Getting my first build started - Rocket Mass Bench Heater on Casters

Got pretty cold this last weekend and had a rough time working in my 1,500 sf pole barn at 35 deg so I started on this project.  Idea is to put this in a corner or behind another workbench when not in use but roll out, attach a flexible tube or maybe a hard pipe and vent the flue gas out a window or a permanent spot in the wall somewhere.  After completion & a little testing, plan to make this into a bench - the seat maybe a bit high in which case I'll add a step, maybe with a hinge to keep it narrow.  I know it's a large workshop, but I'm hoping for just enough heat to knock the chill down a bit.

Not sure if the dimensions are going to work, so I'd like some thoughts on this before it's all semi permanent and thank you all in advance for all the prior knowledge sharing.

If the pictures don't show up, here are the details;

Feed tube; 4.5x3.5x9
Burn tunnel;  4.5 tall, 3.5 wide, 16 long
Heat riser; ~41.5 tall
Barrel; 43 tall
flue; 3 x 5' sections of 5" duct with 2 x 180 bends

FYI, I do have the 2007 Ianto book and we've watched hours of RMH videos from multiple builders, but haven't seen any smaller versions.

Welcome to Permies, and to rocket science!

What you show is essentially a 4" diameter J-tube system, which is at the extreme small end of what can work for a rocket mass heater. Most who are not experienced in RMH construction have difficulty making such a small system work.

I appreciate that you are using masonry for the feed and burn tunnel; given the small size of the system, you will probably not get the very high temperatures that quickly destroy metal in the riser and other parts of the core. What kind of firebrick are you using? Heavy dense ones, or soft light ones? Hard firebrick will pull heat out of a small system as fast as it is generated, and you may have difficulty getting a strong fire going. The core needs to be insulated as much as possible so the firebrick can build up heat.

A 4" J-tube is going to be difficult to reach in to clean ash. I think you need to add an access port at the base of the feed tube so you can reach in with a tool to rake the floor of the burn tunnel.

Considering the system size, the 2" gap between riser and barrel is probably sufficient. The 15' of duct is problematic, though. I don't think it will allow a strong enough draft to work well. I would advise switching to a "half-barrel bell" covered in cob or other mass. This will have essentially zero drag, and allow the best possible airflow.

Addressing your overall situation, I think you would be much better off, both in reliability of function and in warmth given, going to a 6" system. If you have some more firebricks, you can make a J-tube core that is not much larger externally than your 4" core. A 6" J-tube can be reachable for cleanout without special ports. For workshop heating, you probably want more instant radiator than storage mass.

A batch box core will give more heat with less frequent loading, but that is a considerably more technical build. I would do a J-tube and use it for a season or so before trying something more ambitious.

For the bell, if you want to use it as a bench, you will need some mass so the surface does not get too hot. Two or three inches of cob, or a layer of old red bricks or any other masonry, should take care of that, and even out the heat release without delaying it too much. A length of rectangular HVAC duct would work as the liner/base of the bell, ideally 12 to 16" wide by 8 to 12" high by whatever length you have.

To be able to keep the barrel you have, you would need a thinner insulation layer on the riser. Ideal but costing some money would be a "5-minute riser" made from a length of 8" stovepipe lined with 1" thick ceramic fiber blanket.

Hi Glenn & thanks for the replies and I have a few questions / clarifications

Regarding the size, I've read that smaller systems are difficult to make work, but what doesn't work?  Are they hard to get started, keeping them going or something else?

The flue piping will get covered with cob in the shape of a bench to sit on.

Glenn Herbert wrote:
What kind of firebrick are you using? Heavy dense ones, or soft light ones?

Not sure, how do I know?  Some say Butter on them and they seem fragile like they're falling apart.  Would this be soft ones?  I can weigh if that'd help.

Glenn Herbert wrote:
The core needs to be insulated as much as possible so the firebrick can build up heat.

Right, not sure how I forgot to mention, but I have yards and yards of nice orange clay and plenty of perlite for the riser insulation.  I also picked up a 15lb tub of fireclay for mortar on the bricks.  I did look into the ceramics and that looks very interesting, but for my first build, I'd like to go traditional and better understand the mechanics of these things.  Living near Houston, there are plenty of refractory places that I'll be visiting for scraps (hopefully) in the future.

Agreed on the batch box and sticking with something more simple.  If I bump the size of the feed, burn tunnel and heat riser up to 6", the flue pipe would be too small, correct?  What if instead of 1 5" duct I ran 2 5" ducts out of the barrel? I have 20' of this pipe sitting around at the moment.

I also have concerns about having two 180 turns in the flue, maybe two runs would help?  I know it's not as good as have a large diameter flue, but hopefully maybe?

In the end I feel like these heaters are for tinkerers like me, I just don't want to have a complete utter fail on my first go, but I have this uncontrollable urge to stubbornly do things a little different.  I will look into the half-barrel bell to see how that'd work.

Thanks again

Small systems might be trickier to start, that would depend on the details, but principally, you run into surface-to-volume effects. The smaller a system is, the more surface there is to create drag and take heat from the combustion stream, relative to the amount of heat generated. Volume is relative to the square of the surface, so combustion quantity is less and generates significantly less heat. At the same time a smaller system has less energy to flow, the surface drag increases. A smaller combustion volume will not be able to get as hot as a larger one because heat is bleeding out of it faster than it can build up, and you may not be able to get to complete combustion temperatures, leaving unburned gases in the exhaust which will condense to creosote.

You know what a regular red brick feels like, right? Hard firebricks feel distinctly heavier than that, and soft insulating firebrick feel considerably lighter. Insulating firebricks can be scratched with a fingernail, while hard bricks take work to cut with a masonry blade on a saw. Insulating firebrick would be ideal for a core, as long as you can be gentle with feeding wood. The feed tube probably wants to be lined with hard firebrick splits, or at least the top courses made of hard firebrick. You do need to mortar to seal all cracks so no cold air gets in where it is not supposed to. The fireclay mortar will work fine as long as you can keep the assembly from vibration or shocks and shifting. It is a sealant, not a cement or glue. A layer of straw-reinforced cob around the core will keep things in place even if you have IFB and additional insulation is not needed.

The 180 bends in a single 5" flue are definitely problematic. It is generally said that parallel flue paths are a bad idea, because they can never be identical and one will always end up with more flow than the other. However, short parallel flues may work fine in your case, as the friction in them will be much less than the friction in the connections before and after. Running three or even four 5' lengths of 5" duct will probably work fine, with a plenum at each end for gases to equalize. A few inches of cob over the assembly and you should get good results. You don't want more than a few inches of cover, as that (like 6" or so) would take hours to conduct heat to the outside and start warming your shop, and keep warming it far into the night after you leave.

A 6" firebrick J-tube can be tricky depending on your material. Insulating firebrick can easily be cut to whatever dimensions you like with a hacksaw, but hard firebricks are constrained by available sizes. If you cannot get a square cross section in the feed and burn tunnel, it is recommended that it is better to have it taller than wide. Standard firebrick dimensions will let you have 4 1/2", 5", or 7" height in various combinations of orientation. Using splits (4 1/2" x 9" x 1 1/4" thick) in the mix, you can get 5 3/4" high which would be close to perfect.

Thanks Glenn, I'll do some head scratching and see what I can come up with.  Got a nice scrapyard nearby so I might pay them a visit to see what materials they have laying around or an nearby HVAC boneyard.

I'll post up again when I start moving forward again