Rocket mass heater questions

OK so my DIY project for the year will be a larger greenhouse to replace my smaller greenhouse.
The new greenhouse will have a footprint of 23 feet by 20 feet I am planning a potato /onion box along the west and north walls 3 feet tall 3 feet wide. Along the east and south walls I am planning a rocket mass stove with bench and herein lies my  two questions.
Question 1. What is the maximum length of the off gassing tube (the duct that warms the bench) is there a maximum length? In the videos I have seen the duct work doubles back and vents at or near the beginning of the burner.

Question 2. I am assuming that there is a incline in the duct work to allow for the outflow of the CO2 assuming this what is the mathmatical breakdown of this ? (i.e. 2 in. over a 2 feet run)

All things being equal the bench in and of itself will have a tin roof sheeting base to avoid digging critters. I plan on creating the main mass of the bench out of dirtcrete as I have in one portion of my garden just the right material for it and will be happy to have it gone so I can cultivate better tilth in that section. Overall the dimensions of the bench will be 2 1/2 feet wide 2 1/2 high tall and roughly 42 feet long elled at 21 feet. Dressing out the bench with stucco or just a regular cob with sawdust instead of straw to give a smoother texture I think. The top of the bench will be used to warm and keep seedlings growing till they can go in pots or the ground in raised beds, and the whole bench as a mass will keep the greenhouse warm. At least this is the plan so far.

Any ideas or suggestions and hopefully answers to my two questions are greatly appreciated.

I'm just learning about rocket mass heaters, but my experience as a mason, having built Russian style flue systems (similar to kang stoves), which are really efficient and work on the same principle as the rocket, tells me that your horizontal run should be relative to the size of your fire, as well as the over all height of the chimney.
The hotter your fire, and the higher your chimney, the faster the exhaust is going to move initially through your horizontal chimney.  I couldn't tell you the optimum ratio for fire size/horizontal/vertical but my rule of thumb has been 1:3 horizontal to vertical given a typical 18' fire box using 6 or 8 inch flue.  I am sure that somebody with more exp. with rockets could give you more accurate advice, but the concept should be good.

Sol wrote:
I'm just learning about rocket mass heaters, but my experience as a mason, having built Russian style flue systems (similar to kang stoves), which are really efficient and work on the same principle as the rocket, tells me that your horizontal run should be relative to the size of your fire, as well as the over all height of the chimney.
The hotter your fire, and the higher your chimney, the faster the exhaust is going to move initially through your horizontal chimney.  I couldn't tell you the optimum ratio for fire size/horizontal/vertical but my rule of thumb has been 1:3 horizontal to vertical given a typical 18' fire box using 6 or 8 inch flue.  I am sure that somebody with more exp. with rockets could give you more accurate advice, but the concept should be good.

thanks for the input. It looks just about right as to videos from cob cottage that I have seen as to the 1:3 rise. If nothing else I could just go with the single flue run without doubling back and vent off at the far end of my bed I suppose. All this is just on paper currently in planning phase.

Sure thing, another detail as far as run/rise in your exhaust pipe goes, I have seen flue systems that actually turn downwards coming out of the fire box, so my gut feeling is that you could probably run level all the way out.

Sol wrote:
Sure thing, another detail as far as run/rise in your exhaust pipe goes, I have seen flue systems that actually turn downwards coming out of the fire box, so my gut feeling is that you could probably run level all the way out.

I've seen the same thing where the exhaust meets the heat riser out flow it takes a little jog downward goes into a tee piece for the ash cleanout then on for the run. I'm thinking that little low point will act as an aid for the ash catchment. At least in my thinking that would be what the low point would be. All in all i'll include the 1:3 rise over the course of the run for the proper flow of CO2.

That sounds like a good idea, although I doubt you will have to use a clean out very often, almost all the ash that gets sucked into the exhaust in a Russian flue ends up getting burned away, like in a self cleaning oven.  The ash that you do collect is the stuff that falls through the grate directly beneath the fire box, rocket stoves don't have  this feature, although it sounds like they get a pretty complete burn as well.  What I would consider doing, is to put a secondary clean out at the first bend of the exhaust pipe, if you ever do get unwanted build up, this is where it will end up.

