Help! Selecting rocket mass stove design.

I've hired a contractor to build an addition for my home on my deck and I need to know what rocket mass design would work best for my needs.

I really want one that will let me look at the fire for aesthetic purposes.
Also, one that allows for sustained heat (thermal mass?) is ideal as well.
My limitation, however, is that I am building on top of an existing deck and, therefore, probably cannot have a stove that is too heavy.

Does anyone have any recommendations based on this situation? Thanks for letting me know.


I'll send you lots of good vibes for any advice.

It sounds like your design constraints are pushing you toward a batch-box-style RMH, built with a lot of perlite mixed into whichever medium you choose for your mass (cob / concrete / other masonry).

The trade-off here is that light-weight means, literally, less mass (i.e. less thermal mass to store the heat) in the unit itself.


In light of the above, here are two options to consider, and do further research on:

*Add framing to your floor so it can support more mass above it.

*Construct a light-weight RMH with a large heat exchange that conveys the heat into a large, insulated, open tank of water somewhere to hold all the excess heat you scavenge.
You can then put loops of your domestic potable and/or hydronic water lines through that tank to preheat them and save some energy.

Greg has it right, you can't have a rocket mass heater without mass. What you can do is locate the mass somewhere that can support it, and/or use material with the greatest possible specific heat (amount of heat that can be stored per degree temperature rise per unit of mass). Water has a higher specific heat than most common materials, with the limitation that it cannot go above 212F or 100C while liquid. You might combine a smallish masonry mass that can get very hot with a surrounding water mass that can absorb heat from the first mass.

Aside from that, adding a couple of piers or posts on solid footings could support whatever mass you want. A batch box is going to be the best way to see the fire, with a door that has glass in it.

Jeff Pollari wrote:I've hired a contractor to build an addition for my home on my deck and I need to know what rocket mass design would work best for my needs...  

Which begs the question: how high is the deck above the ground? One option being, reinforced concrete filled CMUs' resting on a concrete pad, rising up to the floor level of the deck via another (elevated) concrete pad to support any version of RMH footprint desired. A compact 6" system size batch-box with thermal mass bench can easily fit onto a 44 to 48 inch wide by 10 foot long concrete pad.

Wow! Thanks for the replies everyone! Many good vibes to each of you!

So, my deck is only 4 feet off the ground. Stacking concrete on a slab is  not a bad idea but IDK how hard it would be to build such a slab considering I'd have to hang out under the deck to dig and pour it.

I'm definitely going to reinforce the floor now that I've started studying designs more seriously. Thanks!

I have a thought though that I'd love some feedback on.

My house is already heated with a big stove which is on the other side of my coming addition. I'm wondering if I can just convert the present stove (pictured below) into a thermal mass rocket stove which connects to the addition through the wall behind it.

Occams' razor suggests I should just use my present stove more efficiently than building a new stove for a new room when the old stove is only a few feet from that wall.

Relatedly, I looked into a heat reclaimer device and the creosote build-up it generates spooked me away from that for sure.


You can see

Jeff Pollari wrote:Wow!
So, my deck is only 4 feet off the ground. Stacking concrete on a slab is  not a bad idea but IDK how hard it would be to build such a slab considering I'd have to hang out under the deck to dig and pour it.

In that case, it's easier if the deck is opened up first, then you have a nice open hole in the deck in which to work standing up straight etc.

Jeff Pollari wrote: My house is already heated with a big stove which is on the other side of my coming addition. I'm wondering if I can just convert the present stove (pictured below) into a thermal mass rocket stove which connects to the addition through the wall behind it.  

A doorway or two in the wall behind your present woodstove will allow the adjacent addition to be heated, assuming it's not a huge room. As far as converting a steel box-stove into an efficient, say batch-box, you basically just end up using the door off the thing, if you want to do it right. It can be done, has been done, but it takes a good deal of work to do the conversion, metal working skills, and etc. and really doesn't save (replace) much in the way of masonry materials. Easier to just build a batch-box / bell brick masonry stove. Have you seen Peter van den Berg's website ? --->
https://batchrocket.eu/en/building

Yep, you're not going to want the decking there anyway, so just cut it out in the footprint you want. You can either build the block piers up between the floor joists, or frame around the footprint. I would give the existing framing some (but not too much) consideration in the new layout - if moving a few inches makes framing easier, do it.

I agree that it would probably be easier and maybe less expensive to just replace the wood stove (which looks to be older and likely not that efficient) with a new masonry RMH. If the existing building is solid enough to support the weight you could build one RMH there and make openings to the new room, or even run a bench through the wall to directly heat both spaces. If the existing framing is not beefy enough, maybe it would be easier to take out the stove and build the new RMH in the addition and open the wall or run the bench through into the existing room.

Byron Campbell wrote:

Jeff Pollari wrote:I've hired a contractor to build an addition for my home on my deck and I need to know what rocket mass design would work best for my needs...  

Which begs the question: how high is the deck above the ground? One option being, reinforced concrete filled CMUs' resting on a concrete pad, rising up to the floor level of the deck via another (elevated) concrete pad to support any version of RMH footprint desired. A compact 6" system size batch-box with thermal mass bench can easily fit onto a 44 to 48 inch wide by 10 foot long concrete pad.

Hello again! Are you suggesting that I could put concrete blocks on my concrete pads to rise up to the floor level under the thermal mass to support the weight? I'm reading the book on the stove and it cautions against building on top of reinforced flooring. But, I do not want to bother with cutting my floor and other stuff.

