RMH length of mass tubing?

Is there a length consideration for the function of a rocket mass heater tubing?  My concerns are:  

1.  Will it fail to draft properly if the length is too long?  

2.  Is there a diminishing return on length, meaning there will simply be a lower amount of heat in a larger mass?  

And a bonus question:  

3.  How often does a RMH need to be tended (adding wood) while burning to keep it going?    

Hi Christopher,
yes, the overall length, roughness (material) and the number of elbows will influence the draft.

1. Yes, that will happen. You want enough length to extract a lot of heat while leaving enough to have a draft. There's some calculations in the Wiesner's book. Also there's some rule of thumb lengths. But it depends on the size and type of heater you're planning

2. If I understand you right your question is whether there's a choice between a small and hot mass and a larger, not so hot mass? It mainly depends on your needs and site, as for example in warm climates a smaller, more reactive mass would be preferable

3. It depends on the size and type. A 6" J-Tube roughly every 20 mins I would say. But you often only have to run it for 1h or 2h. With a Batch Box it's liek a normal wood stove, depending again on the size.

Hi Christopher;
An 8" J-Tube can push 50' of pipe with deductions for any bends.
A 6" J-Tube can push 30' of pipe.
An 8" once heated up will burn for 60 minutes.
A 6" around 30 minutes.
A  Batchbox has a door and will easily have coals for 2-3 hours.

If you try to push it too far, your rocket will stall and smoke back into your home, (But at least it will not blow up on the launch pad...)

Almost all new builds have changed from a piped mass over to using Stratification chambers (Bells)
They are much lighter, easier to build, take up less space, and you never need to clean out ash-filled pipes.

Rocket Mass Heaters in an insulated home are only lit a few times a day or less depending on weather conditions.
An RMH in a larger area can be burned back to back all day if wanted, I call these, Extreme Burning Batchboxes.
I run two of my four Batchboxes as extreme burners, lit off in the morning they burn all day until evening and then are allowed to go out for the night.
Extreme burning RMH cores will require more maintenance, as bricks move or mortar cracks, I expect to do partial rebuilds every other year or so on them.

thomas rubino wrote:An RMH in a larger area can be burned back to back all day if wanted, I call these, Extreme Burning Batchboxes.
I run two of my four Batchboxes as extreme burners, lit off in the morning they burn all day until evening and then are allowed to go out for the night.
Extreme burning RMH cores will require more maintenance, as bricks move or mortar cracks, I expect to do partial rebuilds every other year or so on them.

Any tips to share on extreme burning a J-tube? With 20' of horizontal run on an 8", I end up running 1/4 to 1/2 loads to keep the barrel giving off comfortable heat. With minimal insulation, temps drop pretty quickly without a fire in the box. The mass is enough to keep the air above freezing overnight when it drops to -25*F out, but I should have added another 10' worth to it.

Am I likely to be getting a full burn if only running three or four wrist sized pieces per load? Exhaust outside is visible, but turns clear within 10' (usually <20*F) so I'm hopeful that means it is moisture freezing and evaporating. Yet on windy days I occasionally get a whiff of smoke, maybe it is just the abandoned box coaling out?

I have an oven thermometer hanging on the rim of the barrel that I usually keep between 100 and 150. It will go over 200 at full burn while I'm doing a stir-fry.

The thermometer on the exhaust ususally runs higher than I like with the short run, hence thinking partial burns are more efficient for extraction in the overall picture. But I don't know how to tell for sure how often the riser temp is reaching complete burn. There are definite signs of creosote in the exhaust stack, though I haven't taken it apart to see how much. This would be building up any time I let the box coal out, no?

Hi Coydon;
With an 8" J-Tube, you can push through 50' of pipe minus deductions for turns.
So you can definitely add extra length next summer.

In your case, I would wrap the barrel with an insulator (Super wool , rock wool, cob...) only on the sides.
Sounds like you cook  on the barrel top, but when done cooking for the day, consider placing a large stone or other easily handled chunk of mass.

Get your mass truly heated well, and you might be able to hold 40F -50F overnight (we hope)

Our 8"  J-Tube could reach 1100F on the barrel top if I pushed it. Normal barrel top temps were 500F-800F.
I did have 2/3rd of the barrel covered in cob, with only the top ring and barrel top exposed.

Coydon Wallham wrote:There are definite signs of creosote in the exhaust stack, though I haven't taken it apart to see how much. This would be building up any time I let the box coal out, no?

Creosote will build up in the chimney during startup and less so during the top of the burn, assuming the thing is running well. During coal-out however, there aren't any tars formed which as a consequence, can't build up in your chimney. All carbons during that latter phase are released as CO. In free air, this decays to CO² and water very quickly, plus... some heat.

