RMH build Idea - one sided bell - that serves as a heated seat back

thanks..will soon change to 6" and see what happens....
for the bell/bench transition; I can probably remove much of the support I now have but will have to leave one center pillar I believe to support the structure of the seat back/bell side. Red pillar in pic, but can probably leave the top edge of the seat a smooth and open transition so the smoke can easily roll over that edge and into the seat.

A pillar in the transition opening isn't a problem, most of the time. Best to make this as narrow as possible, for example with a round or square steel duct. As long as it supports the above structure, you will be OK.

so, made 3 changes
- increased pipe to 6" single wall black pipe (after pic was taken-replaced dryer vent). Exhaust outside at end was 80F
- increased gap at top of bell to metal cap from 2" to 3.5" to get more heat over to the back - which seems to have worked. sitting on the seat was a pleasure and soothing for back
- removed the archway support structure under the seat and replaced with single metal tube


The fire was approx 30 mins long . and got the heater up to what I would believe is medium working temps?  will do a longer fire next and take new readings.

see pics at various times in the burn
temps at the tip of burn sticks - 600-1000F

very approximate temps in the heater
220F at the metal cap
115F +/- at the upper seat back -which is just about perfect - and stayed nice and hot for hour or two
95 at the lower seat back
90 on the metal seat itself
exhaust outside 80F

dare I say, this is working perfectly.....

These seem like nice working temperatures. Not sure where or if I'd change them....I assume these are in range and proper for this build.

The exhaust is breathable with slightest hint of wood.

Only parts one wouldn't want to touch are the top metal plate and the bottom of the firebox.

thanks again to all who chipped in with help and ideas. Maybe someone else can use the info as a guide to build same/similar. So far, I'd say this is a winner. At least for my needs. It came out very similar to the original mind vision. these heaters are quite the machine. The rocket action makes it feel alive. And the heat absorption and slow release also seems human and natural. RMH's 20 year history was worthwhile I'd say...

UPDATE - still working well - pleased with the heater


Experimented with a short small fire to see what temps could be had from a handful of semi-wet, real world wood: freshly fallen twigs and semi dry soft and hardwood.

The temps didn't reach 'full' temps as previous fires, but just hot enough for a nice sitting session. of 3-4 hr duration

Temps are similar/same, as a larger fire in terms of proportional heating throughout the heater. (The highest fire temps from previous fires have been 300F on top and 190 upper seat back)



temps for this small fire
.5 hr -      200 metal top   -   180 upper seat back    -   100 seat
1 hr  -      180                        140                                       90
1.5    -     160                        140                                        90

equalibrium for top and seat back happens after 2 hrs
2 hr    -    130                         130                                      85

these temps remain for another 1-2 hrs


Lessons learned so far:
1- Realize that the firebox and burn tunnel is a bit small - it ended at 5x5.5" due to brick size, but I believe the extra inch is desired because when larger fires are made, there is a tendency to show a reduced draw/draft. The smaller opening seems to become air constricted/blocked, a bit, due in part from too many sticks and some developing coals which take up space in the burn tunnel entrance. Combined with using semi wet wood and there can be occasional back smoke - I suspect that fully dried wood would partially solve this, but suspect the burn tunnel is also a bit smaller than ideal.

A small fire however, like the one above, if also kept towards the front/middle of the firebox, works perfectly with no smoke back
The key is to start the fire up front in the box to warm up the tunnel, and then once it takes hold on a few stick tips, move it back to the center, then gradually introduce larger sticks and keep the entire fire up towards the front/middle - used a brick to take up some of the undesired back firebox.



2  The metal top needs some work.

One of the edge mortar joints cracked. Tried high temp sealant as well as high temp mortar and neither was flexible enough. The metal expanded at too much or at different a rate.

It hasnt spread and doesnt smoke, but will want to change the top configuration for next season. The idea is to cut the metal top to fit, set it on insulation and then border it with a thin 1x2" brick course, then set another piece of metal on that brick border ,and use the high temp caulk like sealant..this will separate the lower hottest plate from the 2nd top plate and should be cool enough that the mortar/sealant tried prior, will hopefully not crack.

200f on the steel plate  above the riser seems very low?

it was a small fire. typical fires get the top to 300F. I build the fires more to focus on the upper back temp and when it gets past 180 or so, I stop feeding. Could probably get the metal higher otherwise.

Carol,

I just noticed now how close is your heater to this flammable wall. If your top capping sealing develops a substantial crack during intense firing, the flames can start a fire.
I remember one time I was experimenting with a cooking stove and placed a cast iron plate on top with a few wet pieces of wood on top to dry them a little. The fire was intense and it cracked the plate and the flames ignited the wood being dried, through a hair thin crack. It was outside so I did not care much.

