Can my chimney be too big?

Hi Guys

I'm making this 8inch RMH, but have difficulty finding af matching chimney. I have found a cheap one that is 12 inches inside.... Will that be super, or is it a problem with too much draw there? I would think it would be super, but would love to be confirmed...

Another thing... can anyone guide me to a good guide to make the combustion part out of standard firebricks (heavy type pizzaoven)... I'm really having a hard time putting them together in a smart way, so i get the ash-collector etc... Would loove to see how this puzzle is made smart... I just want it simple.... for the simplicity of it as im building myself and noone else to spare with.

Hope you will help me out.

Thanks

Michael

So if i'm reading this correctly the 30cm chimney will be too wide for a 20cm system right? The chimney will be close to 8m. tall to rise to 1m above roof top.

Michael Brix wrote:So if i'm reading this correctly the 30cm chimney will be too wide for a 20cm system right? The chimney will be close to 8m. tall to rise to 1m above roof top.

Not necessarily. In an improperly working system, it could prove to be too big. Or in a properly working system which has the gases flowing too fast, or a system which doesn't have enough heat exchange surface, it could reveal usefull. It all depends onto the rate of slowing. And the remaining heat in the chimney. A big chimney with 120c° in it would have rather fast a gas flow While a 6 inch with 30 or 40C° could stall altogether. It's all mater of compromises.

Michael Brix : Welcome to Permies.com . Lets look at it this way, we have an efficient /clean burn then we try to balance the heat produced with an

adequate Thermal Mass. In a prefect world we would leave just enough residual heat to maintain a good 'Draft' inside and at the exit to our chimney!

If we do not have enough Thermal Mass the temperature of the Exiting Exhaust Gases will be high and the 'draft' strong and quick !


With a large diameter final Vertical Chimney the exiting exhaust gas speed will slow down, decreasing 'draft' and our final exhaust temps will be lower -

This makes a more efficient system.


You can do a simple experiment with two funnels a short large diameter funnel, and a long skinny funnel like you would use to add transmission fluid.

One would expect that the long skinny one would have more friction losses and be a slower pour - but your test will prove the long funnel works better

FASTER ! This is a generalized view of what happens in your chimney !


Some Rocketeers get lucky with time and materials with their very 1st build, some people have to spend a long time learning the idiosyncrasies of Their

Rocket Mass Heater RMH, - guess who finally is the more knowledgeable one to talk too ! For the Crafts ! Big AL

Thank you so much for taking time to explain this so thoroughly... i think i'm understanding the mechanisms... but so... If i make an 8inch system - 6 meters of exhaust tubing inside of the mass - 3 meters forward to an elbow going 3 meters back and up the chimney.... Do you guys THINK a 12 inch chimney (indoor isokern of 9 meters tall). will be a good idea here? - or should i work harder or pay more to try and find a 8 inch chimney?

I'm sorry to say, we don't know. What's your prevailing winds, how strong. What is your insulation etc.

We can't tell instead of you, if it will work. I advocate strongly trial and error.

Tonight, i've run successfully a 7 inch batch, with down low secondary air, and a profile that Peter van den berg is not too keen on. With 8.21 M² of metal bell. Which Peter said is too much.


The best way is trial and error. Burn several barrels, and see what maximum ISA you can get. Would it be half barrel bells " a la Matthewwalker" ; or straight tower bells as Peter and i do. Then extrapolate what kind of system you can do. If your sytem is running fine with all metal ducts and bells. Then, it will run fine with cob on the same surface, providing you don't have too many elbows.

Phew - just working trial and error buying a 1000$ chimney is expensive erroring... I was hoping i could get close to buying a chimney that will PROBABLY work...
Im making by the way a regular oil drum and using high quality fire bricks (lightweight kind). Insulation with perlite... Engine part will burn well i'm sure... Just much in doubt about the chimney... I was hoping some people could at least advise me on which size they would buy if they were making a system like mine...

too many variables to be answered!

Check this!

http://s65.photobucket.com/user/mremine/library/NYC%20Rocket%20Stove%20Build/

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

Thanks for the help.

For someone else facing the same problem as me, i can confirm having talked to two other separate RMH-owners that have the same dimensions mentioned and have it running smoothly.. So these dimensions are certainly easily able to function very nicely together.