A 4.5 ton (100K btu) 95% furnace & 4K HP was recently installed in my home. The install was a MOVE from the previous loc. and required new return installed and furnace output patched into the old ductwork.
The plenum out of the new furnace (feeds out the bottom) is roughly 20x20". Upon opening the floor the installer encountered two 2x4" joists traversing the area of the plenum. Not wanting to cut the joists he decided to take a 16" round, squish it slightly into an oval 14" across (the clearance between two joists) and use that to couple the ductwork to the plenum. You can see a photo of the plenum exit at http://files.raywired.com/plenum_exit.jpg. By simple math, this leaves nearly 50% of the plenum output area blocked.
On the bottom side, there's about 18" straight to a 90 deg elbow and then that feeds the old ductwork through a reducer. (http://files.raywired.com/plenum_down.jpg)
The installer assures me that the 16" round is plenty big enough to carry the 1600+ CFM. But I'm wondering - isn't this gonna be a hell of a lot of turbulence and thus noise, and perhaps an efficiency hit? I'm the sort of person who is quite sensitive to noise, and in this case I'm probably more concerned about the noise aspect than the efficiency question.
The furnace (AS) calculates CFM and reports it on its controller board. It indicates it's moving 1600 CFM, so I guess whatever the SP is it isn't beyond the capacity of the blower to cope. I have some leaky ducts, so I'm not sure how much the present SP would really tell me (unless it was high despite the leaks ?)
The old furnace sat on top of a good sized box that drove 3 ducts out the various sides. The box was torn out when the old furnace went away. The new setup seems noisier than the old, in terms of vent noise and I wonder if this plenum exit is related to the issue. (granted more airflow might be a factor). The main return will need some work too-- it seems to transmit a 300 Hz whine into the house when the blower is running at a particular speed (blower is var. speed electronic drive).
Perhaps the next phase of this project might be to have the entire duct system looked at to get it optimized for the heat pump and the loads around the house, but also with an eye to reducing the noise in the system. Re. the subj of this post though, I don't know enough about this stuff to know how much I should be concerned about this plenum issue. I guess I was expecting a full 20" square shot coming off the furnace, through a smooth (radiused) 90 turn, and routing over to the old box. I'm running on logical intuition here, but without any real-world knowledge or experience to back it up.
Many thanks for any and all constructive commentary!
P.S., minor point, there is no rubber boot decoupling the bottom of the furnace from the duct. I was told the install space height was too tight to allow that. I asked about decoupling the duct in the crawl with a rubber shoulder. Is this a problem from a trade practices / permit point of view (fire etc.)?