Owners are often puzzled by a smell that appears only when the fan is on. The fan is meant to clear the air, yet the coach suddenly smells like the black tank. The explanation is simple physics: an exhaust fan pushes air out of a small, fairly tight box, and the same amount of air has to come back in. If the easiest path is a slightly leaky toilet seal or a drain with a dry trap, the fan pulls tank air straight into the room.
The effect is stronger in newer, better-sealed rigs and with high-powered fans that have several speed settings. It is also stronger on a windy day, when air moving across the roof can lower pressure over the coach or push down on the plumbing vent.
Fan set to exhaust versus fan set to intake
Many roof fans can reverse direction. Each mode has trade-offs for odor.
Exhaust (blowing out)
- Removes cooking steam and shower humidity
- Creates negative pressure inside the coach
- Can draw tank air through weak seals and dry traps
- Needs an open window on the far side to balance airflow
Intake (pulling in)
- Pushes air into the coach and out through gaps
- Keeps tank air from being pulled up through drains
- Can pull in campfire smoke, dust or generator exhaust from outside
- Less effective at clearing steam from one spot
A simple test
Close everything
Shut windows and other vents, and make sure the toilet bowl has a little water in it.
Run the fan on exhaust at high speed
Wait a few minutes. If the smell appears, note where it is strongest: bathroom, kitchen or a cabinet.
Open a window across the coach
Crack a window on the opposite side and wait again. If the smell fades, pressure was the trigger.
Find the weak point
Pressure only reveals a leak; it does not cause one. Inspect the toilet seal, refill traps and check under-sink valves where the smell was strongest.
Questions to bring to an RV service visit
- Is the toilet seal holding water overnight, and is a replacement kit available for this model?
- Are the under-sink air admittance valves closing properly?
- Is the black tank vent clear all the way to the roof, and is the cap intact?
- Would a different vent cap that draws air out of the stack help on this rig?
- Is anything about the fan installation or the coach's seals unusual for this floor plan?
Common questions
Is it safe to run the fan if it causes a smell?
Usually, as long as fuel-burning appliances are not affected. Open a window opposite the fan and fix the weak seal or trap. If exhaust smells appear, stop and have it checked.
General information for the Denver area, not a quote or professional advice. In an emergency, call 911.