sending exhaust out the wall vs. the roof

This is a topic that has come up so often in the last couple of years, that I want to make a thread just for this topic and then I can point people to this thread.

Consider that the rocket mass heater is a passive system. There are no fans or electric air pumps to move the exhaust through the system. We are using the pump that comes with the heat riser and the bell. At the same time, people want to have a lot of ducts with a lot of turns to get the heat out. More duct requires a bigger pump. Or more than one pump.

429) If you go through the roof, you have added a vertical pipe. If it has a wee bit of warm to it, it will act as a second pump! Yay!

429.1) If you move that vertical pipe near the bell, the pipe gets warmer (by collecting radiant heat from the bell) and the pump, therefore, becomes stronger.

429.2) The heat used for this secondary pump goes out the roof. Damn!

430) If you go out the wall, you can extract every bit of heat before shooting the exhaust outside. Yay!

430.1) You can make an outdoor chimney. If you have extracted all the heat out that you can, then the exhaust is room temperature, 72 degrees. And if the temperature outside is, say, 50 degrees, then you can use that temperature differential to help with a secondary pump.

430.2) If the wind blows toward your wall, it could force the whole system to run backwards - oh no!

430.2.1) There have been techniques to mitigate a lot of this: put a T on your wall exit; put a triple-T on your exit (looks like an H); route your exit into a shed (mitigates gusts); add an exterior riser and bell; add an exterior air intake on the same wall.

431) Another point to keep in mind is that if your home is full of warm air, then the air in the house wants to go up. And if your house has dozens of teeny tiny paths where air can go up, then your house is trying to suck air up .... that means that air needs to come from some where. And look! There's that great big pipe to the outside that you call a rocket mass heater. So the house in this scenario is trying to make your rocket mass heater run backwards! Of course, if you have a chimney through the roof and the exit is higher than the highest point in the roof, then the warm air in the chimney will have a stronger pull than the warm air in the house and the rocket mass heater will go forward!

432) Kitchen fans, bathroom fans and any other fans that blow inside air to the outside will also try to make the rocket mass heater run backwards.

433) you can make your rocket mass heater pump more powerful by:

433.1) making a taller riser

433.2) make a riser that is more insulated

433.3) use less duct

433.4) use less turns in the duct

434) exterior air intake such that the air intake can be sealed to get for the stove from ONLY the outside.

434.1) Suppose you don't have an excellent rocket mass heater and instead you have a "pretty good" rocket mass heater. In such a case, it smokes back about 1% of the time. If you had an exterior air intake, then the smoke back will not enter the house.

434.2) Eliminates all of the concerns of 431 and 432

434.3) Mitigates most of 430.2



(okay, these are my quick notes. what have i left out?)

paul wheaton wrote:This is a topic that has come up so often in the last couple of years, that I want to make a thread just for this topic and then I can point people to this thread.

I love it! This is something I have spent some thought on....

I will add some other things (armchair thought most of it not tested).

1) Long pipes have resistance, I have experimented with bells (well one anyway) and been really happy with the results.
good things about the bell:
a) if built right this can act as a second pump. (the primary barrel is a type of bell).
b) can pull a lot of heat from the exhaust flow in a small space. (instead of running the pipe in two directions with the resistance of a 180 it can be straight)
c) has less resistance from one end to the other. (not sure about this, but I think the pump effect makes up for the loss anyway)
d) easy cheap to build (I used an old water heater core - free)
e) The bell can be located in a second room with a straight pipe under the floor to get it there. The loss due to resistance should not be too bad in a straight run.
f) I think (could be wrong) that more heat can be removed from the warm gas because the exhaust port is at the lowest part of the bell.
g) easier to clean than a long pipe.

Now the above doesn't seem to say much about how things are exhausted, but I think it gives more freedom in how exhausting is done.

Anyway, Assuming that the end result is room temp air.... I would like to suggest using that room temp exhaust to warm the earth within the mass used for heating the home (that 20 feet). The general idea is to build on a slope of some sort and a lot of the annualized heating systems use this to ventilate the home without loosing to much heat. Why not use the same idea for heat exhaust? The exhaust goes out of the home underground at about room temp through a pipe that slopes down through the earth mass to outside the the "umbrella" and into a rock pit. The earth temp along this path should vary from room temp to outside temp... or rather seasonal average (59deg or so). The rocks would help to condense any water left in the exhaust too. This cooling effect should pull (your second pump) the exhaust out.

Another thing to think about, may be having more than one route for exhaust depending on the situation (wind temp differential from inside to out or whatever). This would:
a) allow a build with different ideas to try without having to rip things apart to try something else
b) allow maximizing the use through all seasons.... not warming as much in shoulder seasons by bypassing some of the heat storage
c) allow dumping heat if it gets too hot in the middle of a burn (should never happen) without having to wet down the feed.