I would recommend that you make proper capping from two layers of bricks: using T-bars and some wool on the first layer and then the second layer. All with mortar.

Thanks for that,,,,Am researching that technique... but one of the issues is that dual layers of bricks will take away most of the instant heat> and this heater is used for say 2-5 hrs at a time and instant heat is important.

to that end and also because of materials on hand and lack of mortaring skills, I wanted to exhaust the idea of dual metal plates first.

If there is one metal plate sitting on top firebrick rim, resting on insulation. This sill separate the metal from the stone and still block 90% of the smoke and a percentage of the heat.

I wonder what that percentage is however. The air gap above that plate (in between it and the top plate) will be one brick width. say 5" . Am trying to estimate what insulation value a 5 inch x 20x20" air gap would impart to the heat that happens to come off and above the first plate. Alot of the heat will be directed onwards into the heater, but a significant amount will pass through first metal plate.

The idea is that I still get some quick heat that way and the 5" air gap should insulate the space enough to lower the temps so that I could then use a standard heat caulk for the top metal piece... And also remove any chance of a hazard etc

max temps on the plate as it sits now is 300F, so the above plate, after being insulated and isolated, ----might be 150-200 max.

this is the idea...

success I believe - two metal 'tops' bottom piece resting on insulation, top piece set in place, outer edge siliconed
used the meeco furnace cement on the top half of the brick work -essentially from the lower back up to top metal - so that when sitting and leaning on the back, do not have to worry about failures . this has held up well so far and was easy to use

if for some reason i'd want to reduce that top metal temp - can cut the caulk line and remove, insert a layer of insulation to set on top of the bottom metal, then replace/resilicone, might reduce 15-20F ?  will see. seems fine as is so far

if anyone sees any concerns, feel free to let me know.  I am aware the ideal fire stick length should be size of feed tube, but im always near the heater and a water hose is right outside at all times. next season will cut all stock appropriate...

but the overall build is almost exactly as i'd envisioned. the size, the amount of heat, the seat temps, the exhaust temps etc. And now that it's all fully sealed and insulated. it drafts very nice with no back smoke so long as i tend the fire a bit and keep it in the front half of the burn box, which is not difficult to do once you get the idea....quite happy with it

Well I am pleased that you are happy with your new stove but those must be the lowest temperatures I have ever seen!

It a true sense a rocket stove should be a super hot, raging bull, burning at max temp inside a super hot and insulated fire box and exiting the riser as a smoke free, super clean burn.

I can only assume you are just trickling in heat from a tiny amount of burning wood for a short period?
Have you tried to get it running flat out?

Like I say if it works for you then that is great…..

I'm burning found sticks and semi dry 3-6mos cut hardwood and soft (aka wet).  misc pieces of ply have also found their way. next year will be dry hard and softwood

outside temps have been 35-60F (late season is when this was completed), so will have to wait till next winter, but even then we only get down to 0F  for a few weeks a year. The temps this heater will be used in will usually be 20-50F outside temps

rocketing sound is there during most of the burn cycle, varying from loud to soft depending on point of burn cycle and amount of sticks...Usually its just 5-7 small sticks (twigs to 3/4" or so)

longest fire has been 1-2 hour burn. But the fire itself has always been 1200F + at the burn start. That is correct I believe. The hottest the metal top (prior when just used single top), was 300F I believe (may be misremembering that but I think its around there). The amount of wood burned and length has been small I guess but its been based on the heated seat temps. It doesnt take much I guess to get those temps in the zone. And also the space is small and the heater is good size for space, so these small fires is all it takes to heat the space. it's a utility work space during after work and over night time period.

It is a clean burn in terms of the exhaust. It's clear and smoke free after the first 15 mins. (at outlet outside of building), it is not clear inside the heater. (you are suggesting that should be the case it seems?)


The space between the top of burn tower and metal top is 3.5" or so. So the temps coming out of the burn tower (which is now fully insulated), are unknown but are reduced when measured at the metal , by a percentage . the temps inside at that point are unknown without a temp sensor inside.....people say the desired tower temps are~ 1000+F correct. Im guessing the temps in there are 500-600F ?  Could I get those up to 1000,if drier fire was burned, for many hours? maybe Don't know till next year. And what would that estimate for the temps of the lower metal top (which I now cant directly measure), am not sure...

I think its Glenn? who has a thermocuple in his heater at the riser? Would love to see him or someone run a test in a proper heater...using 'wet wood/found sticks' vs dry wood and see what the difference is at the top of burn tower....I've never heard or seen such a test, but that would give me an estimate for the increase in temps I'd expect when dry wood is burned in the future...

