bath exhaust fan?

16 years ago

We had a Panasonic exhaust fan installed in the master bathroom of a house that we visit every few months, and at the same time had the existing fans extended to vent out the roof. It was common practice here for builders to just vent into the attic. The electricians had trouble with the new fan, and ended up wiring it to the overhead light switch.

Since then, on two visits I've noticed a slight sewer gas sort of odor in that bathroom. This past weekend, the odor was strong some times and not at other times. A plumber came today, but couldn't find anything except a need for fresh caulk at the base of 2 toilets. The odor was not apparent then, but it had returned when I checked a couple of hours later.

There appears to be a correlation between running the new exhaust fan and the smell. Is it possible to install a bath exhaust fan backwards, so that it blows into the room instead of drawing air out? The ceiling is too high for me to feel the fan with my hand, but a dryer sheet held up to the exhaust fan on a pole was not drawn to it, it only fluttered a little. I'm wondering if the exhaust fan outlet on the roof is near enough to the plumbing vent to draw sewer gas back into the house. Does this seem plausible?

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Comments (9)

cobraguy
16 years ago
Is the odor more noticable with your fan running?
badin
Original Author
16 years ago
Yes, the odor seems to be worse when the fan is or has been running. The odor was very strong a couple of nights ago (5 - 9 p.m.) while/after I'd used the new fan to "air out" the room (windows open in the master bedroom) but when I got up around 2 a.m. to check on my son, there was no odor.
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Hmmm, that's interesting. We have a fan unit in the attic that vents 2 bathrooms that are side by side. From the unit, the vent duct goes straight up to the roof. You could certainly insulate the duct work, I am pretty sure we did that mostly for noise (we used rigid ducts and you hear the turbulence, not the fan). We have a cathedral type ceiling in both bathrooms, and we mounted the vent on the vertical wall above the shower head, routed to the motor, then as above (does that make sense to you?). This does not eliminate the heat loss into the duct, but as it is on a vertical wall, rather than the ceiling, maybe it is less like a "chimney", although I am sure we loose heat there too. This routing avoids running what sounds like a "heat conduit" between the roof and ceiling. But now I think I will go outside in the morning and look at our roof after over a foot of snow...
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Thanks for the suggestions. Text communication being what it is, I may have been misunderstood on a couple of points. The reason for not wanting to vent near the porch or under windows has nothing to do with appearance from the street, but rather what I thought was a more obvious objection to moisture and odor wafting in through open windows during the warmer months, or making the porch a less enticing place to relax or entertain when the bath is in use. I confess to being a bit fussy about details, so if I did go through the roof, I wanted the vent to be in the same line as the other two bath vents, which are a couple feet apart in the same shingle row, thus the offset in the attic. Another Q/A site suggests that the worst two places to vent a bath is 1) into the attic, even if under a ridge vent, and 2) under a vented soffit. The site "expert" instead recommends going through the roof, though the bath in this case is on the highest floor. The two existing baths have vented through the roof for many years with no apparent issues. The runs are relatively short, the duct is insulated and the vents themselves have dampers. Enough warmth seems to escape to keep snow clear around the vents. The clothes dryer also vents through the roof as the alternative would be to vent directly out to the front porch, which would create a nice steamy fog on cool days, not to mention the moisture rising up through the porch ceiling vents. Again, no apparent issues after years of use. Having said all of this, I appreciate the concern about running a duct from the basement to the roof. If I can find a path to an acceptable point along the rimjoist, I'll take it.
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I think you mean the vent duct is uninsulated? Another possible cause is not leaving the fan running for at least 30 minutes after showering/bathing. That's why many are installed with timer switches. You need to keep air running through the line for awhile so that the moisture that condenses in the ducting gets absorbed into the warmer and less-humid air that passes through afterward. It also helps to open a window, however slightly, in another part of the house so that you get adequate airflow out the vent. If your house is tight and there isn't an air opening to the outside, the vent won't be able to exhaust much air. (It's not like it can suck so hard as to create a vacuum in the house). Air will only exhaust to the extent the outflow is matched by an inflow.
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I do not like soffit bath fan vents. My house had one, and you could tell in the attic that the wood was exposed to more moisture there. All of my soffit is the vented aluminum. The window near that exhaust was also more worn. If you exhaust it out of a soffit, it will either wear out a nearby window, the nearby soild soffit, or if the soffit is vented it will go up into the attic. I went through the roof. Get a long feeler drill bit, go into attic, drill bit in appropriate spot, leave bit in and go on roof and you can see where the center of your hole should be. Learn how to install the vent on the shingles, it is similar to the plumbing vent flashings. It is easier to put the flex duct through the hole and attach to vent, then install the vent on the roof.
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heimert
16 years ago
Or the fan could be pulling air out that is replaced from some source of sewer gas, such as a p-trap that doesn't have any water in it because it's dried out from lack of use.

Why would the attic have a sewer gas smell when the stacks shouldn't be vented inside the attic?
badin
Original Author
16 years ago
Heimert, your suggestion about the exhausted air being replaced from some other source of sewer gas occured to me also. However, I can't figure out the source. I've run water in both sinks & the tub, and the shower has been used a dozen times now.

I'm sorry, but I don't understand your question about the attic. The plumbing vent stacks vent out the roof. It was the original exhaust fans that were vented into the attic.
cobraguy
16 years ago
badin, I would say at this point it's time to get up on a ladder and see for sure what direction the fan is running. It very well may be running backwards. If that's the problem, I would sure want to know why it's picking up the odors in the first place.
davidandkasie
16 years ago
there was another thread onthis board recently about this same issue. turned out to be a dead animal inthe attic next to the fan. running the fan "stirred" up the odor and resulted in smelling it in the house. you may need to check inthe attic as well.
normel
16 years ago
Also, look for a floor drain somewhere that may have dried out allowing sewer gases to be drawn in.
beacher003
16 years ago
I would tend to agree with cobraguy about the fan running in reverse. If it's drawing in air and the vent cap is on the roof it's most likely getting the smell from the vent stack. I have a roof level deck on my house and can sometimes get the smell from one of my vent stacks.
If you can access the fan in the attic, disconnect the hose and with the fan running check the air flow. It should be blowing out of the fan housing and into the attic.
Most vent caps have flappers on them to prevent back flow so that might not be the issue.
homebound
16 years ago
Possibly a gunky drain + overflow in the sink. If so, the odor would be more noticeable right after water goes down the drain. Pour a dash of bleach down the drain and see if the odor continues.

(BTW, do folks really install bath fans backwards??)

bath exhaust fan?

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cobraguy

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bath exhaust fan?

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cobraguy

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badinOriginal Author

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