Len, could you explain the bell itself a bit more. I can't quite get it. You explained the benefits well but not its construction.

Greg Harvey wrote:Len, could you explain the bell itself a bit more. I can't quite get it. You explained the benefits well but not its construction.

A bell (this being a common term in masonry heaters, so I forget not everyone knows what it is) is just a container with an in and out. The out is always on the bottom, the in can be just about anywhere, but is commonly bottom as well. The idea is that the warmest air moves to the top and looses it heat to that surface. The cooler gas falls as it shrinks and is exhausted. The gas flow can be slower too. In a pipe often the warmest air is in the centre with a layer of air insulating it from the pipe walls requiring a long pipe to get all the heat out of it. Even putting a T in the middle of a horizontal exhaust pipe will give a hot spot at the T's top (assuming the top is blocked .

Most people make them tall and skinny, but I made mine low and long. I took a barrel and put it on it's side. I cut the top flat and put patio slabs on top instead to form a bench. Because the bottom was round, I had to put the input above the exhaust. I had about 400lbs of mass over top of the unit (in patio slabs). I did not get to adding sides or insulating the bottom, but I think that would help.

So glad to find this! Ok, Paul, here is my list of questions concerning the riser.
I noticed that Erica and Ernie's riser (video) was made of whole firebrick and not insulated. Are whole firebricks sufficient without being additionally insulated? Won't they eventually get too warm and deter the cooling effect on the outside of the riser inside the bell?
When collecting my supplies to build my RMH, I obtained a quarter inch thick 8" steel pipe to use as my riser, having read that the metal pipe used as a riser will burn out overtime because of the the stresses of extreme temperatures. My plan was to use perlite/clay as insulation around that riser as thick as possible within the bell. I found a 15 gallon barrel a 35 gallon barrel and a 55 gallon barrel. I thought maybe if I used one of the smaller barrels around the 8 inch steel riser and insulated the riser with perlite/clay, I would get a longer-lasting riser with that configuration. And of course, then next I had read that the thicker metal takes longer to warm up and charge the system. Overkill? I guess.
So then I had opportunity to come to the workshops! I feel bad for those who weren't able to make it. You missed out on a great learning experience. Yet, having had the experience of the workshops I now have a new set of questions. ) So much of those workshops was highly experimental that my questions are limited. But I have read about wrapping the riser in Rock wool. How do the insulating qualities compare between perlite/clay and rock wool? What about using the Dura board? I am also concerned about attaching the insulation with metal screen or wire where such high temperatures are involved. And have you yet cast a riser using Erica's ingredients? Or are the risers you're using made of the same material? It didn't seem to be the same to me.
I love the idea of the shippable core and I am definitely on board with that. I also would like to work on designing a more solid configuration that would piece together or that would be solidly cast to thwart those weak areas we saw. I think given the weight of the material will be sufficient for a thicker core.
Oh! One more question. This one concerns the second bell. Sending a pic of a design I'm considering. I am thinking that this will work just fine. I am concerned with the second bell because it is a 16 foot bench. Will the second bell capture that much heat so that it would radiate or am I just overkill?
Thanks again Paul!
Meg
PS
I hope you like my pineapple story~

Meg Keeney wrote:
Oh! One more question. This one concerns the second bell. Sending a pic of a design I'm considering. I am thinking that this will work just fine. I am concerned with the second bell because it is a 16 foot bench. Will the second bell capture that much heat so that it would radiate or am I just overkill?

I would zig (or zag) the flue in the bench to be right against the wall... maybe using square or rectangular pipe for more surface area in contact with the wall. Then instead of a "U" shape I would put the second barrel at the other end of the 16ft.

My reasoning is:
a) cob does not conduct heat that well. It is a good heat storage, but only medium heat conductor. You are obviously using the masonry wall as part of the heat storage and so I would want to get my heat to that wall. It will radiate into the second room (green house).

b) The second bell will pull heat out of the exhaust gasses and so the second 16ft may be too much.

So out of the barrel, 45deg. towards wall, 45deg. to go along wall 16 ft. 90deg through wall into second barrel. Exhaust from there. That would be my best guess.

In any case I would build the metal parts as they would sit outside and try it out first to make sure it draws right... before doing a permanent install.

Meg,

I want to keep this thread focused on sending exhaust through the wall vs. the roof. If you start a new thread about these other things and direct me to that thread, I will gladly address it!

Once the air intake was installed with the pebble style rocket mass heater in my office, it has been very cold every day. So here are a few quick observations:

I prefer to not use it. So far. I find that when i make a fully closed system with the air coming from the outdoors, there gets to be a sort of "tang" to the air. I suspect there is a leak somewhere. The pebble style is prone to leaks.

