Smell from ceiling exhaust fans?

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Old 11-06-19, 11:00 PM
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Smell from ceiling exhaust fans?

The smell comes from the bathroom ceiling fan or sometimes from the adjacent laundry room ceiling fan (both on main floor of 2-story home). Or sometimes both at the same time.

It smells like dead animal or rotten urine smell and is overpoweringly strong

The smell appears suddenly and lasts from about 0.5-2 hours before completely dissipating. Turning the fan on will temporarily remove the smell completely.

The frequency is about 1-2 times per month.

The fans are drywalled in so I cannot remove them without damage. I have used a snake camera from the inside (fan flap) and from the outside vent that show nothing inside the entire venting lengths. Both vents have intact bird screens on the outside.

What next?
 
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Old 11-07-19, 05:30 AM
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It is very possible the smell is not coming from the vent fans. Have you checked the crawl space for a dead animal?
 
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Old 11-07-19, 05:37 AM
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There is no crawl space as it is in between floors. There is a 1 foot space to allow venting though. Also, why would the smell only happen in short intervals over long periods of time? I may have to take the drywall off to find out I guess.
 
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Old 11-07-19, 10:01 AM
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The one foot space you mention. Is that your crawl space or the joist bay between floors?
 
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Old 11-07-19, 12:45 PM
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Actually a better way to describe it would be that the ceiling in the bathroom and adjacent laundry room is 1ft lower than the rest of the ceilings on the main floor. I assume to make room for fan ductwork.
 
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Old 11-07-19, 01:52 PM
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mice in the duct work or in the joist space.
 
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Old 11-07-19, 02:19 PM
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Why would it only last for about an hour and then dissapear?
 
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Old 11-07-19, 05:22 PM
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Weather. All it takes is wind from a certain direction or changing air pressure to force air into the living space.
 
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Old 11-07-19, 07:49 PM
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I thought that as well but then it is absent for months at a time so I doubt that explanation at this point. Also against that explanation is the fact that it sometimes comes from the bathroom and sometimes from the laundry even though they are both vented to the outside. Thanks for the ideas though. It is a mystery. Might have to dig through the drywall to find out what it is.
 
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Old 11-08-19, 06:31 AM
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You still haven't mentioned looking in the crawl space underneath.
 
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Old 11-08-19, 07:39 AM
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Sorry, dumb questions.
Is this a multi-unit home?
Being on the lower level (assuming there is a floor above) puts those fans near what we call the neutral zone (NZ). In winter all home experience a natural air flow with outside air coming in the lower openings and exiting the upper. But the NZ shifts up and down as the home experiences any change in infiltration or exfiltration. Open a window and it can shift. That shift could make those fans a source of intake.

Wild guess, I'll watch for your input.

Bud
 
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Old 11-08-19, 11:01 PM
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Single home with 3 levels (basement, ground floor, upper floor). The bathroom fan and laundry room fan on the ground floor share the same crawl space (1 foot) for ductwork. Sometimes the smell comes from the bathroom fan. Sometimes from the laundry, sometimes from both.

Again, the fact that the smell doesn't happen for months and then appears suddenly and dissapears is confusing an doesn't fit the positive/negative pressure idea.

I am not even sure who to call to investigate. HVAC/ductwork guys or pest control??
 
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