Too much draft with bypass open sucking flames into the pipe.

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Flames 15’ up a stack means that the flue temps near the stove were well over the 500 needed to activate the catalyst. Should have shut the bypass sooner. That’s the fix here. Monitor flue temperatures.

As mentioned above my more seasoned experts, I close that bypass when flue temps hit 500 internally measured. The cat meter is fun but I think it’s pretty slow to respond.

Another “trick” is to load the fuel so that a split blocks the direct path of flame to the bypass opening. Put one on top in the middle.
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Thank you all for your replies. I had the chimney inspected with a camera yesterday and they said it looked fine. They said it was clean and that I did a good job cleaning it but thought that I probably loosened a piece of creasote up and that it caught fire inside of there somewhere. They said shutting the tstat down and closing the bypass wouldn’t of stopped an actual chimney fire if a full fledged one was happening.

I’ll have to monitor flu temps a little closer. I’m thankful that my chimney isn’t warped and seems perfectly fine per camera inspection.
Great. You can sleep well tonight knowing the fire in the metal box in your home is still safe.

You can buy a Condar flue probe online easily. Less than $20 I believe.
An Auber digital one is more realtime.
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