Comment by hinkley
3 hours ago
I suspect bathrooms aren't big enough to buffer the air pressure but it seems like we should design the air handling so the “fan” is always on in the bathrooms. Maybe a split between several places and taking a bit from the cold air return for the rest.
The system is always on and moving air as a whole, and the water closet and bathroom have an intake duct that sucks in humid air that goes to the heat exchanger for exhaust. I have a little humidity sensor hooked up to Home Assistant that kicks the fan in the exchanger into a higher gear and returns to auto when humidity returns to baseline.
All rooms in the house have an intake or exhaust duct depending on requirements.
There is also a small control panel next to the thermostat in the living room that controls the whole system for when, ahem, your number two's are particularly odorous (or you're using the kitchen to cook for 6).