It's now my second year with a RMH. I originally planned with my first system as an 8" and bought 8" circular firebrick piping for the heat riser. I then realised it was difficult to get 8" exhaust where I live and the rest of the system is 6". i.e. The burn tunnel is is 6" and so is the exhaust. So my current system is a hybrid 8/6"

I'm now planning my second system in another building. And I still have enough 8" firebrick tube for the heat riser. I've read that since if you get a really efficient burn, that actually the gasses reduce and so its possible to use a 6" exhaust for a 8" system. So I'm thinking of this time trying with a 8" burn tunnel.  I have wondered if whether having a 6" burn tunnel doesn't really give enough  fuel to fill up that 8" heat riser and take advantage of it. What are your thoughts on this?

So what is the school of thought on this around gasses reducing. Have they already reduced enough once they are exiting the barrel drums to drop down from 8" to 6" exhaust?

Cheers,

Rob

Rob, i have done it, from 139mm, to 111mm. Worked vaguely ok.

Done it on 150 to 111. Worse. Still worked. This is with J tubes.

I have done it with a batch, having all the piping the same as the system tube, except the transition going through the wall, about 70cm long, which was still 111mm. Worked surprisingly well.

Then i moved place. And going through a proper chimney makes a world of difference. Now i go from 220mm batch system, to 20cm square chimney.

So try hard to find the proper piping for your system.

I think part of what can make a reduced chimney work well is to have a tall internal chimney, so that you are getting significant draft assistance from it. In extreme cases, it might even be that a tall warm chimney needs to be smaller to avoid pulling too hard and blowing the flame off the wood, or pulling the gases through the heat riser before they have finished burning. Obviously the details are critical here, and I wouldn't recommend reducing the cross section unless you had optimal conditions.

Thanks for this. Greatly appreciate the comments