The air intake comes in low, then goes up to about six inches over the wood feed. It is metal and (currently) uninsulated. So when it is 20 degrees outside and 70 degrees inside, this makes that air intake do the thermo-siphon thing. And it proceeds to PUMOP air into the system. I think that the cold air is much smaller than the room air, so this means that more oxygen hits the fire. The fire ROARS. I think the whole system ends up going TOO FAST. Heat is coming out of the mass!

The metal air intake is COLD. It is cooling the room. I think this will be mitigated by insulating it.

I think that on a day where it is 50 it might be helpful to have the extra push. Especially when getting the fire started. So it would be good to have it uninsulated with the door closed on the top. If we want the fire to be unattended, we can put the insulation back and leave it like that.

When the temperature outside is 20, we can just shut the air intake completely off until we want it for some reason.


Onto another topic:

429.3) On a cold day, the warmth of the room and the warmth of the mass make the vertical rise keep working. So it will pump all of the heat out of the mass to the outdoors.

paul wheaton wrote:Once the air intake was installed with the pebble style rocket mass heater in my office, it has been very cold every day. So here are a few quick observations:

I prefer to not use it. So far. I find that when i make a fully closed system with the air coming from the outdoors, there gets to be a sort of "tang" to the air. I suspect there is a leak somewhere. The pebble style is prone to leaks.

The air intake comes in low, then goes up to about six inches over the wood feed. It is metal and (currently) uninsulated. So when it is 20 degrees outside and 70 degrees inside, this makes that air intake do the thermo-siphon thing. And it proceeds to PUMOP air into the system. I think that the cold air is much smaller than the room air, so this means that more oxygen hits the fire. The fire ROARS. I think the whole system ends up going TOO FAST. Heat is coming out of the mass!

The metal air intake is COLD. It is cooling the room. I think this will be mitigated by insulating it.

I think that on a day where it is 50 it might be helpful to have the extra push. Especially when getting the fire started. So it would be good to have it uninsulated with the door closed on the top. If we want the fire to be unattended, we can put the insulation back and leave it like that.

When the temperature outside is 20, we can just shut the air intake completely off until we want it for some reason.


Onto another topic:

429.3) On a cold day, the warmth of the room and the warmth of the mass make the vertical rise keep working. So it will pump all of the heat out of the mass to the outdoors.

I have two thoughts on this. right now my house has a 'Direct Vent' propane wall heater. The incoming combustion air comes down the outside of a specifically manufactured 2 wall duct system. The exhaust gas goes out the inside column of that same duct. Two things happen, the incoming air gets pre-warmed and the exhaust air is cooled down enough to be able to go through the wall penetration without having to maintain huge fire clearances to combustibles. One caveat is that this is a fan powered furnace which pulls the room air through it, so it doesn't overheat but also it doesn't try to accelerate the air movement through the ignition chamber.

Second thought, use the blast gates that I previously mentioned to shut off air flow to the mass and then 'dump' the extra heat load directly to the outside through Erica's diversion duct innovation. You would 'damp' down the fire by covering the feed tube all or partial to control the fire enough to keep it simmering some. With that small a burn chamber I kind of doubt that any fuel load would last long enough to make it to the next burn cycle. There are still some inefficiencies but without resorting to 21st technology like electricity and catalytic converters, your options tend to be limited.

Try it you may like it.

If you send the exhaust out the wall and then extend it to above the roof line the you wouldn't have to worry about wind.

Couldn't find this in my search:

How long of a horizontal exhaust run (with a little rise) can one run on a RMH?

We are getting ready to build a long Earthship/PassivHaus Hybrid home and it is almost 90 feet long. We will utilize an standard/existing Earthship floor plan with the great room on the East end and all the beds and baths in line to the West.

Would like to build the RMH in the Great room, on the earth berm North wall, and run the exhaust ducting West through each of the rooms. I realize the last room, prior to exhaust gasses exiting, will get the least amount of heat but every little bit helps.

What'a we think?
Too far to run?
Put a second, smaller, RMH in the Master bed (furthest West room)and run it East to help heat the other rooms when necessary (we all usually like our rooms cool when we sleep)?
Concentrate effort/heat in the great room?
It's an Earthship chances are we won't need but the one in the Great room?

Thanks,
Michael

Michael Johnson wrote:Couldn't find this in my search:

How long of a horizontal exhaust run (with a little rise) can one run on a RMH?

