts branch off the large plenum.
first the plenum has to be large enough to flow the required air.
then the branch ducts need to be large enough to flow the air the room needs. some rooms need more branch ducts in them to get enough air to condition the room
so... if you have a large plenum with only 4 small branches in it, it will not flow enough air out the 4 branches. making the branch ducts larger will flow more air into the rooms... or cutting more branches into the plenum will flow more air...
understand?