Bathroom fan exhaust near fresh air intake

A homeowner asked whether it's fine to run a bathroom fan's exhaust duct right next to the intake for a home's fresh-air system, figuring the bathroom fan runs so rarely it shouldn't matter. The answer from building science pros: no, spacing rules apply regardless of how often the fan runs.

Building codes set specific distances for this. The International Mechanical Code requires at least 10 feet between an exhaust outlet and an air intake, and at least 3 feet between an exhaust outlet and any operable opening, like a window that can be opened. These rules exist because even occasional bursts of moist, odorous bathroom air getting pulled back into the house through an intake defeats the purpose of bringing in fresh air. It doesn't matter that the fan only runs sometimes — the two vents still have to be kept apart to meet code.

For someone dealing with this in their own house, one workaround suggested is routing the bathroom fan duct out through the roof instead of the wall, which can make it easier to keep the required distance from the intake without cutting a new hole. Whether the intake in question feeds combustion air for a gas furnace or fresh air for general house ventilation also matters, since the setup and consequences differ. If you're placing or troubleshooting bath fan and intake vents on an addition, remodel, or new ventilation system, it's worth checking these clearance rules before finalizing duct routing.