The correct answer is "It depends" The two numbers I have heard tossed around are 50ft for 8inch dia. and 30ft for 6inch dia. Remember that these are absolute maximums with no turns at all. Not practical lengths. I am not sure if they were tested with a chimney on the end of that or what height if so. In other words, at those lengths, I do not know what the design final temperature was. While not a RMH, I have seen a very long exhaust path going downhill away from a mass heater (Masonry) where the end temperature was low enough that the exhaust did not rise at all.

I think all of these were best case scenarios though. They worked as and when tested, but are not sure to work when the wind is from the wrong direction, the temperature is to hot/cold, the operator is not holding their mouth right or whatever.

We are getting ready to build a long Earthship/PassivHaus Hybrid home and it is almost 90 feet long. We will utilize an standard/existing Earthship floor plan with the great room on the East end and all the beds and baths in line to the West.

Wow! that is long. What is the other dimension? It has been a while since I looked through their plans. I like the ideas used but pretty much decided it was not the route for me to go. I would like something much smaller anyway and also am thinking that the earthship design may not be the best for our climate either. It doesn't get that cold here on the coast, but it is wet. I would like to try a small wofati design to see how well it deals with the wetter climate, but am also thinking cob. I have the steel parts for a RMH, but will need to redo the core I suspect before I use it full time. I am doing the other way of trying to get as much heat out of it in a short space using a bell style bench.

Would like to build the RMH in the Great room, on the earth berm North wall, and run the exhaust ducting West through each of the rooms. I realize the last room, prior to exhaust gasses exiting, will get the least amount of heat but every little bit helps.

My first thought is that you will be loosing a lot of heat to the earth berm. Yes it will help charge it up, but that berm/heat store is lossy. The idea of annualized heating is that the mass is so big anyway that it doesn't matter.... but when trying to heat it with wood, it will suck up a lot of heat before returning it.

Assuming a reasonably massive wall between your great room and the next room, I would put the RMH barrel against that. (built right into it even, though that has not been tested ) This might be the time to feature a partly sunken living room around the RMH so the feed and barrel exhaust can be below grade. Then run the exhaust across to the next wall and into another barrel. I don't know if another riser would help or not, but even without it would function as a bell. The under floor flue could be insulated so the heat would only be used in the bell(s) as you go. As I said I would make these a part of the walls... figuring they would be cob or some other high mass material. These bells could be as easy as a T in the pipe with one section of pipe going straight up in the wall and then capped.

I do not know if you could extend the length of the horizontal flue by insulating it or over sizing it (or both). The whole idea above is just a quick thought. The general thought is that it is not good to oversize and that may actually make the maximum distance less, I have seen cases where the horizontal flue is reduced as the flue gases cool and contract. In fact, I do this on my unit with the final outlet 1/2 the area as the riser.... but my horizontal flue is a bell where the flue gas is purposely slowed down to extract as much heat in a small area/volume as possible.

What'a we think?
Too far to run?
Put a second, smaller, RMH in the Master bed (furthest West room)and run it East to help heat the other rooms when necessary (we all usually like our rooms cool when we sleep)?
Concentrate effort/heat in the great room?
It's an Earthship chances are we won't need but the one in the Great room?

Thanks,
Michael

With the standard tested (as in known to work) RMH, 90ft is too long. I think in many ways you are going in untested pathways. The best scenario I can think of is to build in such a way that various things can be tried and discarded or abandoned. For example a channel in the floor that can be covered and reopened. A flue in the middle as part of a wall or fully exposed so it can be dismantled or added later if needed. The most promising and sure way would be to put one at each end on the inside wall of the end rooms and have a central exhaust flue. This can be done with a bell with two inlet and one outlet for example. The "centre" could be offset and use an 8inch unit in the great room and a 6inch in the master bedroom. If the two horizontal flues lined up, you could try exhausting from the master bedroom by not using the centre outlet and using the master bedroom as a flue instead of a heater. If you can find the right demolition, you may be able to pick up a lot of normal heater vent to try some of these things outside. (though that will probably be 5inch so you may need to run two in parallel. Lots of people don't like that idea, just so you know )

Len,

Thank you so much for the answer. I really appreciate the breakdown.

Looks like we will build the RMH in the great room and see how it performs over all, throughout the home. If there is the need for the extra heat (or we simply desire the ambiance) I will build another RMH in the master and run it through the other 3 kids rooms before exiting out. It is early on enough that we will work the great room unit into the initial build out.

Love this forum! Thank you again for all the advice and your valued opinion on things.

Michael

Len Ovens wrote:

Michael Johnson wrote:Couldn't find this in my search:

How long of a horizontal exhaust run (with a little rise) can one run on a RMH?

The correct answer is "It depends" ...

I think all of these were best case scenarios though. They worked as and when tested, but are not sure to work when the wind is from the wrong direction, the temperature is to hot/cold, the operator is not holding their mouth right or whatever.

