How to find heating ducts leaks in crawlspace?

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.
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peter wrote:

Smoke test? Tony
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peter wrote:

That crawlspace smell might also be mold growing in your ducts. Have you had them cleaned?
R
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Are the ducts insulated? If not, they should be.
Perhaps a better way to attack this problem is to get some mastic duct sealant that you brush on and seal every joint in the duct system. Then insulate!
Yeah, it's a lot of work, but you'd spend much of that time trying to find all the leaks.
Between sealing the leaks and insulating the ducts, you'll save a bunch of money on your heating bills.
--
Doug Boulter

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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.

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peter wrote:

Infrared picture? Tony
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Take the floor registers out and seal around the register boots with clear silicone. Wait a day before putting the floor registers back in so you don't seal the registers to the boots.
Stretch
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