8” or 6” chimney for 8” system?

I am building my first RMH and am enjoying the process.  I wish it was done, but the journey is part of the fun, right?

Anyway, I’ve read Ianto’s book, still reading Erika & Ernie’s book, bought the 3D Plans - Pebble Style Rocket Mass Heater, and Matt Walker’s plans for the Superhot J Tube SuperHot J Eight Inch Core Cut Plan.

So my system will have an 8” core, 8” ducts through the mass.  My questions are:

Do I need an 8” chimney like it says I need in Ianto & E&E’s books?

Or do I only need a 6” chimney like it says in the 3D Pebble Style plans?

Why do you so answer?

I’m not sure if this has already been answered here on the forums elsewhere.  If so, please post the link. Otherwise, thanks in advance for your help on this.

Hi Matthew,

I recently asked the same question and Peter van den Berg replied that you shouldn't build larger than your flue size. That answer can also be found deep in the archives on the Experimenter's Corner site. Of course putting in a larger flue may be possible, albeit expensive. Good luck with your build.

Craig

I would say it depends on your draft conditions, for a J-tube system. I have constant positive draft in my house, and the 8" J-tube with bell works excellently with a 6" stovepipe and external chimney. A batch box as Peter works with requires more constant CSA. Of course, a 6" batch box delivers heat about as fast as an 8" J-tube...

I would say if you have a choice then definitely go for a system size chimney.

In good draft conditions, a system size chimney for a J-tube may allow too much draft and require significant damping at the feed to burn properly and not suck heat up too fast. I find I can cover 3/4 or more of my feed tube, and still get strong flow with my 6" chimney and 8" system.

Glenn Herbert wrote:In good draft conditions, a system size chimney for a J-tube may allow too much draft and require significant damping at the feed to burn properly and not suck heat up too fast. I find I can cover 3/4 or more of my feed tube, and still get strong flow with my 6" chimney and 8" system.

Thanks Glen!  

Would you mind telling me the length of your ducting in the mass and chimney length?  

Does the length of the mass ductwork affect the answer to my question?

Would a system with a longer bench benefit from a larger chimney?  

Will building my mockup in the front yard answer that question?

Thanks again everyone. I welcome more comments.

I don't have ducting in my mass, I have a masonry bell. The ISA is comparable to a 6" batch box recommendation.

My chimney is about 17' total height from burn tunnel floor or 16' from feed tube top. The 6" stovepipe starts at floor level, up 7', horizontal 4' through the wall, and up 10' insulated to above the roofline.

If you have a ducted mass, you might need a full size chimney; I don't have experience using that style. I am sure the length of ducting and number of bends would affect the draft. The pebble style RMH at Wheaton labs has a ducted mass and uses a 6" chimney.

If you build a mockup that includes the ducting and chimney layout your final build will have, it will probably give you a good idea of how it will  work. The draft conditions in your house are a variable that can't be addressed this way. If you currently have a woodburning stove, you may be able to have a good idea of this.

My same question -- 8" system exhausting into a 6" flue. I've got a basement (2900 sq/ft) with about 30' of 6" flue (bottom half tile flue liners and top half insulated SS flue pipe) straight out the roof. The flue is pretty much in the center of the basement so -- perfect. It seems that I saw somewhere that an 8" system can be safely reduced to a 6" flue due to various restrictions in the system, going around corners and such like, which effectively reduced the need for that 8" flue.

It's not the restrictions in the system that allow use of a 6" chimney on an 8" J-tube system, but the fact that an 8" core has a large volume of very hot gases running through it, which by the time they get to the chimney are much cooler and smaller volume so the 6" stack does not choke the flow more than the main part of the system does.

A chimney like yours sounds ideal for this use case, with the caveat that a basement location may be impacted by the whole-house stack effect where the house above heater level is trying to compete with the chimney for airflow. If you have a leaky upper part of your house, this could be an issue. A nice tight house will probably be fine with a setup like yours, especially if you had a woodstove in the basement and it worked with that chimney.

Glenn Herbert wrote:It's not the restrictions in the system that allow use of a 6" chimney on an 8" J-tube system, but the fact that an 8" core has a large volume of very hot gases running through it, which by the time they get to the chimney are much cooler and smaller volume so the 6" stack does not choke the flow more than the main part of the system does.