We are getting ready to build a long Earthship/PassivHaus Hybrid home and it is almost 90 feet long. We will utilize an standard/existing Earthship floor plan with the great room on the East end and all the beds and baths in line to the West.

Wow! that is long. ...

Assuming a reasonably massive wall between your great room and the next room, I would put the RMH barrel against that. (built right into it even, though that has not been tested ) ...

Has been tested (burying the barrel in cob or earthen walls). Short answer is:
- Anything formal-looking (like plasters) tends to crack in the heat from the barrel. At least, use flexible fiberglass gasket or mineral-wool insulation around any hot metal embedded in masonry, or you will crack the masonry over time. They expand at different rates.

- Heat accumulation keeps the barrel from downdrafting inside like it is supposed to, reducing performance draft. That ties in with your longer questions about length and exhaust. The nominal lengths for horizonal are for the heat-exchange bench itself; for the exhaust chimney afterwards, a vertical chimney is your most reliable option, and there may be limits to how long it can be. We are already stretching the limits of what masonry heaters generally consider best practice, with exhaust in the 80-150 F range (depending on the builder; we aim for above 100 F on all of ours). Most heater masons try to stay above 200 F to avoid the worst-case dew-point of water. Higher dew points + damper wood in humid climates might necessitate a hotter exhaust. That means shorter bench, short/vertical chimney, and/or reheat the exhaust in the waste heat above/behind the barrel and then insulate the heck out of the chimney to conserve what little heat is left.

- and it's a right royal pain to dig out if anything goes wrong under the barrel.

As I learn more about the overall traditions of masonry heaters, I'm beginning to really appreciate the recommendation that all structural / load-bearing members be separate from the fiddly heat-exchange channels and their linings. At some point in the future, you or someone you trust may need to do structural repairs, or get in and reconfigure / repair / remodel the heat-exchange components, or just give the whole thing a good dissection and cleaning to suss out what might need fixin'. (remember these designs originate in Europe and China, where many buildings are expected to last for centuries, and families may inherit their ancestors' good and bad investments.)
If your fiddly heater bits and your massive structural bits are separate, ideally with room to slide a chisel in between, you have a managable project instead of a cat-in-the-hat, multiple do-over nightmare. You also have much better luck finding a qualified repairman for one or the other, since few people are experts in both.

The more often you pull apart someone else's work, the more you begin to appreciate little conventions like predictably-spaced stud framing, non-combustible hearth supports (why is there tarpaper under the brick hearth for this woodstove, where I was about to start building a high-temperature masonry firebox?), mortar that is softer than the bricks it supports, etc.
If you are doing a lot of improvisation in your home, and you want it at some point to be a comfortable place to retire rather than an ongoing project playground / do-over patch, I might recommend working in some improv-friendly, non-toxic material such as earthen masonry, and learning as much as you can about the material and the traditional detailing that makes it work well. Being able to dig back in and renovate something that didn't work out is a very good feature in a first-time building.

I also recommend the 'build your barn first' strategy - not because you should care more for the animals, but because your first large building project may only be fit for animals once it's done. You will learn a tremendous amount in the process - every building teaches you something. So if you can plan on a smaller-scale outbuilding or shed first to test the design concepts, you may be much happier with the final result. You'll also have a place to hang out when things get delayed on the big one. (The big one may also go quicker overall, since you will have solved a lot of problems ahead of time. And you may even have eliminated a few unworkable or unreasonably difficult ways to solve those problems, and found easier or quicker ones. So when you get to the big building you won't be wasting a lot of time correcting mistakes, or using a solution that's way harder than it needs to be over a very large area of a project.)
For earthships, you definitely want to try the whole compressing-earth-into-tires thing in real life before committing to a particular length of wall built in that method, for example. Too large a building might exceed your lifetime capacity for that sort of labor, if you are planning to do it with your own hands. If your dream really does call for that much space, you'll want a budget for the man-hours or equipment it's going to take to get it done, in time to still be alive to enjoy it.

Backyard Testing:
You can lay out a rocket mass heater in bare dirt and test-fire it. The length of pipe seems to matter more than the thermal mass for test-firing: thermal mass comes into play over hours, not minutes, as it soaks up or releases heat.
Lay out the length of bench + exhaust, with the pipes at the approximately correct heights, and see how it does.
If your exhaust won't go upward, but you are putting the heater in a tall building, you may need a lot of outside air supply or you could get bogged down with negative building pressure at ground level that keeps the stove from ever working right. Sometimes these pressure dynamics are solve-able in magical alternative ways; sometimes the stove just has to be shorter and hotter to push past the house draft dynamics.

