PSA: that 20% soot rule for masonry chimneys is actually a hard limit

I used to think the 20% soot depth rule was just a suggestion you could eyeball. Then last Tuesday I watched a YouTube video from the CSIA where they showed a chimney that tested at 22% and had a chimney fire start in 40 seconds during a demo. That video used an infrared camera to track the heat spike. I grabbed my own gauge the next day and checked three jobs I had done that week. Two of them were at 18% and I had left them thinking they were fine. Now I carry a depth probe on every job and I measure before I leave. Has anyone else had a close call from trusting the look test over the actual measurement?

2 comments

2 Comments

derekgibson15d ago

Oh come on, that CSIA video is a setup if I've ever seen one. They didn't say what kind of wood was burned or how dry it was, and they probably packed that chimney full of creosote for the demo. Real world conditions are way different. I've been sweeping chimneys for 27 years and I can count on one hand the times I've seen a fire start from soot buildup. Most of the time it's from a bird nest or a cracked flue tile. That 20% number is a guideline from the insurance industry to cover their own backsides, not a magic line where your chimney suddenly explodes.

the_reese15d ago

People latch onto these "expert" numbers like they're gospel and ignore the actual messy reality of how things work. It's the same mindset where folks trust a thermostat reading more than their own shivering skin.

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