The center of my barrel sits about 3 inches over the top of the riser. Running a full burn it will max out my IR gun at 1090+*F. That's on the outside of a heavy gauge barrel. Maximum temperatures occur at the junction of the burn tunnel and the riser where secondary ignition should be happening. Discussion on materials to use to build the burn tunnel and riser I've seen mention that clay that can fail at 2200*F is borderline, best to have something that goes up over 2400. The PTJ crew got the overkiln up over 2000*F, and that was from gases that had already passed through the riser.

I'm curious if there is a definite number for what constitutes a complete burn. The two most socially important benefits to RMHs are the lack of creosote buildup that would present a fire hazard, and the lack of particulate matter in the final exhaust that exits the system that would contribute to environmental air pollution. If we don't ensure that these two features are present, it detracts from efforts to convince regulators and insurers (as well as genuinely concerned individuals) to be open to their adoption.

Once fully primed, exit gasses should be free of visible smoke for sure. The statement about smelling a bit woody makes me wonder. My chimney is too high to get a reliable whiff, but I don't recall anyone mentioning any sort of smell, just that it is basically steam. Thermocouple readings would sure be helpful.

yea it is a can of worms imo. people have different goals. I dont see how RMH could ever be insurable because RMH means 100's of different things depending on the design/materials/location etc. It would have to be standardized and severely limited, then tested by Gov, etc. at which point the usefulness in many peoples applications may no longer work or be within budget. The materials for these heaters is already approaching cost of traditional stoves. the beauty of rmh is its customization and diy aspects imo. I specifically built this for its unique applicaiton and use case and that is partly why its the way it is and has less than ideal burn temps. That was not even in the equation other than to get things close enough to correct, to insure proper draft and decently low temps so exhaust could go right out side wall with no treatments, and to that end, the build is exceeding expectations.

So rmh community members who are pursuing that ideal/goal of 'complete burn', have a interest in the top of tower burn temperature. Same with those who believe in the globe, or 'pollution' etc. Total can o worms there. Compare a day of US military weapons' affect on air water and soil just for one of thousands of pollution starting points, thenask ones'self, whether a rmh with only 80% efficiency vs a finely tuned 95% example is worth a seconds thought. ......And after building and learning a bit from this, I believe there are some downsides to too hot a burn, that are not mentioned for whatever reason. How they wear out the heater so quick, how it makes even high temp cements and other mortars very precarious to unusable. etc.....Everything is a tradeoff and calculus equation in this less than ideal world.

the other mentioned drawback of a less than ideal burn is creosote. I'll keep an eye on it, but it looks and smells clear to me. although i could because the exit exhaust is just above head height, I dont smell it enough to know. But times I have, depending on point in the burn and type of wood etc, it ranges from no smell, to hint of wood. Which compared to a metal wood stove, is far better, so this would equate to a much less likelihood of creosote buildup. will see. I think with dry wood I can get temps at the top tower inside to be 6-700F ?(although wont be able to confirm). But even if its currently 4-500, I dont have an issue with it. Part of the sales pitch for rmh is that you can burn found sticks and misc building materials etc, and while I dont plan to do a lot misc materials, I will be cleaning the property of fallen twigs/branches and putting them right in there. That is real world use case, and it does work fine for me so far, it seems.

Just for fun and personal edification,  I'd like to know the difference of burning the twigs and wet wood VS, ideal dry wood though, in terms of top of tower burn temp...

I think it should be kept in mind that concerns over stove exhaust are not restricted to political polemics and claims about ambiguous scientific communities. A random town I stopped at in the Lewis & Clark National Forest had a short history posted that explained the size of the tiny settlement was restricted because certain climate/wind conditions had caused the smoke from residents' stoves and fireplaces to linger, choke out the town and cause evacuations. This was a random experience, I can only guess how common something like this has been, not to mention things like smog and urban centers during the industrial revolution.

RMHs have been approved for building code in places. They have also been accepted by insurance companies, although that seems like less of a concern to me. People who may find regulations coming from the UN based on theory something they can dismiss will be less likely to do so for codes based upon centuries of real life experiences. The point to me is to promote the development of robust designs as a group of supportive individuals so that we don't provide the Department of Making You Sad with an excuse send out more bureaucrats.

BTW carol, I was just reading up on a professional earthen builder that tried to put a metal plate on top of their masonry bell/riser, but couldn't get it to seal without leaking. Good job figuring out a solution here, you may have to consider giving workshops now...