Sol wrote:
That sounds like a good idea, although I doubt you will have to use a clean out very often, almost all the ash that gets sucked into the exhaust in a Russian flue ends up getting burned away, like in a self cleaning oven.  The ash that you do collect is the stuff that falls through the grate directly beneath the fire box, rocket stoves don't have  this feature, although it sounds like they get a pretty complete burn as well.  What I would consider doing, is to put a secondary clean out at the first bend of the exhaust pipe, if you ever do get unwanted build up, this is where it will end up.

yup I have a tee piece designed there as well and at the final bend where the duct work goes out the green house wall. Overall I've over engineered the thing on paper which kinda runs in my family as we don't like going back to correct faults later.

cool, you over engineer and plan ahead, not a bad way to go about things.  I typically like to tinker and learn from mistakes.  Here is one of my concerns with the rocket stove based on a mistake I made.  A flue system can be built with 2 flues, one a more direct vent for starting fires, the second is the long system that winds through the mass heating it.  I made the mistake of using 1/2 inch steel stock as a damper plate/flue switch at the back of the fire box.  The fire gets so hot because the Coriolis effect of the burn turns the whole thing into a rocket engine (like flames 12 feet long!)  That steel damper plate has burned away over the years, there is nothing left. I'm worried that a similar thing could happen with a rocket, that the top of the barrel will eventually fail after years of rocket blast.  It's already got me thinking about building modular rocket stoves from ceramic precast components to avoid this scenario. 

Sol wrote:
cool, you over engineer and plan ahead, not a bad way to go about things.  I typically like to tinker and learn from mistakes.  Here is one of my concerns with the rocket stove based on a mistake I made.  A flue system can be built with 2 flues, one a more direct vent for starting fires, the second is the long system that winds through the mass heating it.  I made the mistake of using 1/2 inch steel stock as a damper plate/flue switch at the back of the fire box.  The fire gets so hot because the Coriolis effect of the burn turns the whole thing into a rocket engine (like flames 12 feet long!)  That steel damper plate has burned away over the years, there is nothing left. I'm worried that a similar thing could happen with a rocket, that the top of the barrel will eventually fail after years of rocket blast.  It's already got me thinking about building modular rocket stoves from ceramic precast components to avoid this scenario. 

I've had the same thought about the barrel top but as the pack-rat i am I always have several 55 gallon drums here and there (they're like Doritos , you can't have just one ! ) if worse comes to worse and one degrades chipping away the cob surround shouldn't prove that difficult if one needs to be replaced. I suppose if one wanted to engineer a double bottom in a barrel that would prove to act as a cushion of sorts to the barrel against heat degrade. the only thing to watch for in that is to make sure the heat riser is still at it's 2 inch minimum distance from the barrel bottom.

Greetings - rather than start a new thread is it ok to add some questions of my own.  I'm just starting down the path investigating these heaters so if this has been bought up before please forgive my ignorance.

We heated with wood in the past and one of the 'no no's' we were always told was not to burn with green wood because of creosote build up and the risk of chimney fires.  Does the same hold true for a rocket mass heater?

On the subject of chimney fires ... has anyone had any experience with one catching fire?  Easier or harder to put out than a conventional wood stove/chimney fire?

Is it feasible to burn scrap wood (both soft and hardwoods) safely.

I've noticed that most people seem to use a steel barrel as part of the combustion? chamber.  Has anyone tried an alternative in cast iron?



Many thanks

Icewalker wrote:
Greetings - rather than start a new thread is it ok to add some questions of my own.  I'm just starting down the path investigating these heaters so if this has been bought up before please forgive my ignorance.

We heated with wood in the past and one of the 'no no's' we were always told was not to burn with green wood because of creosote build up and the risk of chimney fires.  Does the same hold true for a rocket mass heater?

On the subject of chimney fires ... has anyone had any experience with one catching fire?  Easier or harder to put out than a conventional wood stove/chimney fire?

Is it feasible to burn scrap wood (both soft and hardwoods) safely.

I've noticed that most people seem to use a steel barrel as part of the combustion? chamber.  Has anyone tried an alternative in cast iron?



Many thanks

well, lots of questions there but i'll take each in turn and see if it helps you out.

