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Length of flue

 
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Hi everyone
I would like to connect a jtube with a steel barrel to the flue. How long can the flue be before it exits the house? I mean is there a max length for an 8 inch system? Does it effect the draw effectiveness of the stove?
 
pollinator
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An 8" system will drive a decent length of flue. Are you talking about the run through the mass or the exit length?
 
Robert Grantham
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Hi Phil
I dont really have time to do the mass at this stage so im planning to run a flue for now to get as much heat into the house as possible. I will probably wait until summer next year to do the mass

Maybe a 6" system would be sufficient for Our place. Its Just going to be used when we are in the house anyway and our place is well insulated

Yeah im Just talking about the portion of flue in the house. Is 4 meters of flue going through the house enough to extract most of the heat from a 6 inch system?
 
Phil Stevens
pollinator
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Ok, that makes sense. Suggestions: go with 6" if the house is well insulated, and consider a stratification chamber or bell made from half barrels. You can pile dirt or gravel over the barrel sections and have some thermal mass that is easy to remove when you want to build something permanent.

An RMH without the mass will send a lot of heat up the chimney and it seems a pity to waste that if you can grab it without too much effort.

Like this

and this

and very much this Drops you into page 4 of Peter's incredibly well documented build, showing the bench layout.
 
Robert Grantham
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Thanks Phil i was looking to make something similar to a liberator.. Metal outer Shell ceramic board riser and burn tube encased by steel in a jtube Type design  i still havnt figured out exactly how to do this though. Im looking for as many ideas as possible.

As I said i dont want to be messing about with mud at this time of year.. Its already cold and wet here. So ill lay out the pipe and maybe get some gravel/pebbles for around the stove pipe

 
rocket scientist
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Robert;
Trying to run uncovered pipes without a mass over them is not going to work as well as you might want.
You will get condensation and a very poor draw.
Those uncovered pipes will shed heat so fast there may not be enough to create a steady draw out the chimney.
Cover with pebbles and it will help but they are a very poor substitute for a real mass or a simple bell.
 
Robert Grantham
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Yeah i think i will go the Bell route. Ive plenty of bricks on hand. How big would you go for a 6" system? I was thinking to vent from the Oil drum into the Bell close to the top via a stove pipe.

The only thing putting me off is knowing how to size the Bell. I remember Reading that for a 6" you would need at least 6 meters of stove pipe to extract the heat via the mass. Im not sure what the eqivalent of this is for the Bell.

Also im not 100% sure the Best way of venting from the Bell to the outside.. Should the pipe be kept as short as possible?
 
Rocket Scientist
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This page will give information for sizing a bell. It is intended for batch boxes, but can be converted to J-tube for an approximately 3/4 sized system, so a 4 1/2" batch box might use the same bell as a 6" J-tube. My experience with an 8" J-tube fits the math for a 6" bell. Mine also works fine with a 6" stovepipe and chimney; you don't need an 8" chimney for an 8" J-tube as long as you have decent natural draft.


The horizontal flue from the bottom of a bell should ideally be fairly short. Mine goes out at the base, up 7', and out the wall about 5' to the insulated chimney pipe.
 
Robert Grantham
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Hi Glenn

Thanks for the link.. Im having a job trying to calculate the ISA of the Bell based On a 4.5" system. It gives the value of a 5" at 3.7m2.. Is it ok to use this figure or do you think it should be slightly smaller? Will this make a difference?

The other issue im having at the moment is trying to imagine how the flue will exit my steel drum. Im planning to build a walker j tube 6" system and use bricks to surround the burn tunnel up to the base of the riser. On top of this ill use a steel barrel as a radiator...

My question is how to i vent from the system to the Bell? Is it ok to come out from the bottom of the barrel? I cant imagine there will be enough space for a 6" stove pipe to fit in the barrel next to the riser.

Also does the exit pipe from the barrel have to be level with the entry into the top of the Bell? Or can this be done with elbows? Im imagining the Bell will sit higher so the stove pipe will have to curve upwards.

