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In-wall rocket mass heater

 
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Hey folks! I'm in the design stage for a greenhouse which will use a wood stove for secondary heating during colder months. The flue pipe pattern will direct the gasses down the length of the wall and back again before exiting at the top, ideally warming the lower half of the wall and letting thermodynamics take care of the rest. As the flue runs only about 25' total, and considering the client has requested a wood stove as the fire box, I thought the system could use a bit of extra room to combust and release heat into the mass before traveling through the wall.

As you can see in these pictures, I am planning to build a hollow into the wall itself which will use a blast gate style baffle to direct flue gases two directions to travel in one of two directions depending on the temperature of the system. The secondary burn chamber is designed with cob baffles to lengthen the distance the flue gasses must travel before entering the pipes. The other side of the wall will be the toilet area for the adjacent potting shed, so this would provide a comfortable temperature in there as well.

As this is the first time I've attempted this configuration for a rocket mass heater, I was wondering if there might be any big design considerations I've overlooked here? If anyone has suggestions, recommendations or questions I'd love to talk about this thing before I build it!
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Rocket Scientist
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Location: Upstate NY, zone 5
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Welcome to Permies, Ross! A rocket mass heater can be a good way to warm a greenhouse, and using an interior wall as the thermal mass could work fine.

I have some questions about your proposal, though. Is it an existing wood stove you show for the firebox? Wood stoves (unless they are very modern high efficiency ones) will not burn well enough to use as part of a rocket mass heater. Only a refractory lined chamber can stand the high temperatures needed, and a wood stove sheds too much heat to work. The firebox needs to be built into the wall or an extension of it. Connecting the firebox to the wall with a stovepipe will not work, and you will not get any secondary burn in the wall cavity. It will just cool the smoke and condense creosote in the passages.

Presuming you want a heater that does not need to be fed every half hour or so, you would want a batch box combustion core. You can find all the information you need to do that properly at batchrocket.eu.

Setting the batch box core into a cavity in the wall would work well, and the most effective as well as easiest way would be to make it a double wall with around an 8" space between the sides, and the exit to the chimney from the bottom of the cavity. A bulge in the wall cavity to give room for the combustion core with heat riser will hold all the workings.
 
ross brubeck
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Thanks for the thorough reply! Very informative. I've looked into batch boxes in years past, but I haven't found a proper application for the method yet. Maybe it's time to take this idea of the shelf and go for a redesign!

Before I commit to that, though,I have some followup questions about the current design.  it is indeed a medium size wood stove pictured in the model. The client would like to try incorporating it into the design of the greenhouse to keep the project cost down - something I'm committed to helping her do to the best of my ability. If we were to insulate the stove - say with firebricks on the inside, and a ceramic blanket around the exposed flue pipe, would that make a significant difference?

I do love the idea of incorporating a large firebox into the wall itself, and I can see it becoming a very distinct and beautiful feature of this greenhouse. As this is a shoestring project though, I'm obligated to optimize the value of what we already have on hand before moving on to something new altogether.
 
Glenn Herbert
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An ordinary woodstove is never going to become an effective batch box RMH. Firebrick inside the firebox could prolong the life of the steel shell, but will still not give the most complete combustion without the precise batch box geometry - the dimensions and proportions have been experimentally determined and anything much different will not work as well. The stovepipe even if insulated will not function fully as a heat riser, and if well insulated and the firebox delivers sufficient heat for a secondary burn, the stovepipe will be destroyed very quickly, maybe in days of use. Stovepipe material is useful for heat riser construction, but only as the outer shell to contain and stabilize ceramic fiber insulation inside it.

A batch box can be made with ordinary firebrick that will cost maybe $200 new, less if you can find a pile of them on Craigslist. This does not include the heat riser, which will work better if made from ceramic fiber blanket inside a large stovepipe (say around $50-100 with new material), but be stronger if made from firebrick splits wrapped in cheaper fiber insulation (say around $200). The ceramic fiber type is known as a "5-minute riser" and is easy and popular these days.
 
ross brubeck
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Awesome! Thanks Glen! I'm feeling more confident about this idea than ever now.

I think the value of putting extra money & work into an experimentally- and field-tested design like this is pretty apparent. In my construction of past RMH's I have had increasing success making my own refractory cement mixes and casting bricks for use in heat risers. I might even feel competent dialing in the precision enough to match the geometry in these diagrams I've been studying on the batchbox.eu site.

I guess that settles it - I'll give the rocket mass wall a quick re-sketchup and post the results here! Stay tuned ~
 
Glenn Herbert
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If you already have some experience with casting refractory material, you should have no problem making a batch box, and probably for a good bit less than the costs I mentioned.

My first multipart core casting worked well aside from a few glitches due to haste, and is still functioning properly after five seasons of heating, with no deterioration in the critical burn tunnel ceiling part. I would advise more sections avoiding long or complex shapes, but making joints to control the location of the cracking that will happen.
 
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