Venting RMH

[matt walker]

Needing to cover part of the feed is totally normal guys, don't sweat it too much. I ended up choking down my feed quite a bit, it's now roughly 5"x5". That allows me to run it open and it almost never creeps, but depending on the fuel, there are still times I need to cover it.

Fuel is certainly the most important factor in whether the fire creeps up the feed. Too wet internally to burn down and fall in before the flame front climbs the wood is one way it can manifest. Wood that has a lot of very dry surface area and not much fuel value can also just plain overload the system and creep can happen that way. I've noticed a huge difference in burning fir vs. maple vs. alder, etc. They all behave differently, and moisture content and split size, in the round, etc. all make a big difference. Sticks are one of the worst offenders for me, unsplit branches and such. They just stay wet inside for a very long time, but usually have really flammable outer layers.

[rjdudley]

That's really good to know. To tell you the truth I'm not sure just how dry most of my wood is.
And Matt, since you now choke the feed to about 5x5 what do you think about redesigning your mold? Maybe a 5x5 feed tube and the burn tunnel/heat riser staying at 7x7?

[matt walker]

Yeah, that's an interesting thought Ray. I think my system may need a little more choking than, say, yours for instance, due to that little window/door I have down low in front. It has some significant air leakage around it which necessitates a smaller feed. How much smaller than yours I can't say. Also, like I said above, different fuel woods and their size/shape play a big part in the way the whole thing works. My neighbor loves his large feed because he likes to lay larger but shorter pieces down in the bottom of the feed, then covers most of the feed with bricks. In practice, choking it with bricks, or in my case a tin camp stock pot, is a pretty good solution. It allows infinite adjustability and keeps the feed open for larger or odd shaped fuel.

In thinking about it, I think one of the main factors is that you are sucking in cold air, and then it's going through the system as hot, expanded gases. So, what fits through a CSA of 52" cold might be way too much for the same CSA when it's hot. By choking down the feed you can meter the air, and how much to do that depends on system temp, type of fuel, etc, etc.