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Post by Donkey on Oct 3, 2012 16:42:23 GMT -8
This IS an important subject and is (with rocket stoves) fertile ground for confusion. The classic answer for all wood-stoves on the planet Earth has been, a taller chimney works better. With Rocket Stoves, this is generally true too. EXCEPT The Wonderful World of Rocket Stovery, for some time, has been dominated by a menagerie of oddities (such as myself), with the goal of getting as much out of these things as possible while miraculously (also), getting them to work (sometimes at all). It's a common thought-form (seems obvious) that retaining as much heat (produced by flame) IN THE HOUSE as possible is a good thing.. To this end, the menagerie has worked to produce low exhaust temperatures, figuring lower temps at the chimney equals more heat forestalled, made useful. Chimneys work on the DIFFERENCE between the internal and external temperatures, the greater the difference, the better the draft. If it's -40F outside, a 40F chimney should work fine. If it's 50F outside, the 40F chimney will fail to operate. Low exhaust temps can bite you in the butt. I've built stoves that refused to draft with a chimney on, classic case of poor design when it comes to hippy-built gizmos.. Fortunately, it was all experiment, so no time lost. Incidentally, I've learned by shaving it too close (obvious in retrospect) that insulated chimneys can help.. A lot. The short answer: Your chimney MUST be hot enough to draft (in all conditions) AND (if so) as as general rule, a taller chimney is better.
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Post by susquedad on Oct 4, 2012 17:34:43 GMT -8
Great topic Donkey. What would really solidify this is if someone were able to come up with mathmatical guidelines as to the relation of CSA to riser height to (in mass) flue length to output gas temp to chimney height. Hmmm, any takers?
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Post by Donkey on Oct 4, 2012 18:20:33 GMT -8
Yep.. It'd be nice if it were distilled to some kind of simple rule-of-thumb... 8 inch system, so many feet of cob, etc. 'Course, it would need to be translatable to bells, hollow benches, water heaters, and all the other ways to distribute heat..
Sounds like rather complicated job.
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morticcio
Full Member
"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
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Post by morticcio on Oct 5, 2012 0:19:29 GMT -8
I'd follow Donkey's rule:
If you're going to adhere to local building codes/regulations when constructing your RMH then stack/chimney height is usually determined by proximity to ridgeline, skylights, adjacent buildings etc.
It might work fine with a 6ft stack but needs to be 12ft to exit safely and to comply with local codes. There again the code might only require an 8ft stack but the RMH needs 10ft plus a cowl to make it work effectively.
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Post by Dan (Upstate NY, USA) on Oct 5, 2012 1:54:35 GMT -8
I always like the try and see method. I used 3 ft sections of triple wall chimney for my wood stove. When I had bad draft problems in high winds I bought another 3ft section and no more problems. Now this is the same chimney I use with my RMH.
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Post by ucghandyman on Oct 29, 2012 9:08:31 GMT -8
So the answer to chimneys is trial, error & experimenting (ie tuning) is still needed, along with experience, expertise, good judgment.
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Post by kirkerik on Nov 3, 2012 12:21:36 GMT -8
any thoughts on reducing the chimney diameter when going vertical out of the horizontal bench run? I currently have a 50 sq/in. system using 8" black pipe for most of it. I have noticed that when burning fiercely in the firebox/feedtube that the gases seem to just slowly billow out of my chimney. The velocity seems very low. (which is good for dwell time of gasses in the stove but it seems more susceptible to down drafts)
The chimney exits horizontally through the strawbale wall under the eave on the leeward side of the house. From the bench there is a short 4-5' rise then the chimney goes through an elbow and takes a turn rising about 45* for about 6-7'. So there is some rise, but not a vertical chimney.
given the gasses are billowing out so slow from the end of the chimney, do you think that reducing the diameter at some point to maybe 6" would be an overall improvement or not?
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Post by kirkerik on Nov 3, 2012 12:25:41 GMT -8
i meant to mention my thoughts on this are alike to the huge, tall and tapering chimneys i see at old factorys. As the gasses cool they take up less space so the chimney tapers to keep the velocity up...
in a small system like a rocket stove does the same principal apply?
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Post by Donkey on Nov 3, 2012 13:39:14 GMT -8
Yes it does. You can reduce pipe size to improve flow. Be careful, if you reduce too early in the game, you stand the risk of blocking flow instead.
The visual coming off the pipe may NOT be a good indicator of much. Water vapor can be pretty lazy looking, while the invisible burn products are simultaneously moving with vigor
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Post by kirkerik on Nov 3, 2012 16:05:48 GMT -8
Yes it does. You can reduce pipe size to improve flow. Be careful, if you reduce too early in the game, you stand the risk of blocking flow instead. great! i will be reducing to 6" from 8" just above the heat riser after running through a 10' bench to and fro. The stack temperature at this height is quite low (~120*F) and i just have a hunch that the volume of an 8" pipe is too much. Besides the lower side of the pipe is much cooler than the top. (pipe at 45* angle) The visual coming off the pipe may NOT be a good indicator of much. Water vapor can be pretty lazy looking, while the invisible burn products are simultaneously moving with vigor Hmmmmm, yes i can see what you mean. still when i place my hand in front of the pipe it feels quite slow, then again it is really hard to tell gas velocity w/out measuring. And what feels slow to me might be too much air to the stove and fire! if only i had a testos, i will just have to trust my nosetos
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Post by kirkerik on Nov 3, 2012 16:28:04 GMT -8
i remember now, last winter while burning i was experimenting by damping the exit of the chimney. I have a plug i trimmed and hinged so i could open while burning and close when i wish to trap the heat inside. i had it counter-weighted w/ a simple pulley and cord. i was closing the flap down and i had to close it quite a bit to see any difference in the firebox/feed tube. i think it was 3/4 shut or more to make a notable difference. This was all within a few minutes time frame. I did not test over an entire charge or burn. I don't recall if i used my gas analyzer at the time? (my olfactory)
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Post by Dan (Upstate NY, USA) on Nov 5, 2012 5:42:09 GMT -8
I have an 8" system that goes through about 22 feet of bench and then tapers to a 6" chimney, works great.
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Post by kirkerik on Nov 5, 2012 13:05:40 GMT -8
Thanks wolf
How tall is your chimney? Is it vertical? Is your feed 50 sq./in ?
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Post by Dan (Upstate NY, USA) on Nov 5, 2012 16:32:44 GMT -8
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Post by kirkerik on Nov 6, 2012 6:05:25 GMT -8
Cool, thanks. That is pretty much my set up except i have ~ 5' vertical rise in 8" black pipe to just above the barrel. Then i'm going to pare down to 6" for the rest of the shot through the roof. That will be about another 9 or 10' of vertical.
I might do 8 to 7" then 7 to 6" just because i have a piece of 7" i would like to burn up. Anyone know if the taper would help? I think i will have plenty of draw regardless.
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