Comments on: How to fix a Rocket Stove http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/ more attainable & sustainable Sat, 11 Feb 2012 16:19:53 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: Mike http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/comment-page-1/#comment-15196 Mike Wed, 25 May 2011 01:57:14 +0000 http://www.tinyhousedesign.com/?p=3756#comment-15196 I think centuries of convention regarding chimneys and flues have blinded us to some basic physics regarding rocket stoves. On intakes and exhausts, the intake need NOT be as large as the exhaust as a LOT of gas is added to the circuit by the combustion process. Keeping the air source smaller will help prevent back drafts as any increase in pressure in the combustion chambers will always take the path of least resistance - the larger pipe. Also on intakes and exhausts, I believe great benefit would be observed if the intake air is sourced in very close proximity to the exhaust air. Air movement around (or over) any object (building) creates high and low pressure areas. While the pressure differentials may be thought of as insignificant, consider the effect when surface area is factored into the force equation. An intake in close proximity to the exhaust will minimize pressure differentials. Sourcing the combustion air from close proximity to the flue exhaust obviously will necessitate an external combustion air inlet and sealing of the circuit to prevent the inlet air pressure from being affected by the building internal air pressure. Another area that I think is mis-considered is designing the flue path through the storage mass. As the gas stream moves through the flue, it constantly becomes cooler, not warmer. As warmer gases rise and cooler gases fall, I believe better draft performance will be realized with a flue sloping downward from the top of the "internal chimney" to the point of gas exit from the flue. I say to the point of exit as heat transfer continues throughout the ENTIRE run of the flue. This means the gases at any point in the flue run, relative to the gases at any other point in the flue run, are ALWAYS COOLER downstream, be it a span of 1 inch or 30 feet. So the flue gases should naturally flow better in a falling flue rather than in a rising flue. I think such a design will considerably reduce the pressure on the secondary combustion chamber or "internal chimney" to push the cooling gases uphill through the flue. No elevation increase in the hot gas circuit should be necessary and, I believe, will actually detract from draft performance, which detracts from the overall performance of the unit. Another advantage of a constantly falling flue is that cold, outside air will have less tendency to flow DOWN into the flue and into the building when the unit is not fired. If you're having performance or draft issues, attempt to somehow equalize the ambient air pressures at your flue exhaust and at your combustion air intake. Also, if your intake and exhaust openings are equally sized, reduce the size of the inlet air path at any point to evaluate reduction of backdraft. If you are an engineering type, build a cheap, simple differential pressure water manometer and determine the differential pressures along the entire unit gas circuit (from air intake to the flue exhaust). Please let me know if you experiment with any of this as I have not proven any of it yet. It just makes sense based on simple physics. I intend to build such a design as soon as I get a round tuit. If you should happen to come across a round tuit before I do, your validation or invalidation of my ideas could save me some time. A lot of frustration with rocket mass heaters cantankerous performance might also be eliminated for many others. I think centuries of convention regarding chimneys and flues have blinded us to some basic physics regarding rocket stoves. On intakes and exhausts, the intake need NOT be as large as the exhaust as a LOT of gas is added to the circuit by the combustion process. Keeping the air source smaller will help prevent back drafts as any increase in pressure in the combustion chambers will always take the path of least resistance – the larger pipe. Also on intakes and exhausts, I believe great benefit would be observed if the intake air is sourced in very close proximity to the exhaust air. Air movement around (or over) any object (building) creates high and low pressure areas. While the pressure differentials may be thought of as insignificant, consider the effect when surface area is factored into the force equation. An intake in close proximity to the exhaust will minimize pressure differentials. Sourcing the combustion air from close proximity to the flue exhaust obviously will necessitate an external combustion air inlet and sealing of the circuit to prevent the inlet air pressure from being affected by the building internal air pressure.
Another area that I think is mis-considered is designing the flue path through the storage mass. As the gas stream moves through the flue, it constantly becomes cooler, not warmer. As warmer gases rise and cooler gases fall, I believe better draft performance will be realized with a flue sloping downward from the top of the “internal chimney” to the point of gas exit from the flue. I say to the point of exit as heat transfer continues throughout the ENTIRE run of the flue. This means the gases at any point in the flue run, relative to the gases at any other point in the flue run, are ALWAYS COOLER downstream, be it a span of 1 inch or 30 feet. So the flue gases should naturally flow better in a falling flue rather than in a rising flue. I think such a design will considerably reduce the pressure on the secondary combustion chamber or “internal chimney” to push the cooling gases uphill through the flue. No elevation increase in the hot gas circuit should be necessary and, I believe, will actually detract from draft performance, which detracts from the overall performance of the unit. Another advantage of a constantly falling flue is that cold, outside air will have less tendency to flow DOWN into the flue and into the building when the unit is not fired.

If you’re having performance or draft issues, attempt to somehow equalize the ambient air pressures at your flue exhaust and at your combustion air intake. Also, if your intake and exhaust openings are equally sized, reduce the size of the inlet air path at any point to evaluate reduction of backdraft. If you are an engineering type, build a cheap, simple differential pressure water manometer and determine the differential pressures along the entire unit gas circuit (from air intake to the flue exhaust).

Please let me know if you experiment with any of this as I have not proven any of it yet. It just makes sense based on simple physics. I intend to build such a design as soon as I get a round tuit. If you should happen to come across a round tuit before I do, your validation or invalidation of my ideas could save me some time. A lot of frustration with rocket mass heaters cantankerous performance might also be eliminated for many others.

