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.
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.
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