The units that I have seen are installed in parallel with the main duct
leaving the furnce air box. Air flows through the humidifier all the time
with the humidifier only adding humidity when called for. There is no loss
in efficiency at all because the humidifier air flow path is a small amount
of extra flow capacity that would not be provided if the humidifier was not
there. A damper is not required but if there was one it would keep air out
in the summer which would keep the unit from getting a bit dusty.
========================================Tnx Cris & Reno
This one definately loops back. There's plenty of room, so adding a damper
will be easy
Understand point that looping back air is not big deal when burning fossile
fuel My HP doesn't loose set-point it until outdoor temp goes below 22,
so I'm using HP most of the winter. I suspect in HP mode, looping any warm
back across the indoor coil probably reduces HP efficiency.
Begining to suspect another problem w/ this mostly HP mode of op. HP mode
air not warm enough to evaporate water very well. In dry cold weather
humidifier is running almost all the time the HP is on which at 22 deg is
continuous. Humidifer is maintaining set-point, but there's a lot of
unevaporated water being wasted.