The furnace is a horizontal unit located under the house. I recently extended one plenum from the furnace. The plenum originally ran under the house, up through an interior wall, and exited the wall near the ceiling (about 7.5' above the floor). I extended the plenum, using a 6" duct, into the attic (1.5') and then horizontally about 12' to terminate at a register in the ceiling. When the heater is on, I hold my hand at the register and feel a "little" warm are, but no air movement. Prior to the mod, I got very good flow and heat from the original location. I get excellent flow from my other registers. Assuming no obstruction, could I have exceeded the capabilities of the unit?