OK, that explains it.

A chimney like yours sounds ideal for this use case, with the caveat that a basement location may be impacted by the whole-house stack effect where the house above heater level is trying to compete with the chimney for airflow. If you have a leaky upper part of your house, this could be an issue. A nice tight house will probably be fine with a setup like yours, especially if you had a woodstove in the basement and it worked with that chimney.

When I built the house I included a mass heater with an 8" flue. It burned great!--but smoked both inside (poorly built, leaky) and outside the house and didn't really heat the space as I had imagined it would. Completely unsatisfactory! So, I took it out and installed the largest Hearthstone heater. it burns great AND heats the space. That same heater in the basement performed poorly -- I wondered about the draft?--that's a lot of chimney, so I installed a draft inducer fan and introduced outside air, neither of which really didn't make much difference in heater performance.

I'd like to get some off-grid heat in the basement, but don't want to invest a lot of time and energy installing a RMH that is going to perform poorly on that flue in the same way the Hearthstone did.

Here's the deconstructed mass heater with Hearthstone heater installed. I took the mass down to a level I could use for a hearth, retaining the two flues.

Glenn Herbert wrote:It's not the restrictions in the system that allow use of a 6" chimney on an 8" J-tube system, but the fact that an 8" core has a large volume of very hot gases running through it, which by the time they get to the chimney are much cooler and smaller volume so the 6" stack does not choke the flow more than the main part of the system does.

Aha!  That's the missing bit of information I needed also.  Upon further research, it's called "Charles Law.  I ran some numbers and came up with the following:

1800-degree gas drops to a quarter of its volume at 150 degrees

1500-degree gas drops to less than a third (29%) of it's volume at 150 degrees

1800-degree gas drops to less than a third (29%)of its volume at 200 degrees

The CSA of 8" pipe is 50 sq. in.
The CSA of 6" pipe is 28 sq. in.  

So the 6" pipe is approximately half the area of the 8, but the cooler gasses are reduced in volume by a factor of 3 or 4.  So the 6" pipe should be fine, right?

This is an interesting question, I have read many hundreds of post on this and other rocket stove sources that openly state the chimney must match the system size.
I can only assume (as I dont know) that the people (builders designers) have tested their conclusive advice but Glenn seems to think otherwise and has put his case forward with confidence and stated his reasoning behind his thoughts.

Without testing one alongside the other I dont see how one could really know?
I can obviously  see the benefits of using a smaller chimney for ease of use and cost.


Funny enough, in my case, my J tube is a 6” system with a 7” chimney but, that has also been openly discussed and using a larger chimney is said to be a good method….. however  in turn, I have not tested my stove with a system size 6” chimney!

Matthew Wagner wrote:So my system will have an 8” core, 8” ducts through the mass.  My questions are:

Do I need an 8” chimney like it says I need in Ianto & E&E’s books?

Or do I only need a 6” chimney like it says in the 3D Pebble Style plans?

Why do you so answer?

In the normal sense of a ducted bench (or bell), you'd need an 8" chimney.

Using an 8" j-tube system with pebble-style bench to exhaust to a 6" chimney has the best papers to work, in my opinion.

My probably overly long answer is down here.

In general: I'd recommend not building larger than the flue's capacity. I know that the gases are cooled down quite a bit before entering the chimney, although that won't be constant during the entire burn. Starting up a cold system would presenting a real challenge without heating the chimney first. And maybe you'd need to heat up the chimney prior to every burn, warm core or not.

There's something important to consider while contemplating the mass of a ducted bench.
While the mass is massive, i.e. cob/stones lasagna, bricks and mortar or even poored concrete, it might be difficult to get a cold mass going because it's extracting lots of heat from the gases over its entire length. In that case, a bypass would be desirable since the chimney could be heated first. The higher the chimney temp, the more draft is created despite a smaller vertical chimney.

While using a ducted bench filled with pebbles the situation is radically different although this isn't that obvious. All pebbles and gravel will touch the ducts sparingly, so not much of the conduction heat is extracted while the exhaust temp behind the barrel is low. With raisings temps the duct is getting (much) hotter in such a way the pebbles in the direct vicinity of the duct will be heated quite well and those will heat its brothers and sisters further away from the heat source.
Seems logical so far I'd say.

