Rocket stove (J-tube) questions

I want to get around my list of materials for building a rocket stove mass heater, but I want to be sure my design is something that will work. This is my j-tube design, and I plan to bring my exhaust out the bottom of 55 gal drum ... ( running horizontal at the bottom ). My questions are.. Is the sizing of j-tube ok? I'm going to put the top of j-tube between 1 1/2"-2" from barrel.......does that seem like it will be ok? Sizing of exhaust ? Thinking of doing it in 6" but can do it in 8" since I haven't bought supplies yet. Going to put exhaust in L-shaped bench with exhaust leaving shop out roof only for better draft... Any red flags?

Danny Markley wrote:My questions are.. Is the sizing of j-tube ok? I'm going to put the top of j-tube between 1 1/2"-2" from barrel.......does that seem like it will be ok? Sizing of exhaust ? Thinking of doing it in 6" but can do it in 8" since I haven't bought supplies yet. Going to put exhaust in L-shaped bench with exhaust leaving shop out roof only for better draft... Any red flags?

Hi Danny, welcome to the forums. To me, the length of the riser is looking a bit short. My preference would be feed / tunnel / riser as 1 / 2 / 4, measured along the centerline of the J. In order to calculate the top gap between riser end and barrel take cross section area of your riser inside and devide that by the square's circumference. The result is the absolute minimum size of the top gap, in your case about 1.68". But since the barrel top probably will be buckling in and the gases need to go through a 180 degree turn, make it wider. Twice the minimum is already much better, avoiding a restriction by doing so.

The exhaust would be better in 8" since the cross section area of your J-tube is closer to an 8" size. And make sure the transition from the barrel to the bench is very spacy. Most of the time a possible restriction is in that area.

You wrote the RMH is for to heat a shop, right? Is that shop used 24/7 or just intermittendly? When the last one is the case, don't build a mass heater. Just bang another barrel on top of the first (without lenthening the riser) in such a way that in the inside an open cylinder is created. The exhaust still at the same place, just half the exhaust diameter above the floor. This will heat your place quickly without charging any mass. A nice warm mass is useless when you aren't staying in that shop overnight...

The length of my riser was determined by the height of 55 gal drums being 33-35". I'm not set on this measurement but would you suggest stacking a couple of layers of block above the tunnel(to set the barrel) in order to make riser higher? Otherwise my riser will be taller than the barrel placed on top of it.. I don't use the shop all the time, but I want to learn how to do this properly on my own shop because my sister wants one built up north in their cabins where they snowmobile for extended periods of time...I wonder if it would be possible to have two separate exits in the heat exchanger? Position the exhaust in such a way, that I could bypass the mass storage when not needed and just go out the double wall pipe thru roof ? Just a thought... I will use 8" pipe for exhaust like suggested. Probably going to research for a few month before I start so no hurry. Thanks for any opinions or help

To get the extra barrel height bricks can be used, or if you have an extra barrel you could make a 1/2 barrel manifold / base out of it, then band-clamp the full barrel on top with an appropriate seal (usually wood stove fiberglass rope gasket held in position with high-temp aluminum tape). For an example of how the 1/2 barrel manifold is done check out the cob rocket build photos on about the latter two thirds of this page:

https://permies.com/forums/posts/list/1000/26232

For a compact footprint cabin rocket mass heater check out these plans that will take all the guess work out of its construction, and it also comes with details of the 1/2 barrel manifold:

https://permies.com/t/40984/rocket-stoves/Rocket-Mass-Heater-Plans-Cabin

I wonder if it would be possible to have two separate exits in the heat exchanger? Position the exhaust in such a way, that I could bypass the mass storage when not needed and just go out the double wall pipe thru roof ?

This much I know: Some people build a gate (usually a slide gate or a rotating disk gate) at the exchange/entrance to the mass area to bypass the mass and go straight up the chimney. This is done for at least two reasons: 1.) To heat the exhaust chimney which aids in better combustion in the preliminary stages of the burn, and to increase the draught. Increasing the draught helps in reducing or eliminating 'smoke back' coming back up your feed chute, and it also helps to create the better ignition conditions to succeed from tinder to kindling to larger wood chunks as rapidly as possible. And 2.) A bypass is also built into the system, sometimes, for the purpose of quickly heating the air in a building instead of 'wasting' the heat to warm the mass, unnecessarily (if a person is only going to be inside for a shorter time and does not need the mass to slowly radiate over time). Just so you know: where this gate would be located is also a good place to have a clean out door, not only to check the gate function, but also to clean out fly ash and to check your burn efficiency (look for carbon[dry black] and/or creosote[gooey black] build up) and to ensure that your gate is functioning. FYI: It is a good idea to have another clean out door where you have your bends in your tunnel to clean out fly ash.

Thanks so much for any and all help during this process..I will probably get that PDF download for those plans. I don't want to reinvent the wheel but only do this project right and once....love learning ang teaching others

My heat riser is 46" tall with the usual 200 liter drum and the 8" exhaust pipe is passing under the barrel just to the side of the horizontal burn chamber.

Hi there, i am currently very interested to find a solution regarding bypassing the mass. I would like to hook on to DANNY MARKLEYS question, what experiences have been made at what exact point the bypass been installed.
I am not sure if i have understood it right, does the 8 inch Cabin RMH pdf from Wisners include details of bypass points, perhaps in the 1/2 Barrel manifold??

The Cabin-8 doesn't employ a thermal mass bypass. But it would be easy enough to add onto the design if desired.

Excuse the rather wordy description, but one way to construct a bypass is to use two 8" Tees, connected together to form an H.

I.e. the half barrel manifold is then connected to the lower left leg of the H, the top of that leg is connected to the thermal mass bench ducting, which runs down the length of the front of the thermal mass bench making a 180 degree turn at the far end, and then returns to and connects to the upper right hand leg of the H. The lower right hand leg of the H is connected to the "as per design drawing clean-out Tee, elbow, and ducting assembly" that leads up into the back of the thermal mass bench and on to the chimney.

Actual bypass valve: The horizontal portion of the H is where the bypass control is located, which can simply be a typical round wood stove exhaust damper; one that rotates 90 degrees to either close or open the bypass.

In actual construction of the Cabin-8, the H assembly is positioned horizontally, of course. Easier to visualize an upstanding H in the how it works description. Hope that helps:)

I'm currently sourcing materials for my first RMH build, and I have a few questions regarding type and number of bricks I will need. I'm planing on using bricks for the fire box and heat riser. There is a company in town that makes every kind of brick from fire brick to refractory brick to super duty refractory brick and I would appreciate any guidance on what the experts recommend. I will include a link to the company so you can look at all of the brick options. Any help would be appreciated
Thanks

https://ssfbs.com/hardFireBrick.php

https://ssfbs.com/insulatingFireBrick.php

After looking at Ernie and Erica's web page i was able to fine a materials list and it says 375 building bricks, 59 fire brick 40 half bricks and two pavers. Hope this will be helpful for others.

The basic grade of hard firebrick will be excellent for the feed tube/burn tunnel area. If you can get splits (typically 4 1/2" x 9" x 1 1/4" thick), they will give less mass to heat up at the start of a burn. The insulating firebrick (lowest grade will be fine) are ideal for the heat riser, and for insulating around the burn tunnel. The quantities will depend on your planned system size - 6" or 8" are most common. Ernie & Erica Wisner have complete plans for several systems for $15-$30. They have a new book coming out this summer:

http:www.ernieanderica.info


Didn't see your edit until I posted... glad you found the info