Has anyone ever experimented with a narrower burn tunnel?

Was just curious if anyone ever experimented with narrowing their burn tunnel in relation to the feed and/or riser?

For instance a 6x4 burn tunnel with riser and feed both at 6x6.5
For 6 inch pipe this would give the CSA of burn tunnel less than 30 and the feed and riser more than 30 which on paper isn't optimal.

I would think it would draw harder but don't know if it would burn clean enough.

I think everything has already been tried... those measurements are the result of lots of experimentation by the rmh innovators and  only place I have heard of a variation from stock specs is to make the burn tunnel 1" deeper to allow for the 1"of fly ash that sits there.

Howdy Jeremy. I built a 4" flue diameter RMH, which is widely regarded as pushing the lower boundary. The burn tunnel is cast refractory and I used Peter Van den Berg's design with trip wire and kick tail. It burns beautifully, draws well, but I need to be careful of two main things: Lighting it after a long idle period when the cob mass is cold and perhaps a little damp (it's in my glasshouse); and the dryness of the fuel. Sometimes sticks that look and feel dry still have a fair bit of water in them and although they burn just fine, all the steam they put out condenses in the flue and can cause stalling and smokeback. I don't think there would be any issue deepening the burn tunnel slightly and that would accommodate ash buildup better. In fact, I'm probably going to cast a new chamber before next winter and will probably do just that. It's 83mm and square, so I may make it 80x100 and see how that goes.

If I were doing it again I might go with a 6" system, but it is a small space and our winters are short and mild...getting shorter and milder as I speak, but there's frost forecast tomorrow night so I'll be firing it up at least one or two more times before next year.

Jeremy, it's been done. That's for sure. And, iirc, it's better to do the burn tunnel tall and narrow, than shallow and wide. I have always heard that the burn tunnel should be the narrowest point in the whole system, if there is a point that has to be narrower.

Tho, it's better to keep the system size constant.

Making the burn tunnel narrow or wide has effects.

First, embers and fuel exposed.

If tall and narrow, less embers and fuel are exposed to air keeping the burn hot,  but might have lots of excess air going over the burn.

Low and wide, more fuel is exposed, and embers too, to incoming air, which might cool off the fire too much.

Low and wide also has the tendency to block from embers accumulation.

Lately, i'm onto a batch and J mixture, in this case, it's real tall and real narrow.

https://permies.com/t/90353/Vertical-batch-latest-attempt

Thanks for the replies. My greenhouse is 12x12 and shares a wall with the house. It also houses a 1200 gallon water tank which takes up most of the space so im trying to keep the rmh as small as possible. Was experimenting with mock ups in there and got the wheels turning but couldn't find anything in my books that deals with the effects of differing the depths and widths inside the heater except for the burn tunnel should always be smaller than or equal to the other dimensions.

Jérémy. I think laying a Bell against your wall, if it's masonry, would be clever.

Something like this,



But may be flatter.  Along those lines.

http://www.heatkit.com/research/2009/lopez-rocket.htm