Rocket Mass Heater Smackdown: J-Tube vs. Batch Box

Join us live this Saturday, October 3, at 10:00 a.m. Mountain Time for a Rocket Mass Heater Smackdown with Paul Wheaton and Matt from Walker Stoves!



What truly qualifies as a rocket mass heater? How do J-tube and batch-box designs compare? Paul and Matt will share their perspectives, discuss different approaches to rocket heater design, and answer questions from viewers.

Paul will also strap on the new “monkey cam” for a live tour of several rocket mass heaters at Wheaton Labs, including the batch boxes in the woodshop and classroom and the experimental “cob hat” heater in the Red Cabin.

Have a question for Paul or Matt? Leave it below for them to tackle during the livestream!

Very cool! I’d love to see and hear the two but am busy building a rocket mass heater for a costumer.
Will there be a video to watch afterwards?

Benjamin Dinkel wrote:Very cool! I’d love to see and hear the two but am busy building a rocket mass heater for a costumer.
Will there be a video to watch afterwards?

probably

Okay, I've got a question, but it's not so general, so I'll understand if you don't want to use it in the video.

Can a fireplace insert be converted to a rocket mass heater?

There is a building here that has what looks like a fireplace with brick chimney, but seems to be an insert of some sort that's supposed to be used like a regular chimney.. Nobody knows what's up with it, but it appears the real bricks used for the wall around it inside and the chimney itself are a facade to a metal (not cast iron) exhaust tube and enclosure for the wood pile area. It does not seem to have been intended to have glass to enclose the fire. It hasn't been used by anyone here. You can actually see the back of the brick wall inside as it seems the whole thing was built into the corner of a rom and you can see/get to the gap between the curve of the bricks and the corner of the room. You can also see the exhaust tube going up the brick chimney. It is doubtful this was ever a functional brick fireplace, and probably always had the metal insert.

The question is: could this be converted to a rocket mass heater? If so, what would be involved?

May,
There are generally two kinds of fireplace. The first is a site built masonry (brick or stone) unit with a brick chimney, hopefully lined with flue tiles. This type can has a stainless steel liner installed in it to allow a woodstove or fireplace insert to be installed in it. I have used such a setup to install a ricket heater and bench in the room. The rocket and its mass bench can be installed on the hearth if there is room or on the floor nearby if the floor can handle the heat and mass. The rocket is not installed in the fireplace, as that would limit the transmission of radiant heat.

The second kind of fireplace that is more common these days is a "zero clearance fireplace" it is an appliance designed to look like a brick fireplace but it has a metal chimney and framework (not masonry) and can't take an insert or woodstove or rocket heater.

I have installed rockets and woodstoves and inserts in fireplaces and using their own insulated stainless steel chimney. The performance (draw) of an insulated stainless steel chimney is vastly superior. It is possible if you have a zero clearance unit you could replace the whole thing with a stainless steel chimney and a rocket heater.

Whatever you do please do not alter anything without having the chimney inspected in person by someone who knows what they are doing. --Uncle Mud

We are embarking on a new build of a garage with living space above in Northeast Ohio. Is it possibly to put a rocket mass heater on the second floor?

Sandee,
Howdy neighbor. It is possible. Supporting the weight may require adding structural support or at least spreading the load across multiple beams and joists, and compromising on how much mass storage you build. My CottageRocket designs use less mass placed closer to the core so the mass is hotter so it stores more heat than a larger mass further down the exhaust stream. Insulating and sealing the space can also help a lot--Uncle Mud

Matt, I enjoyed your recent “What I Got Wrong” YouTube video, and I am delighted to see you and Paul on this very timely livestream!

After four years of war and resilience-building R&D, I finally caught the interest of Ukrainian army engineers. It’s your ceramic fiberboard core designs that convinced them. We needed a lightweight, portable/shippable, reproducible design that could be built quickly and easily by an average handyman. These engineers (I think half of Ukraine must be made up of engineers) are very concerned about carbon emissions. It seems like the batch box fiberboard design wins over J-tube.  Can you confirm and elaborate?

