RMH pipe sizing question

It seems that an 8" RMH denotes the diameter of the heat riser. In a perfect world would all the other tube/ducting be 8" as well, including the chimney? I have a spot that I would like to try and build my first RMH, and the only thing that I know I am starting with is a good 20' tall 6" insulated Selkirk type chimney. So should I work backwards from 6" chimney; would that make all the other ducting and heat riser 6" as well?

thanks
Alan

Theoretically, yes, you should work backwards and build a 6" diameter system.

However, you first need to evaluate your intended use and needs and decide if a 6" system will be big enough to give the heat you need. It would be cheaper to build what you need now even if it means replacing existing chimney pipe, rather than having to scramble to rework, replace or supplement something that turns out to be too small.

So what is your climate, building characteristics and desired heating use?

You could also check the draft, and see if a 8 could be built with a 6 chimney. If it is insulated, that's a good starting point.

What i am using to check what a system can cope with, heat exctraction wise, is barrels, but i tend to preffer bells over benches.

https://permies.com/t/44806/rocket-stoves/Cobbling-workshop-heater

Allen; By selkirk chimney do you mean metalbestos? Or rather is it an indoor 20' 6" chimney or outdoor ? If indoor and exposed you could remove all the 6" to just below the roof jack replace it with uninsulated 8" inside and reduce to 6" to go thru the roof. Building an 8" system and the trying to push it thru 20' of 6" thats outside probably won't work very well. Building a 6" system if your space needs an 8" system is also not a good idea , you just won't be happy. Once a pipe goes vertical it creates its own draft ,it is possible that your current 6" could handle an 8" rmh but it will be a big MAYBE SO ... as glenn said we need more information.

Glenn Herbert wrote:Theoretically, yes, you should work backwards and build a 6" diameter system.

However, you first need to evaluate your intended use and needs and decide if a 6" system will be big enough to give the heat you need. It would be cheaper to build what you need now even if it means replacing existing chimney pipe, rather than having to scramble to rework, replace or supplement something that turns out to be too small.

So what is your climate, building characteristics and desired heating use?

I do live in a cold climate (Nova Scotia) and I don't have a huge space to fit the RMH into. The heating area is about 1,500 sq'. I have to fit it in a space that is about 9'x3', there is lots of head room with 9' ceiling but from what I have read snaking the ducting up through several switch backs is not great. Having said that it has to be better than burning 4 cord a year on an old box stove.

ab

thomas rubino wrote:Allen; By selkirk chimney do you mean metalbestos? Or rather is it an indoor 20' 6" chimney or outdoor ? If indoor and exposed you could remove all the 6" to just below the roof jack replace it with uninsulated 8" inside and reduce to 6" to go thru the roof. Building an 8" system and the trying to push it thru 20' of 6" thats outside probably won't work very well. Building a 6" system if your space needs an 8" system is also not a good idea , you just won't be happy. Once a pipe goes vertical it creates its own draft ,it is possible that your current 6" could handle an 8" rmh but it will be a big MAYBE SO ... as glenn said we need more information.

The chimney is indoor in the center of the house, it runs through two stories, not boxed in, but with proper clearances. It does get warm at times, but way too much heat just leaves through that nice insulated pipe.

ab

If you have limited footprint but ample headroom, you might consider a bell type of mass rather than a bench. This is effectively a large masonry box with the hot air duct and the chimney exhaust both at the bottom. Hot air rises and stratifies at the top, and only the coolest air sinks to the bottom and goes up the chimney. You can also build it so the heat riser is inside the bell rather than enclosed in a barrel and ducted to the bell.
Your chimney draft situation sounds like it may be good enough to run an 8" system, but that is still not a sure thing. You might consider a 6" batch box system, as the batch box puts out heat much faster than the same size J-tube. It is a load it up and burn it all (about 1 hour) style instead of feeding a few sticks regularly like the J-tube. Each has its pros and cons.

Yep, footprint wise, and northwards wise, you need a batch.

I find the latest batch from Radek looks superb.




http://donkey32.proboards.com/thread/1330/story-bells-20cm-batch-proposal?page=3


And the latest side batch rocket from Hendrik, is fantastic too, despite it's short heat riser

http://donkey32.proboards.com/post/16159/thread

http://www.ecologieforum.eu/viewtopic.php?f=3&t=4610