 
Glenn Herbert
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The recommended way to handle airflow from barrel to mass is to have a hollow space around the base of the barrel and the burn tunnel, giving as much room as possible near the exit below the barrel. This transition from barrel space to exit is called the manifold.

For a bell that is separate from the barrel, you would go straight from the base of the barrel space to the base of the bell, not into the top of the bell. Arrange the inlet and outlet of the bell so that incoming gases do not point straight at the exit, and hot gases will naturally rise and cooler gases fall to the exit. Sometimes a short partition to divert the inlet upwards is used.

Another common method is to simply build the bell around the heat riser and dispense with the barrel. If you do this, you would benefit from installing a metal access panel in the bell for maintenance of the riser, and instant heating. A fault of some small bell RMHs has been the absence of a fast radiation source, thus making the users keep feeding it for hours until the space warms and then having the space overheat because there was too much heat stored in the mass.
 
thomas rubino
rocket scientist
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Robert;
Here are some links showing my first RMH and its transition area being built.
https://permies.com/t/99389/build#819705
https://permies.com/t/74035/Draw-strong#615801

This is the best one
https://permies.com/t/61657/Flue-exhaust-transition-plenum-pictures
 
Robert Grantham
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Glenn Herbert wrote:

Another common method is to simply build the bell around the heat riser and dispense with the barrel. If you do this, you would benefit from installing a metal access panel in the bell for maintenance of the riser, and instant heating. A fault of some small bell RMHs has been the absence of a fast radiation source, thus making the users keep feeding it for hours until the space warms and then having the space overheat because there was too much heat stored in the mass.



Yes thank you i was thinking this. My other thought as you said would be to brick all the way around the jtube.. For quicker radiated heat into the room i thought to add a steel or cast Iron plate to the top of the brick Bell.

I thought this then might be Able to be used as a cooktop.. However considering the distance from the top of the riser it May not be hot enough
 
Robert Grantham
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thomas rubino wrote:Robert;
Here are some links showing my first RMH and its transition area being built.
https://permies.com/t/99389/build#819705
https://permies.com/t/74035/Draw-strong#615801

This is the best one
https://permies.com/t/61657/Flue-exhaust-transition-plenum-pictures



Thanks Thomas im considering bricking up over the riser and including the jtube in the Bell im a Total noob and yeah this idea Just seems more doable for me
 
Glenn Herbert
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Experienced people say that a steel plate over the riser tends to warp, and thus leak around the edges. Cast iron is more stable.

The efficacy of a metal cooktop would depend on the distance above the riser. To be at a usable height, it would probably end up only a few inches above and thus be okay.
 
Robert Grantham
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Glenn Herbert wrote:This page will give information for sizing a bell. It is intended for batch boxes, but can be converted to J-tube for an approximately 3/4 sized system, so a 4 1/2" batch box might use the same bell as a 6" J-tube. My experience with an 8" J-tube fits the math for a 6" bell



So Just to clarify Glenn do you mean that an 8" jtube should have the same size as a 6" batch box? So for my 6" it should be the same as a 4.5"? Do you think i could go with the measurement given for the 5" batch?

Ive calculated the inner surface area to be 5m2 which would be roughly 1m2 per wall including the top which im planning to make from metal. Does this make sense or have i miscalculated? It Just seems quite a small Bell to me.

Also as a seperate question ( and im sorry to be constantly picking your brain) is the Bell is ok to be made from standard bricks? Can i use normal mortar or is refractory motar needed?

Many thanks

Rob
 
pollinator
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Hi Rob, I have a few different styles of rocket stove on my youtube channel, some more basic than others but this style might suit your requirements ?
 
Robert Grantham
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Fox James wrote:Hi Rob, I have a few different styles of rocket stove on my youtube channel, some more basic than others but this style might suit your requirements ?



Hi James

Is this a batch box design you have made?
 
Fox James
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No it is a J tube with a glass front and a few mods but I though the  design and small mass might interest you!
It started life as a standard J tube, I have other videos showing it in a more basic J tube configuration.  
The wood is just dropped in the top as required rather than loading up a big batch.
 
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