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Originally Posted by ericjeeper
I never really understood the term cold fire.How does the fire know the temperature of the surrounding elements?
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Same way your engines behave when they are cold.
It's not the wood that burns, its the gasses. That's why when you look closely at the fire, you'll see the flames are above the surface. Same with candles and such. The wick of the candle chars, but it's the gassified wax that's doing the burning. The only time the wick is burning is when the fire is first lit and things are cold.
In a hot firebox (and by hot, I mean over a thousand degrees usually), the combustion gasses stay hot and burning. This gives you a longer burn time for a given quantity of fuel. So you get more heat from that quantity of heat. You also get much less smoke, as more of the fuel is burned up.
In a cold firebox, the combustion process gets extinguished very quickly because the cold box draws the heat of the combustion process away so strongly that the fire itself cannot be sustained. So you get much less heat from a given quantity of fuel. You also get a lot more smoke (smoke is just incomplete combustion) and creosote buildup.
For really neat burn efficiency, you add in a secondary combustion chamber. This is a place where those really hot exhaust gasses get to hang out and do even more burning. The temperatures here are in the 1,500-2,000 degree range typically. Special firebricking and such is required to handle these heats. Units using this sort of approach are virtually invisible when burning as there is nothing to see but some heat shimmering. Expensive! but you use a lot less fuel for the same amount of heat.
You will not find a real secondary combustion chamber on an outdoor wood boiler. If one were there, it would burn through in a few hours.
