Design for a J-tube Rocket Cooktop with Bypass to a Stratification Chamber

Hello all, this month I'm moving to Wheaton Labs to start building my own wofati. Since it will be a fairly small space, building both a rocket cooktop and RMH seems like overkill. So I was thinking about a rocket cooktop which has the option to exhaust directly up the chimney when it's warm out, or into a bench to charge a thermal mass when it's needed. I'm aware of Matt Walker's Tiny Cook Stove design but I feel a lot more comfortable with the idea of a J tube rather than the batch box/double shoebox designs... the more fool-proof I can make my first complete build, the less foolish I think I'll feel once I start cooking and heating with it! I have a small roll of Superwool that I was going to use for a 5 minute riser with a normal barrel RMH setup, as well as 2 barrels, a decent number of dense firebricks and splits, and I talked to Paul about buying some of his insulated firebricks.

Being taller I find a 40" work surface is better ergonomically for me than the typical 34-36". If I make a 6" system I expect I need that full 28"sq CSA going underneath the cooktop, but I'm not sure if a 4"x7" channel is pushing my luck. That would allow a 36" riser, 12" feed, and 18" burn tunnel if I follow Erica and Ernie's 1-1.5-3+ ratio for the J.  With the burn tunnel of 18", I could pprobably recess the feed into the front a bit, the way Uncle Mud does with his Cottage Rocket design. Anything sticking out is just more opportunity for me to stub my toes, or having to lean over it as I cook. I could also bury the burn tunnel a bit below floor level to get a higher riser/feed ratio.

Once the exhaust has run the U under the glass and starts to go back down, I'm guessing is where the bypass would be inserted. I could probably save myself a lot of time trying to figure it out by buying Matt's plans if it includes that portion of it, I will have to check and see. If anyone has had experience with building a bypass I'd love to see any pictures or suggestions you have. Perhaps an L-shaped bracket, which has a hole cut into each face but offset would work? You would slide it to one side and the hole that goes directly to the exhaust pipe is exposed from the back of the cooktop where the U ends and starts to drop down say a brick or two, so you keep the hottest gases at the cook surface. Then sliding the bypass the other way closes that hole and opens the other hole which continues down into the bench, which then feeds back into the exhaust?

I was thinking about a mass as an L-shaped couch in the corner, built with adobe/bricks and finished with cob as a stratification chamber/bell as I've heard that makes for a more evenly warm top, compared to trying to position the ducting pipe at the right heights. I haven't looked it up yet but I believe there is an internal surface area not including the bottom that you want to match to the size of the system. I could make this an 8" system for more mass but that makes fitting that CSA between the cooktop and riser tougher, and having that extra heat hitting the glass feels like asking for thermal shock issues.

Of course there is also the original plan of making both a 6" RMH in the corner, as well as a rocket cooktop using a batch box design. But since I want to have an enclosed front porch area where the chimney exits the house, having two chimneys is double the cost and hassle. So I'm hoping a combo setup will do the trick, if I can figure out the bypass and ratios. From everything I've seen though, a batch box feed certainly seems to reduce the need for a riser, and comparing to Allerton Abbey that cooktop could be built much taller and perhaps tall enough to avoid a competing chimney with the feed, and I'm just being too concerned about the 1-1.5-3 ratio.

Any comments/suggestions are greatly appreciated. I'll have a couple months to test stack bricks as I'm building to get an idea about layouts if anyone has experience with it.

I think you could have a step-down area next to your J-tube feed for tending the fire, and the main cooktop access at right angles to that for cooking convenience. I built my RMH between what was to be dining room and 8" sunken living room, one 4" step down, and as plans simplified, it became a sunken feed area big enough for comfortable foot access but out of traffic flow. It has become second nature for me to step in for fire tending.

A bypass doesn't need to be complex. A simple opening with slide damper to seal it except when needed will serve fine; you don't need to prevent gases from taking the long route through the mass. I would venture to guess that even in summer it will not be bad to have the mass warmed some, especially if it is connected to the ATI part of the mass. You might even have an alternate or additional gas path deep in the mass for charging it for winter instead of daily heating.

Thanks Glenn, the RMH will be near the front door and a step down big enough for my size 16 feet might be too much in the way for that high traffic area, but sinking the burn tunnel a bit below the floor level is certainly an option.

I had looked at some dampers and was mostly seeing metal ones, and wasn't sure about how to insulate them where they could still operate. I suppose a metal damper near the base of the exhaust would pre-heat it a bit to help draft, I was figuring the gases might still be very hot and there would be a bit of lost heat there. I can try it and find out! I suppose there wouldn't be a need for a second damper into the mass, if the first to the exhaust was already open most gases would probably take that route out.

As far as charging the ATI mass, I was reading 'Passive Annual Heat Storage' which goes into details about the air tubes, and those really are key to charging the mass over time. I'm not sure if those pipes are allowed in a wofati being plastic pipes (I can't recall if Paul and I talked about it in one of the podcasts), but it's definitely a big part of the design including getting fresh air that has the temperatures moderated by the mass. Cracking a window or designing a drafty house when it's -10F outside doesn't sound very pleasant! Of course the mass will also charge from interior air being higher temp than the mass too. I was planning on leaving a 2" air gap between the bench and the wall to avoid any chances of the wood walls getting too hot, but the floor of the stratification chamber isn't insulated, so that would absorb heat every time it's fired up and not bypassing.