An ACHR News piece on multifamily air makes a point that applies well beyond large apartment buildings. Its summary describes exhaust, make-up air, ventilation, filtration, and building leakage as parts that interact, and it warns that "getting one wrong can affect the entire building." If you share a wall, a floor, a chase, or a return path with a neighbor, your house has the same problem in a smaller package.
Name the Five Parts First
Start with a sheet of paper and a pen, not a fan. Write down every device that moves air out of the envelope. Bath fans, kitchen hoods, and the dryer vent are the usual suspects. Then write down what replaces that air. In a tight house, replacement air arrives through gaps and under doors, and that is the make-up air question. If it comes from a neighbor's unit, a basement, or an attic, you have a path you did not plan.
Ventilation is the deliberate supply or exhaust you added on purpose, often a balanced fresh-air unit or a heat recovery ventilator. Filtration is the filter in the air handler and any inline media. Leakage is everything else: the gap above a partition wall, the unsealed rim joist, the attic hatch without weatherstripping, the plumbing chase that connects two floors. Each item on the sheet is a potential path. The sheet matters more than any single reading.
Measure the Pressure, Not Just the Airflow
A digital differential pressure gauge, sometimes sold as a manometer, costs little and tells you whether the room is pulling air in or pushing it out. Set one hose to the room you care about and the other to the outdoors or a reference space, with all exterior doors and windows closed. Run the bath fan, then the kitchen hood, then both together. Note the reading at each step. A reading that goes more negative when a fan runs means that room is short of replacement air and will draw from wherever it can reach.
Doors matter more than most people expect. A bedroom door with a quarter-inch undercut behaves differently from one sealed to the floor. Check the undercuts, the register sizes, and whether a return grille is open in a room that should be closed off. Ceiling fixtures with unsealed penetrations into an attic can move conditioned air upward during a pressure swing. Use a smoke pencil or an incense stick at each suspected gap and watch which way the smoke leans.
Check the Combustion Side Before You Change Anything
Any gas water heater, gas furnace, or boiler that vents through a chimney or flue belongs on the list. An exhaust fan can pull combustion gases back down a flue when the house is depressurized enough, and that is a safety issue, not just a comfort issue. If you have combustion appliances, confirm they draft properly with the exhaust running, and keep a working carbon monoxide alarm on every level. Treat any sign of backdrafting as a reason to stop and call a licensed technician.
Write the Map Before You Replace Anything
Once the list and the readings are on paper, sketch the house in plan view and draw arrows for each airflow you found. Mark the paths that connect your space to a neighbor's unit, a common corridor, or an unconditioned attic. That drawing is what a technician needs to size make-up air or propose a balanced ventilation strategy, and it keeps you from adding a second exhaust fan that makes the first one's problem worse. If you rent or share ownership, the same map gives you a precise question to bring to whoever manages the building.
Before you buy new equipment or upgrade a filter bank, review what your policy says about coverage for home systems and appliances. Know what is covered before a fan or ventilator fails, not after.