Make-up air ducting on twin furnaces
I have a pair of furnaces in a new 2-storey house. But I'm confused about the fresh air/make-up ducting.
The upstairs furnace has a fresh air intake directly into the cold plenum.
The main floor furnace has a fresh air intake directly into the cold plenum. The main furnace also has a separate fresh air intake that goes through a damper, make-up air heater into that same plenum. That damper opens up when the hood fan interlock is triggered.
What I'm confused about is the un-conditioned fresh air into the main furnace. I realize with driers, bath fans, etc, you don't want a negative pressure in the house, but it the make up air heater line doesn't make sense since the furnace could also suck from the regular fresh-air line as well.
Is a residential furnace typically connected that way? It seems like it would make more sense to not have the electric damper, and have a make-up air heater on both fresh air intakes. That way incoming air is always pre-heated if necessary.