Peter van den Berg wrote:
Creosote will build up in the chimney during startup and less so during the top of the burn, assuming the thing is running well. During coal-out however, there aren't any tars formed which as a consequence, can't build up in your chimney. All carbons during that latter phase are released as CO. In free air, this decays to CO² and water very quickly, plus... some heat.

Hmm, I'd come to think that the excess creosote from my old conventional wood burner was from damping it down to extend the burn as long as possible before going to bed, and associated that phase with a long coal-out. I guess that was usually right after a refill of the box, so somewhere between startup and the top of the burn, it having coals from the last batch? Or is the creosote in that case simply more of a product of limited air flow?

thomas rubino wrote:
An 8" J-Tube can push 50' of pipe with deductions for any bends.
A 6" J-Tube can push 30' of pipe.
An 8" once heated up will burn for 60 minutes.
A 6" around 30 minutes.
A  Batchbox has a door and will easily have coals for 2-3 hours.

If you try to push it too far, your rocket will stall and smoke back into your home, (But at least it will not blow up on the launch pad...)

Almost all new builds have changed from a piped mass over to using Stratification chambers (Bells)
They are much lighter, easier to build, take up less space, and you never need to clean out ash-filled pipes.
...........

A quick related question that arose from reading the last line about not needing to clean out ash-filled pipes (if using stratification chamber)....

Correct me if I missed a design fundamental in previous documents, but it appears that most builds have the exhaust piping/stratification chamber at about the same elevation as the burn chamber, even as the two are separated by the riser column and chamber/barrel.  The ratio of an 8" ID system pushing through 50' of pipe *with deductions for bends* would need to be modified further if the piping/stratification chamber is positioned, say, at 8" lower elevation than that of the burn chamber, correct?  Because one would be asking the system to push  heated air/exhaust DOWNWARD even further before turning 90 degrees (angle) to follow the horizontal exhaust run.  My interest here would be possibly having a walk-way with stone/brick mass under which a stratification chamber would be running instead of a raised bench.  Doable or just nuts?

thomas rubino wrote:Hi Coydon;
With an 8" J-Tube, you can push through 50' of pipe minus deductions for turns.
So you can definitely add extra length next summer.

In your case, I would wrap the barrel with an insulator (Super wool , rock wool, cob...) only on the sides.
Sounds like you cook  on the barrel top, but when done cooking for the day, consider placing a large stone or other easily handled chunk of mass.

Get your mass truly heated well, and you might be able to hold 40F -50F overnight (we hope)

Our 8"  J-Tube could reach 1100F on the barrel top if I pushed it. Normal barrel top temps were 500F-800F.
I did have 2/3rd of the barrel covered in cob, with only the top ring and barrel top exposed.

With the Yurpi so thin on insulation and light on mass, the run was kept short to prevent a cold plug. Given it was also my only shelter with practical heat source for an entire winter, the fudge factor was exaggerated. Since it has never hinted at a smoke back, I'll try the extension, but only after a second shelter is up and another dragon hatched for the Weyr.

I don't think wrapping the barrel would help me out. I am immediately cold when the fire starts to coal out. The mass stays above 70* until morning on the coldest of nights, but can't be felt in the air temp outside the covers of the bed over it. It is the barrel heat that keeps things livable, particularly during this cold snap where it was below 0*F for days. Once the room is warm, I do place pending firewood around the barrel to preheat and evaporate surface moisture, and have buckets of water on top, although that is usually for melting snow to replenish water supplies.

When we have had typical Montana winter temps here, (teens and twenties F),  the existing mass has held the room temp overnight above 55 fairly reliably, providing the heater was run the entire previous day.

Hey John;
My studio dragon does precisely what you are asking about.
Although it is now a Batchbox rather than a J-Tube, the floor or the box is apx 18 " higher than the pipes in my transition area.
As a J-Tube, it was less than 12".
I do plan to upgrade that entire system, hopefully this summer.
Removing the entire solid mass and building a double-skin stratification bell in its place.

Coydon Wallham wrote:Hmm, I'd come to think that the excess creosote from my old conventional wood burner was from damping it down to extend the burn as long as possible before going to bed, and associated that phase with a long coal-out. I guess that was usually right after a refill of the box, so somewhere between startup and the top of the burn, it having coals from the last batch? Or is the creosote in that case simply more of a product of limited air flow?