You can then move the entire, proven design into the space, and test it again before cobbing it in.
This process could eliminate several unworkable designs, and save you a lot of effort cutting new holes in walls and roofs, or moving the (heavy and brittle masonry) firebox incrementally closer to a workable distance from the chimney.

-Erica W

If you are going to use tires for your wall, what I have learned by experiment is you can save time and effort by cutting one side out of the tire before filling and tamping it with dirt. If you put a piece of plastic sheeting in the bottom and turn the removed side wall over and put it on top the dirt will not leak out.

I developed thei method initialy because I wanted to step them back on a green house wall and plant in the exposed area.

Im very exited by your claims about sending exhaust out the wall, so I have been experimenting with different set ups.

As you can see, this stove is build outside, which gives the opportunity to have the duct run horizontal with almost no resistance, with no smoke or dust inside.

Riser is build with 1,5 cm thick refractory cement insulated with 5 cm vermiculite.
Effective riser hight 60 cm
6" riser and duct

The stove will burn nicely for some minutes, but then the fire creaps up the sticks. The chimney-guys have their easy answer

So whats wrong here?

hey ole....

that setup looks super nice.

what are the ratios of feed tube, burn tunnel, riser?

from the pictures it looks like it s not a J tube but a L tube. it looks like the tube ob the feed tube does not rise about the height/level of the top of the burn-tunnel. you have lots of wood sticking out of the feedtube. i think, it should be better INSIDE the feed tube.

if your fire runs nicely for a few minutes, then i don t think draft/chimney is the issue

The chimney is the downward-pointing elbow right next to the heat riser. As long as the inside of the building is warmer than outdoors, there will always be a small amount of draft, but there can never be a strong draft, and when the flames climb the sticks, they can overpower the proper draft.

The system needs a real chimney to function reliably. I don't think anything else is going to work, except perhaps a batch box (which wouldn't have the need for fighting sticks' reverse chimney effect), though a batch box is said to require a good chimney to function well.

Thank you Tobias
Riser is 60cm effective, 90cm from buttom of burnchamber to the top of the horizontal tubing.
Riser is square 6x6"
Feed is 5x6" and 9" deep
Tunnel is 5" between feed and riser
Now I added a picture from the recent build with a more clear J-tube.
Well, after a short time fire creeps up the sticks...

Glenn: Do you mean to say that this system would function if it was placed indoor?
I mean, somebody made functioning horizontal systems, didnt they?
Did you run horizontal systems?

Ole Blente wrote:Thank you Tobias
Riser is 60cm effective, 90cm from buttom of burnchamber to the top of the horizontal tubing.
Riser is square 6x6"
Feed is 5x6" and 9" deep
Tunnel is 5" between feed and riser
Now I added a picture from the recent build with a more clear J-tube.
Well, after a short time fire creeps up the sticks...

Glenn: Do you mean to say that this system would function if it was placed indoor?
I mean, somebody made functioning horizontal systems, didnt they?
Did you run horizontal systems?

Ole, from what I can see it would appear your burn going up the sticks could be mitigated by a vertical chimney end. The only other thing I see that may be causing some issues is your flat exhaust piping, maybe others can chime in but I am under the impression that it's desirable to have a slight uphill slope  to the exhaust ducting. I'm not sure if it's different due to your unique build or not. How does it work as far as the mass accepting and releasing heat?

From what i have read in the forums: (1) The heat riser can only push the exhaust so far in the horizontal pipe without an up draft from the chimney. The length of cold mass may be making a cold plug that it can not push through.  (2) the heat that accumulates above your feed tube acts as a chimney as in a Rumford fireplace.  Several experts have commented that more space between the feed tube and the first vertical surface is needed to prevent this draw back up the feed tube. (3) It appears that you are trying to push the exhaust from the riser directly into the horizontal run. The standard RMH has the riser above the level of the bench and then drop down to the horizontal run which prevents the system from running backwards.

My suggestions for the next rebuild: If your concern is keeping the feed tube out ot the weather put the horizontal run to the right.  Extend the masonry enclosure above the height of the bench which will probably be part way up in front of the window then draft down to the horizontal run. The outlet then can run upwards along side the down draft  to give it enough heat to draw through the horizontal run.

thank you for the photos and infos... how long is the horizontal pipe?

i don t see and bell/barrel. how is the setup with that? i think, if one wants a wall outlet (no chimney), then a strong pump-effect from the barrel around the heatriser is needed.

Tobias and Hans:
Im sure that a higher heatriser, which Hans surgest, would improve draft.
But how would a drum/bend down increase draft?
Im inclined to think, that the hot gasses look for a way upwards?

ole,

is your horizontal directly linked to the heatriser? i mean, without a bell/barrel like shown here:

https://richsoil.com/rocket-stove-mass-heater.jsp



it has something to do with expansion and shrinking of hot gasses. the hot gasses from the riser hit the surface of the barrel, shed parts of their energy there and then shrink. hot gasses from riser push them through the horizonatl pipe. something like that.