But the consequence of all this is that the bench' heat extraction will be quite low during the critical startup. So more of the heat is reaching the vertical chimney like this, ramping up the draft and thereby combustion rate. At some point, depending on fuel, wind conditions, rate of combustion and all that, the duct will get hot enough to get the heat extraction cycle going. By that time, the chimney temp will be reassuringly ramped up already. In fact, there's an automatic bypass incorporated in a ducted bench with a pebbles infill.

One example is the pebble-style 8" J-tube at Paul's place. At first, the suction fo the core turned out to be too much for the system, chimney temp did get far too hot so they scaled the feed tube down a bit in order to have a better balance between heat production and extraction. Now it's presented as the best drawing rocket mass heater of all. Another way to tame it down a bit would be lengthening of the duct inside the mass. Heat extraction would still be poorly at the start but that would be (probably even more than) compensated for later during the burn.

Now what I would say about a smaller chimney diameter as compared to system size: use a ducted bench, pebbles-style. Not a bell chamber or massive ducted bench, I'd fully expect this to be very, very reluctant to run properly, if at all. The pebbles-style  bench would have no problems in that it heats up the chimney *before* heating the bench, like a bypass would do.

I think this is a case where the draft of the cold system makes a critical difference. My chimney, even though it runs out through the wall 8' from the floor and up 8' from there, always draws positively and never needs to be preheated. I have a massive bell (sized about for a 6" batch box), and my 8" J-tube runs great from start to finish. I do always cover the feed 3/4 or more while the fire is going.

Glenn Herbert wrote:I think this is a case where the draft of the cold system makes a critical difference. My chimney, even though it runs out through the wall 8' from the floor and up 8' from there, always draws positively and never needs to be preheated. I have a massive bell (sized about for a 6" batch box), and my 8" J-tube runs great from start to finish. I do always cover the feed 3/4 or more while the fire is going.

Aahhh... There's the catch. Your bell is sized for a 6" batch box, as is your chimney. There's a sort of concensus that a batchrocket is potentially about twice as powerful as a J-tube of the same size. I got the impression you ran a complete 8" system on a 6" chimney but that isn't the case. So, this is a viable combination and could be adapted by making a much shorter duct run in the bench so there would be enough heat left in the chimney to drive the system.
Sounds reasonable to me.

As it seems an 8" J-tube has roughly equivalent power as a 6" batch box, I believe my system is properly sized for my J-tube. It could take a somewhat larger bell (or a second small bell as I plan to add), as the exhaust can get hot enough in hard running to make the stovepipe to the chimney uncomfortably hot - too hot to touch for more than a second or two.

I would be wary of using a 6" chimney on an 8" ducted solid mass J-tube, as that will have plenty of friction and the restriction may tip it over to poor draft. At the least I would shorten the duct run a bit from theoretical max. If you have an existing chimney and can verify that it has positive draft when stone cold, I think you would be okay with the larger system.

So the conclusion is ‘yes’ you can use a 6” chimney on an 8” J tube core but … only while using a large free flowing bell or similar open system.

So the answer is ‘yes’ but the question remains… would a 8” chimney work any better?

On my system, I don't believe an 8" chimney would be beneficial. I already cover the feed tube at least 3/4 while burning and still have fierce draft. Probably if I left the feed uncovered I would need an 8" chimney, but that would mean sucking vast quantities of extra air through the system, cooling the fire to no purpose.

Ok thanks Glenn, so I guess what I am trying to get at, is this something we should be recommending?

I ask because there is an awful lot of info out there, like books, videos and hundreds of forum post that state the opposite to your findings!
Not necessarily in an exact sense but as a general term …. a 8” system requires a 8” chimney.

As a group of enthusiasts I am sure that we are all keen to progress and learn by others experiences but, we also have a responsibility to offer the correct advice.

So Just to be clear…. With this new information, we can safely say a 8” J tube connected to a large bell ‘can’ employ a 6” chimney?
Or are we now saying, in fact a 8” J tube connected to a large ball ‘should’ employ a 6” chimney?

"It depends..."

I would say that if good draft is present (existing chimney) or can be expected from the situation, a 6" chimney would be preferred. If uncertain, use an 8" chimney.