As to your first question about green wood and creosote buildup. I wouldn't use green wood as a rule of thumb but this is not for the buildup but rather from a combustion standpoint. An RMH (rocket mass heater) is designed in such a way to to burn fuel completely leaving nothing at the end but CO2 and steam> the jet and convection that occurs at the top of the heat riser/bottom of the barrel is so forceful that there would be little or no chance for buildup of creosote I would think and any that did buildup would be quickly burned away. You want to start with a good dry wood to get your riser as hot as possible that helps create the rocket portion of the combustion after that your golden and if you wanted to add a bit of green wood I wouldn't think it would be bad juju.

On chimney fires of a RMH I don't see how this would be possible as everything is sealed and by the time the flue gases get to the outflow pipe they are sufficiently cooled as to be non combustible. I suppose there could be the possibility of a fire further up the outflow pipe but again as all the combustion occurs well back in the riser/barrel portion there is little or no oxygen in the outflow pipe to supply a fire of any size or circumstance.

I built of proof of concept mock up this morning of a RMH that will be attached to a 40 foot long mass bench and i used some scrap oak and pine and it worked like a dream.

Most of the vids for RMHs that I have seen have used the standard sized 55 gallon steel barrels some have been smaller and I suppose if one wanted to use cast iron you could. My only concern with that would be the extreme weight of a cast iron barrel that size, granted it would be a helluva lot sturdier than a steel barrel and would certainly stay warmer after a fire a bit longer I just kinda think it would be overkill as I have never heard of one of the standard steel barrels failing

Hope this helps you out.

I have noticed that the RMH book by Ianto Evans is available in pdf format for 13.00 and believe it would be great resource for you if you wanted to do further research on the subject.

thatchickencoopguy wrote:
well, lots of questions there but i'll take each in turn and see if it helps you out.

As to your first question about green wood and creosote buildup. I wouldn't use green wood as a rule of thumb but this is not for the buildup but rather from a combustion standpoint. An RMH (rocket mass heater) is designed in such a way to to burn fuel completely leaving nothing at the end but CO2 and steam> the jet and convection that occurs at the top of the heat riser/bottom of the barrel is so forceful that there would be little or no chance for buildup of creosote I would think and any that did buildup would be quickly burned away. You want to start with a good dry wood to get your riser as hot as possible that helps create the rocket portion of the combustion after that your golden and if you wanted to add a bit of green wood I wouldn't think it would be bad juju.

On chimney fires of a RMH I don't see how this would be possible as everything is sealed and by the time the flue gases get to the outflow pipe they are sufficiently cooled as to be non combustible. I suppose there could be the possibility of a fire further up the outflow pipe but again as all the combustion occurs well back in the riser/barrel portion there is little or no oxygen in the outflow pipe to supply a fire of any size or circumstance.

I built of proof of concept mock up this morning of a RMH that will be attached to a 40 foot long mass bench and i used some scrap oak and pine and it worked like a dream.

Most of the vids for RMHs that I have seen have used the standard sized 55 gallon steel barrels some have been smaller and I suppose if one wanted to use cast iron you could. My only concern with that would be the extreme weight of a cast iron barrel that size, granted it would be a helluva lot sturdier than a steel barrel and would certainly stay warmer after a fire a bit longer I just kinda think it would be overkill as I have never heard of one of the standard steel barrels failing

Hope this helps you out.

I have noticed that the RMH book by Ianto Evans is available in pdf format for 13.00 and believe it would be great resource for you if you wanted to do further research on the subject.

Thanks for your responses ... extremely usefuly BTW.

I'm going to be buying the RMH book way before venturing into the actual building process. 


Best regards

Jeff

Icewalker wrote:
I've noticed that most people seem to use a steel barrel as part of the combustion? chamber.  Has anyone tried an alternative in cast iron?

I would recommend building a mock-up to test; but off the top of my pointy little head I don't think cast iron would be as efficient as a steel barrel.  As I understand it, using the steel barrel has two effects:

1. it allows for the rapid release of heat into the living space.  By its nature, cast iron doesn't transfer heat as quickly as the steel does -- an important consideration when its 40 degrees in the living room and you want to warm things up quickly.

2. The rapid release of heat helps the "thermal pump" action of the RMH: as the combustion gasses cool they sink.  For every pound of gas that sinks another pound of gas must be pulled in; every pound of gas that gets pulled in must push a pound of gas out through the chimney.  Since cast iron doesn't transfer heat as easily, using that instead of a steel drum may interfere with this "thermal pump" action.