There is no bell here. Only free flow through the ducts. And at least what comes out during good burn the first few minutes, is slow flowing moist 40 degree celcius.

then you should consider adding a chimney. i think, just some flexible metal pipe taped to your exhaust might help for this moment. if you have pieces of pipe at hand, please give it a try. and check if you see any difference.

i m very positive that when it starts now like it is, then it will run much better with some chimney.

The main reason for the typical barrel is because the mass bench and ducts have to be lower than the top of the heat riser. The radiating barrel mitigates the harm of trying to make the flow go downwards. If the duct can be higher, there is less inherent resistance to flow.

I think the main issues are that the heat riser may not be tall enough for a really good draft effect (but it might be good enough), and the lack of a chimney never allows for an inherent draft driver. I would likewise advise trying a piece of stovepipe extending up, just to see what the difference is.

The reason the original rocket mass heaters work without any chimney is due to the unique geographical setting, with a constant breeze in one direction in a coastal valley, and the outlets always put on the downwind side. Most places in the world will not allow that to work reliably. A good strong J-tube can overcome neutral conditions, but not negative conditions.

Glenn: You bring in some valuable information about existing horizontal systems. Thank you!
Yes, you might be right, that the system could benefit from a higher riser. And yes, like also
Tobias proposes, I will try a chimney.

Glenn Herbert wrote:The reason the original rocket mass heaters work without any chimney is due to the unique geographical setting, with a constant breeze in one direction in a coastal valley, and the outlets always put on the downwind side. Most places in the world will not allow that to work reliably. A good strong J-tube can overcome neutral conditions, but not negative conditions.

Is this set in stone, or are there tweaks that can overcome that tendency?

For example, might a double-barrel Heat Riser have the power to reliably push the exhaust through a mass and out a horizontal exhaust port?

Might bell-type Mass be better able to extract the heat while retaining thrust in contrast to winding piping?

Is there a sort of 'exhaust closet' that could be constructed around the exhaust, with plenty of ventilation [semi-tight wattle, for example] but capable of buffering the impact of shifts in the wind? Maybe just a shrubbery windbreak?

Ole Blente wrote:Tobias and Hans:
Im sure that a higher heatriser, which Hans suggest, would improve draft.
But how would a drum/bend down increase draft?
Im inclined to think, that the hot gasses look for a way upwards?

The drum or down bend before the horizontal run helps in two ways. (1) it acts as a trap to prevent the cold air in the horizontal run from flowing into the heat rizer making the system run backwards. As you have been experiencing the feed tube when hot acts as a chimney pulling the cold air out of the bench.  (2) After the smoke stops burning in the heat rizer it starts to cool and becomes heavier and wants to fall. The down bend or barrel allows that to happen. therefore increasing the draft.

Observing your system it appears that you could dismantle the first part of your bench and put a P trap on the inside where the heat would be conserved in the room. A P trap in water plumbing dips down so that water traps air from flowing back out the entrance. What you want is for the P trap to go up so that hot air will trap the cold air from flowing back out the entrance. This would add the resistance of 4 elbows but if you go back to the beginning of this thread you will read that the pumping action of the heat rising and falling overcomes that resistance. If it works then it will become a part of your heat storage when covered.

For the exit, again this was covered earlier in the thread, You can put a T on the exit with an opening pointing down so cold air can drop out and an opening pointing up with a short length of pipe to help warm air rise and reduce the chance of air blowing back into the bench. Even so cold air will tend to circulate through your bench and cool it down when the fire is out. You could put gate valves on the intake and exhaust to prevent that.

i m thinking about the WHY one does not want to have a chimney.

in our case it s privacy. it s not allowed to have woodstoves in every place. one could think about concealing a RMH-chimney by building an accepted, legal kind of BBQ/smoker/pizza-oven outside and sneaking the RMH-chimney into the chimney of that structure.

aesthetics might be a reason. but you can find ways to make a chimney that actually looks good.

cost as a reason? probably yes, if you build a chimney from the best new parts available. but considering the whole building costs, i think that a normal chimney would fit into most budgets. and i m pretty sure that even a chimney from suboptimal salvaged parts is better than none at all.

Kurt: What is a dobble barrel heatriser?

Tobias:
- Cost is a point here. Chimney starts where your stove ends. Thats an extra, which doesnt give any heat bonus.
- Estetics yes.
- Without chimney you break with old stiffened ways of thinking.
- With a chimney you must manually stop the draft, when the stove cools down, to prevent loosing all gained warmth.
- Only a chimney can catch a chimneyfire. A horizontal piping will never be able to perform like a heatriser.