To reinforce the heater, can I just stack CMUs on the concrete pad to reach the floor joists or do I go for the plywood or do I need something else. thanks! -Jeff

Glenn Herbert wrote:Yep, you're not going to want the decking there anyway, so just cut it out in the footprint you want. You can either build the block piers up between the floor joists, or frame around the footprint. I would give the existing framing some (but not too much) consideration in the new layout - if moving a few inches makes framing easier, do it.

I agree that it would probably be easier and maybe less expensive to just replace the wood stove (which looks to be older and likely not that efficient) with a new masonry RMH. If the existing building is solid enough to support the weight you could build one RMH there and make openings to the new room, or even run a bench through the wall to directly heat both spaces. If the existing framing is not beefy enough, maybe it would be easier to take out the stove and build the new RMH in the addition and open the wall or run the bench through into the existing room.

I like your suggestion of placing a part of the bench in the new room, however, I'm not sure how I would begin with cutting through a wall in my house. The wall is load bearing... soo that could be an issue. Do you have recommended sources where I can learn more?

Byron Campbell wrote:

Jeff Pollari wrote:Wow!
So, my deck is only 4 feet off the ground. Stacking concrete on a slab is  not a bad idea but IDK how hard it would be to build such a slab considering I'd have to hang out under the deck to dig and pour it.

In that case, it's easier if the deck is opened up first, then you have a nice open hole in the deck in which to work standing up straight etc.

Jeff Pollari wrote: My house is already heated with a big stove which is on the other side of my coming addition. I'm wondering if I can just convert the present stove (pictured below) into a thermal mass rocket stove which connects to the addition through the wall behind it.  

A doorway or two in the wall behind your present woodstove will allow the adjacent addition to be heated, assuming it's not a huge room. As far as converting a steel box-stove into an efficient, say batch-box, you basically just end up using the door off the thing, if you want to do it right. It can be done, has been done, but it takes a good deal of work to do the conversion, metal working skills, and etc. and really doesn't save (replace) much in the way of masonry materials. Easier to just build a batch-box / bell brick masonry stove. Have you seen Peter van den Berg's website ? --->
https://batchrocket.eu/en/building

thanks to the link for the batch box. It reminds me of a thermal mass design that I found on YouTube where the guy had a bench that he vented his rocket stove into that was just open air covered by large slabs of stone. The bottom of the bench had a hole where the cooled air would exit up the flu. Are you familiar with that type of design? I've looked all over for that video but can't find it.

Glenn Herbert wrote:Yep, you're not going to want the decking there anyway, so just cut it out in the footprint you want. You can either build the block piers up between the floor joists, or frame around the footprint. I would give the existing framing some (but not too much) consideration in the new layout - if moving a few inches makes framing easier, do it.

So do I just get concrete blocks and put them on the basement's concrete slab until a stack is between each floor joist? You also said 'frame around the footprint' does that mean go around the whole thing with blocks? thank you so much for clarifying!

That would be a Matt Walker design open bell bench.
https://walkerstoves.com/index.html

Any of the "bell" style bench designs would do what you describe. Peter van den Berg's website mentioned above has a lot of information on bells.

For the foundation, you could just stack blocks up to floor level. I would then drop a piece of rebar down the cores of each stack and fill the cores with concrete, to make it good and strong. You will get better support and more safety if you cut out the decking at each stack and run the blocks up to a couple of inches above the top of the floor. Then the mass can be continuous, supported directly by the piers, and safely separated from the wood.

To lay out the footings, I would suggest deciding on mass placement, drilling holes in the decking where you want piers, and dropping a plumb line down to locate the center of each pier. Cutting out the decking to clear the piers will make stacking and filling easy.

Glenn Herbert wrote:Any of the "bell" style bench designs would do what you describe. Peter van den Berg's website mentioned above has a lot of information on bells.

For the foundation, you could just stack blocks up to floor level. I would then drop a piece of rebar down the cores of each stack and fill the cores with concrete, to make it good and strong. You will get better support and more safety if you cut out the decking at each stack and run the blocks up to a couple of inches above the top of the floor. Then the mass can be continuous, supported directly by the piers, and safely separated from the wood.

To lay out the footings, I would suggest deciding on mass placement, drilling holes in the decking where you want piers, and dropping a plumb line down to locate the center of each pier. Cutting out the decking to clear the piers will make stacking and filling easy.

Thank you! I will do that! Do you happen to know how efficient the bell style stove (or walker's riserless core) is compared to the conventional rocket mass heater with the barrel heat riser and ducts inside the bench?

I can't find any info comparing the two heaters on your recommended cite nor walkers'.

There is no inherent difference in efficiency between J-tube and batch box, nor between bell and piped mass. Your actual efficiency will depend on materials used and how well you follow recommended proportions.

Glenn Herbert wrote:There is no inherent difference in efficiency between J-tube and batch box, nor between bell and piped mass. Your actual efficiency will depend on materials used and how well you follow recommended proportions.

great to hear. Thank you.... However,  I'm not sure why there would be no difference. Doesn't the piped mass mean a great amount of thermal energy is being stored than in a bell?

Not necessarily; there are limits to the amount of ducting you can have without reducing draft too much, while a bell can have greater internal volume so that hot gases are able to stay inside longer and transfer more of their heat to the mass. Only the coolest gas exits the bell since the outlet is at the bottom. Any particular version may be more or less efficient than another with different characteristics.