]]> By: Jocelyn East http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/comment-page-1/#comment-5413 Jocelyn East Sat, 03 Apr 2010 22:16:10 +0000 http://www.tinyhousedesign.com/?p=3756#comment-5413 Your exhaust pipe should be gently sloping upwards all the way to the chimney. It looks from the pictures that your pipe is sloping down in some spots and I can't see your chimney on the outside. Your pipe should be at lease 2 feet higher then the highest point of your roof. I saw that you have a clean out under the bed, you may want to burn some news paper to warm up the pipe to create a draft before lighting up the stove. Good luck Your exhaust pipe should be gently sloping upwards all the way to the chimney. It looks from the pictures that your pipe is sloping down in some spots and I can’t see your chimney on the outside. Your pipe should be at lease 2 feet higher then the highest point of your roof. I saw that you have a clean out under the bed, you may want to burn some news paper to warm up the pipe to create a draft before lighting up the stove.
Good luck

]]> By: Michael Janzen http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/comment-page-1/#comment-4456 Michael Janzen Tue, 09 Feb 2010 07:19:08 +0000 http://www.tinyhousedesign.com/?p=3756#comment-4456 That's a really interesting idea Emerson. I think it would work fine since the inlet and exit openings are what control how much air can pass through. The hot air would just pass a little slower through the three parallel pipes. Good thinking. That’s a really interesting idea Emerson. I think it would work fine since the inlet and exit openings are what control how much air can pass through. The hot air would just pass a little slower through the three parallel pipes. Good thinking.

]]> By: Emerson White http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/comment-page-1/#comment-4455 Emerson White Tue, 09 Feb 2010 07:15:11 +0000 http://www.tinyhousedesign.com/?p=3756#comment-4455 Another thought that occurred to me. If the fan can't happen for whatever reason and the duct work within the cob must be changed running the three longitudinal ducts in parallel rather than running them in series might help more than cutting two of them out all together and allow the whole bed apparatus to pick up some heat before it is lost to the atmosphere. Another thought that occurred to me. If the fan can’t happen for whatever reason and the duct work within the cob must be changed running the three longitudinal ducts in parallel rather than running them in series might help more than cutting two of them out all together and allow the whole bed apparatus to pick up some heat before it is lost to the atmosphere.

]]> By: Meagan http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/comment-page-1/#comment-4405 Meagan Fri, 05 Feb 2010 23:48:42 +0000 http://www.tinyhousedesign.com/?p=3756#comment-4405 Brian, I have a good friend who is really into rocket stoves and knows people~ who may be able to help guide you in the right direction. Below is a direct link to his site permies.com, already searched posts on rocket stoves, for a start. You could post your question/concerns with pics and probably get an answer back quickly. May your rocket stove be revamped & working soon. http://www.permies.com/ ~M Brian, I have a good friend who is really into rocket stoves and knows people~ who may be able to help guide you in the right direction. Below is a direct link to his site permies.com, already searched posts on rocket stoves, for a start. You could post your question/concerns with pics and probably get an answer back quickly. May your rocket stove be revamped & working soon.

http://www.permies.com/

~M

]]> By: Michael Janzen http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/comment-page-1/#comment-4400 Michael Janzen Fri, 05 Feb 2010 22:12:27 +0000 http://www.tinyhousedesign.com/?p=3756#comment-4400 A fan is an excellent idea Malcolm and Emerson. I'll pass it onto Brian. A fan is an excellent idea Malcolm and Emerson. I’ll pass it onto Brian.

]]> By: Malcolm White http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/comment-page-1/#comment-4397 Malcolm White Fri, 05 Feb 2010 19:43:21 +0000 http://www.tinyhousedesign.com/?p=3756#comment-4397 Couldn't Brian install a small solar powered exhaust fan on the top of his chimney stack to increase the draw? There are some 12volt units available at marine supply stores that could perhaps work. Worst case he would need a seperate solar panel, a battery and a 12 volt fan. We would not be taking about much wattage required either so the solar panel could be pretty small. I would think this would be a lot cheaper than replacing his rocket stove altogether. Couldn’t Brian install a small solar powered exhaust fan on the top of his chimney stack to increase the draw? There are some 12volt units available at marine supply stores that could perhaps work. Worst case he would need a seperate solar panel, a battery and a 12 volt fan. We would not be taking about much wattage required either so the solar panel could be pretty small. I would think this would be a lot cheaper than replacing his rocket stove altogether.

]]> By: Emerson White http://www.tinyhousedesign.com/2010/02/04/how-to-fix-a-rocket-stove/comment-page-1/#comment-4384 Emerson White Fri, 05 Feb 2010 07:04:20 +0000 http://www.tinyhousedesign.com/?p=3756#comment-4384 The "rocket" actually generates considerable updraft, if the internal chimney isn't close enough to the top surface of the drum too much heat will stay in the exhaust and it will not draft internally as well. An exhaust blower could be used to fix the problem too but it would have a small electrical cost. The “rocket” actually generates considerable updraft, if the internal chimney isn’t close enough to the top surface of the drum too much heat will stay in the exhaust and it will not draft internally as well. An exhaust blower could be used to fix the problem too but it would have a small electrical cost.

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