A quirky detour:
We try to repurpose war debris in all our Resilient Village Ukraine work. Any advice on recycling steel ammo cases, various sizes, to encase the fiberboard or build a manifold? I've been wanting to get in touch to make sure I'm on track here for plans to help train current combat troops (including some of our own American vets) and especially veterans to help them establish much-needed businesses.

We're facing another winter with massive attacks on our infrastructure, and probably a mass exodus of people out of Kyiv’s high rises and into the countryside as these daily attacks continue to ratchet up.

Cob & brick & lots of war-created urbanite is not an option at this point, but I plan to work my way back up to the full RMH system as we finally start developing off-grid “invincible communities” between Kyiv and the Chernobyl Exclusion Zone.

I look forward to diving in deeper with the Permies community over the next few months, no doubt with many more quirky Qs.

Thanks for all you guys do to conquer the world the right way.

Looking forward to this livestream. My question has to do with the height of the exhaust. I know with a traditional wood burning stove the top of the stovepipe should be higher than the peak of the roof, but I'm wondering if this is the same for a RMH since it burns so much cleaner, and if you 'kiss the barrel' with the stove pipe, would you still need the height to get a proper draw?
Thanks for your input.

We are live!

Tim, install a rocket exhaust to the same code specs as any other wood burning heaters. The rules are there to reduce sudden over the roof downdrafts as much as any other reason. Rockets can be more susceptible to downdrafts.

Great stuff! I think I've actually come around to the J-tube design due to this. I've never built or used or seen a rocket mass heater, but just from the looks - I like to see the fire!

But it seems that both Paul and Matt agree that the J-tube is so much simpler to build and operate that most beginners should probably start with that. As much as I'd like to imagine I will learn the ins and outs of running it correctly, what if friends or family visit? Concerns I hadn't thought about.

Edit:

Another thought I had that came up: Paul says the new "cob hat" design is amazing. I wonder how well a RMH would do with JUST the cabin style (mass inside barrel), just cob head, both combined, vs. the traditional bench design? I imagine if the default footprint for a RMH is just the barrel vs. the barrel and the bench, and it still gets like 80% of the mass effect (heh), that'd be a great trade off for many builds especially in smaller spaces?

I also immediately wondered if you could put the "mass head" INSIDE the barrel? What if you terminate the pipe a little short and put 2 layers of bricks/firebricks above it, but still within the barrel?

I have built J tubes that allow you to see the fire from three directions ……

James Fox,
Thank you for sharing your fantastic work. Your glass is so much better than doom scrolling my phone. I didnt think i was looking forward to it getting cold enough to light my rocket, but now I am a little bit. --Mud

M Pearson,

The Cob Hat is an evolution of the smaller hat on a rocket I built in a school bus in 2013.  My CottageRocket design I started using in 2016 has the mass inside the barrel(s) surrounding the core. As you guessed, we can store a lot more heat per mass where we can get the mass hotter. 200 to 400 pounds of mass reaching 600 degrees (f) or more in a CottageRocket core does about as well as a 2000 to 4000 lb mass bench that only reaches an average of about 130 degrees (f). It was designed to heat my daughter's treehouse without collapsing the wood floor (or setting it on fire) but we ended up using the first CottageRocket to heat our 1200 square foot doublewide trailer in Cleeveland Ohio for three years. If the house is big enough to hold a 5 ton mass bench it is probably too big to keep warm over night with a CottageRocket, but it works great for a cottage or bus or trailer. The Cob Hat combined with a small mass bench is a great combo in a Tinyhouse partly because a rocket heater with a fully exposed barrel will heat the room up before the mass is fully charged, so people stop feeding the fire early and wake up cold.    

Thanks, that's great. I found your CottageRocket playlist on Youtube and will be looking into it. My application of a RMH would be in a small cabin, so efficient space/weight use is very much appreciated. Also simplicity of build & operation as this would be my first RMH build.

Andrés Bernal wrote:

I would like to understand better why all-masonry works at all.  or how much of a difference it makes to have a metal radiant barrel top to shed heat rapidly, whether into a cob hat or to warm up your ceiling.  And suppose you had to have a “refractory“ surface to meet code, can you get away with having a thin layer of that and then an air space and then a cob hat above that?