Hans:
"(1) it acts as a trap to prevent the cold air in the horizontal run from flowing into the heat rizer making the system run backwards."
Did you experience this, or is it a thought? From my experience the warmth from the heat riser gently pushes the colder gasses out the horizontal flue, keeping the right direction all the time.
"(2) After the smoke stops burning in the heat rizer it starts to cool and becomes heavier and wants to fall."
Yes there is a temperature fall from heatriser to the exit of drum. And yet: did you ever fire your system without drum? The gasses leave strait into the heavens. Apperently there is no such thing as "wanting to fall" in the drum. The gasses have to be pushed downwards, through it. The drum is an obstackle, creating resistence in the system, that has to be overcome by the force of the heatriser.
Thats the reason I have all tubing (exept the exit) horizontal, in order to have no resistance occuring from temperature variations.
I didnt experience flow back at any moment. Also wind is of no concern in this area.
The exit is pointed downward to avoid cold air to creap in and cool down the system, when not operated.

I definitly will try the system with some few meters of chimney, to see, if it will overcome the fire creaping up the feed. Allthough I have little hope with the soft wood.
Instead I suppose I will have to leave the J-tube for a L-tube, for which there might be enough draft.

The stove will burn nicely for some minutes, but then the fire creaps up the sticks. The chimney-guys have their easy answer  

So whats wrong here?  

This was your original question.

What i am giving as an answer is based on 70+ years of heating with wood and the observations that have been made in the past on this forum..

I believe what is wrong there is the feed tube gets hotter than the rizer after a few minutes and the fire wants to burn in the direction of cooler air down and hotter air up. Also as mentioned before the feed tube placement creates a rizer effect on the outside of the feed tube.

Yes an L tube feed would probably solve the problem but you loose the self feed advantage.  I would try to make a convertible feed that starts as an L untill the rizer gets very hot then can be switched to a J tube feed when the draft is strong enough to keep the feed tube and wood cool.

The gasses have to be pushed downwards, through it. The drum is an obstackle, creating resistence in the system, that has to be overcome by the force of the heatriser.
Thats the reason I have all tubing (exept the exit) horizontal, in order to have no resistance occuring from temperature variations.
I didnt experience flow back at any moment.

With the flames in the picture coming up the wood and around the wood draft has to be coming from somewhere. I don't see any other source but flow back through the bench. Yes the drum acts as an obstacle but that works both ways. It is also an obstacle to flow back. Yes the hot gasses from the rizer want to keep on going up but if you observe smoke it eventually cools and falls. That is why I recommended adding a rise just inside  the building; this would give a more forceful push down and out the horizontal and resist the flow back. I believe the theory of the RMH is that you want complete combustion with a powerful push but a slow flow so that the heat is mostly absorbed by the mass. Therefore resistance and push have to be balanced so that the system flows i the desired direction.

You could start with stacking the feed bricks as an L then when the system gets up to temperature restacking them as a J and see if it works as is.

Ole Blente wrote:Kurt: What is a dobble barrel heatriser?

It's a heat riser formed of two barrels welded end to end.

Kurt: Ok, ofcourse, dobble barrel A very smart thing. From a thermal point of wiev, the bigger the better. Like in ancient thermal baths, which were heated beneath the flors and through the walls. But still no experience showed me any power produces through the barrels. As Hans also confirms, the barrel is an obstackle. But with dobble barrel the gasses have more time to get rid of the heat and therefore can flow out beneath with the least resistance.

Hans: "...draft has to be coming from somewhere" Its difficult to get a good picture; but after 5-10 minutes burn, I see the draft working correctly beneath and flames building higher up the sticks starting to point upwards at the same time. So some air is getting sucked down and some just enters and then is hauled upwards without getting into the riser. This just worsens until the draft stops. It becomes an open fire on top of the feed. Air is sucked in from outside, not downwards through the hot riser.

Now I just tryed with a short chimney. It works better, so its obvious, that the J-feed needs more draft, than my system delivers.  Offcourse it would be sad to let go of the self feeding. But ericting a 5 meter high dobble insulated steel chimney, as would be required here, doesnt feel good. Maybe a fan would be a way out?


Im still wondering: Where are the people, who advocate J-feed and horizontal outlet? I really would like to see their systems running!

Ianto Evans, the original developer of the rocket mass heater, was able to use horizontal outlets in his heaters because of the unique situation where he lives and builds most of his houses. He advocates small cob cottages, which have few tall spaces to compete with the RMH draft, and his location gets constant breezes from one direction.