Humidifier Install - Aprilaire 600 (okay to reduce 6" bypass to 4"?)

I'm replacing an old Honeywell TrueSteam with a new Aprilaire 600. The Honeywell has a 1" bypass duct that connects to the furnace's supply plenum, directly above my furnace (upflow). The Aprilaire calls for a 6" bypass duct. Unfortunately, the supply plenum above my furnace is too small for a 6" circular cut. I could fit a 4" hole, which would require a 6" to 4" reducer. Would this significantly reduce the efficiency of the humidifier? Is there a chance of this setup damaging my new humidifier or A/C system? Alternatively, a rectangular duct would fit. Would a 6" to 10" x 3.25" duct be a better option?

Thanks in advance for any recommendations or guidance.

 

Yes. Much less of a reduction than a 4" connection.

A 4" connection would work. It wouldn't damage anything but from the reduced air flow..... you'd have less moisture transferred to the air and more wasted water coming out of the drain.

 

Went with the 10” x 6” rectangular ducting. Still working on the install, but got this duct installed and it looks pretty clean. Thanks to PJMax for the recommendation!

 

You're welcome.
Looking forward to hearing about the completed job and lots of humidity in the house.