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Why? Because they back-puff into your house, filling it with smoke.
I've read many articles all over the web about back-puffing wood stoves. Many provide half-answers. Many just want to rip you off.
Lots of people suggest increasing the chimney height, preventing negative pressure in the house, and blah blah blah.
Yes, there are many valid points.
But lots of people
have tried ALL the suggestions to no avail. Consequently, we know the available advice is incomplete, and for some people,
Vermont Castings wood stoves NEVER function satisfactorily. Stove manufacturers like to imply operator error; however, at least in some
cases it is the manufacturers who err.
So here's the deal:
Vermont Castings Dutchwest wood stoves back-puff due to preventable flow/combustion instabilities inherent to its design. The air manifold
is designed to keep the glass clean RATHER THAN insert air into the fire. So air falls down the window glass to the fire, or maybe it gets swept up
through the catalytic combuster w/o ever seeing the fire. Falling air - stokes the fire. Rising air - starves the fire.
If air falls, then it gets the fire going, and the rising hot
gases sweep the inlet air up the chimney. This starves the fire, so the fire dies down. So the air again falls down the glass, and POOF it reignites the
fire. You can see this effect if you watch the flame through the glass. It is a flow instability. And that's all it is.
True, the instability goes away if you remove various resistance to flow, such as the catalytic combustor. Yes, this gets the flow going and the
stove is very stabale. But the whole low-flow thing is part of
combustion efficiency. We want the fire to burn slow, hot, and completely.
So here's what ya do. Inside the stove is the inlet air manifold that routes the air from the inlet holes and distributes it (primarily) above the
glass. Remove the screws holding this manifold in place. Then let it tip back into the stove. It will only tip a very small amount.
The resulting open screw holes and the small crack at the manifold base (resulting from tipping) allow air to enter low near the fire. This disrupts
the falling/rising air instability. The air simply rises through/against the fire, and the fire burns much smoother.
I also drilled a couple 3/8" holes on the glass-side face of the manifold, maybe a couple inches from the bottom.
In my experience, these modifications reduce
backpuffing by around 98%. (Almost completely...) Additionally, because the air isn't being swept up the chimney w/o seeing the fire, less air is
needed to sustain the fire.
Two other things to always do: (1) keep the catalytic combuster clean. You can suck it out with a shop vac. (2) When adding fuel, always push
the coals toward the glass and add new fuel behind it. This keeps the fire nearest the inlet air and prevents flow instabilities.
So what do I know about instabilities? Well actually, in my earlier years, I specialized in stability of boiling water nuclear power plants.
It turns out they can be unstable IF the following primary points are true. (1) Exit flow is restricted. (2) Inlet flow is less restricted.
(3) Flow of water through the reator has a power feedback effect. (4) They are large. All of these conditions exist on the Vermont Castings
Dutchwest Wood Stoves. Relative to other wood stoves they are large; however, this effect would be diminished is air was introduced uniformly
into the stove through a sparger rather than a manifold on one side of the stove. This sort of thing is so obvious you can see it with your mouth.
But alas, keeping the glass clean takes priority. (Thank god there's no glass on nuclear power plant reactors.)
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