Because your bathroom fan creates negative pressure in relation to the rest
of the house and your crawlspace, it will draw air in any way possible. I
would look for leaks around the outside of the air duct opening into the
room, under the wall (behind the baseboard where pipes and wires come up
through holes), around the toilet, around the pipe holes in the wall under
the sink, under the toilet tank, around the front edge of the tub, electric
outlets/switches, anywhere there is a possibility of air being drawn up from
the crawlspace. Seal ALL the joints in the ducts along with insulating them,
and spread heavy weight poly over the entire floor of the crawlspace taping
the joints and running up the walls to seal the dirt floor. Ventilating the
crawlspace may help depending on your climate.

"peter" wrote in message
news:k90ze.9077$kh3.3749@trnddc03...
If I close my bathroom door and then turn on the ventilation fan, the air
being sucked out of the heating vent has a crawlspace odor (I've been down
there many times, so I recognize the smell).

I suspect there is a leak in the heating ducts in the crawlspace. However,
when I went down there to take a quick look around, I didn't see anything
obvious.

Is there a good way to locate the leak(s) quickly? I thought about
injecting
smoke into the heating system and look for the smoke in the crawlspace.
But
that would also fill my house with smoke. I suppose I could turn on the
heat
and go over every inch of ducts in the crawlspace to feel for warm air,
but
this could take a long time and I may not catch all the leaks.