Quick Overfire

[MrCampbellAHS]

We are obviously early in the burn season, and we feel like we've finally figured out our GM 60, but I've noticed over the last couple of all night burns that the catalyst gets really close to over fire a bit quicker than we're used to. Last night we even got the over fire smell pretty strong. I shut the air intake back and opened the bypass to get it under control pretty quick, but I had similar issues this evening even on a smaller load out. I'm wondering if it's a leaky door gasket? The one thing I didn't do in my pre-season check was the dollar bill test and I'm thinking logically that a leaking door gasket would be introducing more air than I need. Thoughts?

 

[PJ Nevada]

You probably don't have enough air. I know, how can that be, the cat is overheating!

Well, wood doesn't burn. I know, you put wood in your stove and watch it burn, right? Well the process is a little more involved. When wood is heated enough it goes through pyrolysis, chemical breakdown. Part of that break down is the gasses that are emitted which burn when mixed with oxygen. Hence the flames dancing above the wood. One of the items remaining after the breakdown is char. The char is what burns as embers, without flames, mostly.

So what causes the catalyst to overheat? When a good strong burning fire is throttled down quickly, the chemical breakdown is still going great guns. The gasses are still being released but there is no oxygen to burn the gasses. When these excessive unburnt gasses enter the catalyst chamber the cat tries to burn them and over heats.

While opening the bypass stops the excessive gasses from entering the cat chamber, it does trap the excessive heat in the cat. The manual from my Yotul Firelight said to open the damper during this condition. Thus burning the gasses at a lower temperature in the stove chamber. The relatively cooler burnt gasses will enter the cat chamber cooling down the catalyst. If the temperature doesn't start to drop, then open the bypass.

I remember this from 30 years ago after cooking my original catalyst in the first year or two. Yotul was kind enough to replace my original cat and the replacement has been going strong ever since. A good 28 years.

So the advise in my Yotul Firelight 12 owners manual was to open the damper in a cat overheat conditions letting more air into the combustion chamber. Then open the bypass if the temp does not drop. I think it even called it "counter intuitive" when describing the process.

So my process is: once I have a good strong fire going with at least some coal base, I close the bypass. When it's time to throttle down the fire, I do it a little at a time. Five to ten minutes. This also helps prevent puffing, which is the unburnt gases collecting in the top of the burn chamber until enough oxygen is present to ignite them.

Disclamer:
So this works for my Yotul. Out of the owners manual. Please check your stoves owners manual, they may have completely other recommendations.