Small(ish), but not tiny, Rocket Mass Heater for Bedroom in Japan: Is a 4" system feasible?

thomas rubino wrote:Hi Paul;
Welcome to Permies! I don't have time this evening to respond but I've linked a post you may enjoy!
https://permies.com/t/122458/Advice-RMH-build-Hokkaido-Japan

S Bengi wrote:Yes 4inch and 3inch is very feasible.
Here is a very informative website. https://batchrocket.eu/en/building#dimension

It sounds like you are USDA zone 9, so a 4inch RMH should be fine.

Paul Muthers wrote:

thomas rubino wrote:Hi Paul;
Welcome to Permies! I don't have time this evening to respond but I've linked a post you may enjoy!
https://permies.com/t/122458/Advice-RMH-build-Hokkaido-Japan

Thank you, very interesting article in that thread about heated flooring (http://www.healthyheating.com/History_of_Radiant_Heating_and_Cooling/History_of_Radiant_Heating_and_Cooling_Part_1.pdf) which is making me consider completely overthrowing my current plans and building complete floor heating instead..  I am already planning on putting tatami against the underside of my floor, holding it in place with bamboo and then covering it in mud/cob, like I did at the other part of the house, so it may almost be feasible to just go and turn the whole space into a smoke ducting area... but I am a bit scared of CO leaks especially since we will be sleeping there....

S Bengi wrote:Yes the riser diameter dictates the exhaust diameter of the system, they should be equal (technically the exhaust can be a bit bigger, but never smaller)

Riser diameter = 4inch
Riser Area/CSA = 12square inch
Bell CSA = Tube CSA times 5 = 60sq inch (aka at least a 8.74inch bell diameter)
Bell Gap = 1in to 2in
Bell Internal Surface Area (top + sides only) = Tube CSA times 300 = 12 * 300 = 3600sq in = 25sq ft
Exhaust/Mass Length = 12ft (after the 1st 90 degree bend, minus 5ft for each additional bend)

I recommend building this system outside and testing it, before putting it inside.

Are you sure that your floor can support the weight of the entire RMH?


You could also go with a radiant in-floor heating system powered by a pellet hot water boiler.

John C Daley wrote:{Paul, do you have the gear to roll your own tubes.
You could use a 'pipe anvil' to make say 48 inch long sections and weld the seams.

Paul Muthers wrote:Just a quick general question I am not quite sure I understand. Does the list of measurements also, roughly, work for normal rocket mass heaters or do they all require the batchrocket design with the secondary air intake (p-channel)?

Paul Muthers wrote:And does the diameter of the heat riser dictate the minimum diameter of the chimney pipe system through the heater mass body? Do you remember how they relate? I haven't seen the chimney system mentioned specifically..

Paul Muthers wrote: Is there some downsides to having an open space, aka stratification system, instead of chimney piping in cob?

Paul Muthers wrote:Just testing out uploading some photos of my work site..

Glenn Herbert wrote:A 4" batch box is easy to build, and would probably even work well without special secondary air supply (at least for the first iteration). I made a dry-stacked mockup of a 4" batch box and it burned beautifully. A properly built one should be even better. A 4" size would need reloading every 15-20 minutes more or less, so I would not put it outside. If you have good draft you will not have to worry about smoke in the room. You would not be running it all night in any case, except maybe on the coldest nights.

I am not sure a 4" system would be big enough to use as an underfloor heater. I think it would work better as a modest bench or "pillar" shaped mass which would radiate across the room. The "cyclone batch rocket" built at Wheaton Labs might serve as a good guide, with modifications based on their experience with it in use. In particular, I would include a metal access panel in the side or front of the mass for some instant heat (also to allow inspection and maintenance without tearing up the masonry.)

Peter van den Berg wrote:

Paul Muthers wrote:Just a quick general question I am not quite sure I understand. Does the list of measurements also, roughly, work for normal rocket mass heaters or do they all require the batchrocket design with the secondary air intake (p-channel)?

Despite the similairity in the names the measurements and (simple) formulas on the batchrocket website apply to batchrockets only. Back in the day of, say, 10 years ago I tried to work out a similar set of rules for the J-tube as well. But the then experts on the Ianto Evans' J-tube design refused to cooperate so I gave up.

Paul Muthers wrote:And does the diameter of the heat riser dictate the minimum diameter of the chimney pipe system through the heater mass body? Do you remember how they relate? I haven't seen the chimney system mentioned specifically..

This is one of the few similairities with the J-tube. The j-tube has a consequent cross section area throughout, which is called the system size. In the batchrocket design the system size is the riser's diameter, which in turn is the same as the chimney exhaust diameter.

In general, the batchrocket system is about twice as powerful as the same size J-tube. Since efficiency is roughly the same and raw power can't be produced out of plain air the former is eating away about twice the fuel weight in any given time frame.

There's another difference: the batchrocket system is very picky about restrictions in the smoke path. If ordinary bricks are available I'd recommend to build a bell system instead of a piped bench.

Peter van den Berg wrote:

Paul Muthers wrote: Is there some downsides to having an open space, aka stratification system, instead of chimney piping in cob?

For building a bell system other skills are required. But it beats the piped system in simplicity at a physics level. In my opinion, the former is vastly superior in extracting heat provided it's wide enough inside. The velocity of the gases will slow down greatly when entering such an open space, allowing gravity to do its job by separating the hotter gases from the cooler ones. Hot gases rise to the top, cooler gases sink to the floor. As such, it could be viewed as being a displacement system, pushing the gases at floor level out.
As a bonus, due to the low gas velocity in the bell system fine dust will settle quietly on the floor of it instead of clogging up horizontal chimney pipes.

Paul Muthers wrote:A bell system sounds interesting, though I may end up not doing either and just going with the earlier suggestions of a nice radiant heating batch rocket, do you know what kind of size restrictions there are for the bell system, attached to, e.g. a 4inch batch rocket system? What would happen if the area was too big?

Peter van den Berg wrote: When planning to build a batch box rocket yourself it's sensible to read the whole of the site very carefully.

Peter van den Berg wrote:

Paul Muthers wrote:A bell system sounds interesting, though I may end up not doing either and just going with the earlier suggestions of a nice radiant heating batch rocket, do you know what kind of size restrictions there are for the bell system, attached to, e.g. a 4inch batch rocket system? What would happen if the area was too big?

The heater will refuse to start up properly, that's all. Oversizing could also mean that the chimney temperature is too low. Which in turn is causing water condensation in the chimney. This looks like this: at first, the fire is ramping up but within appromately 20 minutes a chimney stall occurs and a tsunami of smoke is streaming into the living space. Keep in mind: burning of wood will produce about half of its weight of water. It's one of the results of the chemical process, the others (assuming complete combustion) are CO² and heat.

The numbers given on https://batchrocket.eu/en/building#dimension are pretty accurate. Of course you could oversize but in that case you'll need a chimney shortcut or bypass of some sort. When planning to build a batch box rocket yourself it's sensible to read the whole of the site very carefully.

Peter van den Berg wrote:@Paul, there's a distinct difference between a p-channel and a floor channel. The p-channel blocks a little bit of the port at the top so no air can get around it. The floor channel's vertical part is far enough from the port to create half a port's width left and right. Regarding a round duct: using it for a p-channel the answer is no, for a floor channel it's a maybe.

The riser works best without restrictions, the whole of the design is very picky about friction in the smoke path, as a matter of fact. So, what you propose means 5(!) orifices in the riser's length. The odd chance that it would work looks very slim to me.