The return duct in the attic, from the last pickup register to where it
comes down through the ceiling into the furnace room, is 162 sq. inches.
In the furnace room, return air passes first through a duct that's 220
sq in (10 x 22), into a duct that's 235 sq in (10 x 23.5), and finally
into a 330 sq in (23.5 x 14) duct that feeds a Honeywell F75 electronic
filter attached to the furnace. Do these larger sizes relative to the
162 sq. in. in the attic cause any problems?
For what it's worh, there is an Aprilaire 560 humidifier on the supply
plenum whose return is into the 220 sq. in. duct.
No, the larger sizes of the other ductwork would if anything improve airflow
thru the smaller duct by reducing other losses to just about zero. The
humidifier has no bearing on anything. Final duct size may be dictated by
the sizes of the stuff it connects to, like the cutout in the furnace
itself, or the size of the aircleaner.
Donald and Scott:
Yes, the final duct sizes are rather close to those of the electronic
filter and the furnace itself.
My concern is that the increased duct size reduces the pressure in these
ducts. The return for the humidifier fits into one of these ducts. So
the airflow through the humidifier would increase, thus reducing the CFM
into the main supply trunk and increasing the amount of time the furnace
runs to satisfy the thermostat setting.
Just as the main supply trunk, the outlet registers, the return
registers and return ducts form a closed loop (if everything is well
sealed), I view the humidifier as a loop within this loop. It adds
moisture, but reduces CFM into the supply trunk.
My ultimate concern is long furnace run times to satisfy the thermostat.
For example, this morning with the power to the furnace off all night,
thus no residual heat in the ducts, the house temperature was 59F. It
took 30 minutes for the temperature at the thermostat to reach 61F, and
1 hour, 50 minutes to reach 68F. Attic temperature, where the ducts
(duct board with 1" insulation) are located, was 37F. That's too long.
Yes, I have located a leak in the return duct that I need to fix. That
will shorten those times, but the humidifier is lengthening them.
Thanks for your comments.
I am printing this for others that will come later to read...and laugh..
Get a correct manual D done. Once that is complete, you can see where you
Any AC contractor worth a damn can check this in a few moments and tell you
BTW...your humidifier has nada to do with your problem.
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