I have a 400 CFM MUA / Fresh Air intake that is comprised of an electric damper + Fantech blower + Electro Controls duct heater that feeds in to the return air just before the filter/furnace/AC.

Several times I’ve wondered what the downsides would be to leaving the damper open so that the the house could suck in fresh outside air as needed even when the MUA is off. In theory there s/b minimal outside air (cold, temperate or hot/humid) coming in when things are in balance and so only when exhausting appliances like clothes dryers are running will any air come in. Reality may be quite different.

Granted, a good chunk of make up air will come in through cracks and stuff anyway (house is about 3,85 ACH0) so this isn’t really needed for low CFM things like bath fans, central vacuum, clothes dryers, etc. However, what is the energy difference between 200 CFM coming in through cracks and 200 CFM coming in through an open damper on the MUA? In theory (:-) the energy penalty s/b the same. In theory air coming in through cracks could create more human discomfort and possibly be less fresh if coming through wall cavities rather than coming in through the open damper. In theory this would be lower static pressure than air coming in through cracks and leaks and so exhausting appliances should work better.

Thoughts?