My definition of coaling out in a masonry heater is when all the fuel is turned into smokeless charcoal, at the end of the burn. So that last question is the correct one, limited air flow. With other words: starving the fire of oxygen, so the stove is turned into a sick smolder generating lots of smoke that is condensing in the flue, waiting to be ignited into a roaring chimney fire. Mass heaters are run differently, with hot and booming fires, at least I hope so.

But I'd think you knew this already, don't you? Please, say yes!

thomas rubino wrote:Hey John;
My studio dragon does precisely what you are asking about.
Although it is now a Batchbox rather than a J-Tube, the floor or the box is apx 18 " higher than the pipes in my transition area.
As a J-Tube, it was less than 12".
I do plan to upgrade that entire system, hopefully this summer.
Removing the entire solid mass and building a double-skin stratification bell in its place.

Wow....very encouraging, Thomas, as I envision my new build!  And yes, I'm more inclined to batch box already, so a batch box + stratification chamber is starting to come into view for my plans.  Thanks!

Peter van den Berg wrote:My definition of coaling out in a masonry heater is when all the fuel is turned into smokeless charcoal, at the end of the burn. So that last question is the correct one, limited air flow. With other words: starving the fire of oxygen, so the stove is turned into a sick smolder generating lots of smoke that is condensing in the flue, waiting to be ignited into a roaring chimney fire. Mass heaters are run differently, with hot and booming fires, at least I hope so.

But I'd think you knew this already, don't you? Please, say yes!

Yes!

I know that a low level burn will inevitably produce many particulates in the exhaust, just trying to understand what happens around the fringes of the extremely good or poor burns.

Rather than introduce more confusion to this thread, I've tried to clarify my question over here

Rebumping this thread with a question.  Someone else mentioned the idea of a stratification chamber essentially squeezed next to a wall, assuming proper fire safeguards.  I was wondering about something similar in which a strat-chamber, roughly 4X8 feet face dimension by 18" thick (standing on the long 8' edge to create a rectangular box) when covered in heat retaining mass (brick) would be feasible. The models below address intake of gases from the combustion chamber/bell, entry into the strat-chamber at a low point near ground level, and distribution and exhaust examples, each a bit different from the other.  The middle example seems to be a more common concept that maximizes heat build-up in the strat chamber whereas the top example would seem to be less effecient, although less prone to stalling.  The bottom example incorporates some baffles to snake the gasses a bit more before arriving at the exhaust.  Please note that i would plan to have a 'priming' T-connector for the elbow at the base of the chimney allowing for paper fires to heat up the flue.  Please let me know your thoughts and how to extrapolate from the "length of mass-embedded pipe" already noted in this thread (related to internal surface area??).  If there is a "been there--done that" example already here on the forum, please direct me to that if possible. Thanks in advance!

Hi John;
The center choice is the proper choice.
The strat chamber is intended to be simple.
The hottest air always goes to the top, displacing the previously hottest air. The coolest air, 140-200F, goes to the bottom, locates your chimney, and rises up and out.
Strat sizes are measured from the walls and ceiling. The floor does not count.
A 4" bypass at the top of the bell will send enough hot air directly up and out the chimney, creating a good draw to prevent stalling.

ISA numbers for batchboxes are listed on Peter's website https://batchrocket.eu/en/building

thomas rubino wrote:.....
A 4" bypass at the top of the bell will send enough hot air directly up and out the chimney, creating a good draw to prevent stalling.

ISA numbers for batchboxes are listed on Peter's website https://batchrocket.eu/en/building

Thanks, Thomas!  That certainly makes the construction easier, not to have the baffled version.  For the 4" by-pass, do you know if they make those stove-pipe dampers in a 4"  like they do for the 6" and 8" stove pipe?  That would make it 'adjustable flow' in case one didn't want to lose more heat than necessary out that by-pass port.  Thanks again.....

You don't want a typical stovepipe damper, as those are designed by code to allow a small flow even when fully closed (to avoid CO backing up at the tail end of a fire). What you want is known as a "blast gate" or slide damper. It totally seals when closed. It should only be open when trying to get draft going, so I see no reason to have it partially open.

Hi John;
Yes, they do make a standard 4" stove pipe damper.
Glenn is correct in that most folks use a 4" sliding blast gate.
I have used both. My studio and shop stoves have a blast gate, but on Shorty, I used the SW damper design.
I like the damper style better than the blast gate.

You can use a traditional stove damper with a bit of modern technology.
A 4" circle of Morgan superwool can be wired onto the damper once it is installed in a pipe, this blocks the cast-in holes designed to avoid Carbon monoxide poisoning.
Small squares of SW are used as the control rod is inserted to seal where the rod goes through the wall of the pipe.
The control rod is marked on the outside so the superwool side is always turned towards the bell.