Vent an 8" Rocket mass heater out a 6" existing chimney?

Hi,

I'm working on an 8" RMH design. I have an existing exterior 6" stainless chimney with a 20' height that I would like to use.

My RMH design has about 35-40 feet of bench run, then would exit up the 6" exterior chimney 20' or so to the roof peak.

With the significant cooling that takes place in the 30-40 bench run (and the shrinkage of the air), is this a possibility?

Has anyone ever tried and 8" RMH tied to an existing narrower exterior chimney on a long run?

I suppose I could hook it all up and test it, but prefer anyone feedback if anyone has tried this.

Thanks much,

MP

Mark Pushner ;by now you have 6 weeks more experience on these pages, and many of the questions you had have been answered. To answer the question you asked if only in part, Ianto Evans one of the original people to intuitively grasp the value of mating Rocket stoves with cob - a poor mans Masonary Stove, built a 8'' system at an early work shop and changed to a 7 '' 1/2 way down the thermal Mass, this was at a time that his decrees were never questioned. You are near the far end of a doable horizontal run on this build, Your exhaust gases may have cooled to the point where at your location it may not draw during cold damp weather and excelent during cold snaps! You have a few to many climate introduced variables to give a good answer ! hope this helps a little ! pyro-maticly yours ! Allen L.

- i found this and can not tell you if this is the same one or a different one where they have gone down from an 8'' to a 7'' , - ' rocket mass heater-exaust can be simpler than chimney ' Allen L.

Yes, you CAN use a 6 inch chimney with an 8 inch system.
You MUST remove enough heat from the system so that the exhaust gasses contract enough that they will fit down the smaller tube easily.
I know, too damn much gray area.. Unfortunately, there isn't yet a clear rule (that I know of) to use for this.
You will have to experiment, if at first it doesn't work, find ways to dump more heat from the system (longer bench, exposed bell systems, etc) and repeat till it does..