Building a 7" J tube steel, air-cooled RMH-style core in a steel bell

I hope to detail here my construction of a 7" steel square tube, AIR-COOLED RMH in a steel bell surrounded by sand within a sheet metal shell.

Here are the materials I've gathered so far shown in the pic below:
- a heating oil tank for the bell which measures 24" x 47" x 60 ". (It will stand on end thus 60" tall.
- some 7" square tube which is 3/16" thick. The burn tunnel piece is 36" long but the centre to centre distance (feed tube to riser) dimension will be 24".  The feed tube will be 17" long so the end to centre of burn tube dimension will be 20.5" and it will thus accept 24" fuel without protruding out of the burn tube. The total heat riser height (above the burn tube) will be 60.5" so the end to centre of burn tube dimension will be 64".  (I found that a heat riser that is as long as possible really helps to boost the draft.)  So my feed tube to burn tunnel to heat riser ratio will be roughly 1: 1.14 : 3.
-the barrel on top is the one I used on my RMH last winter. It will not be high as shown but will be cut a bit shorter.

Hope to post pics along the way as I build it as well as track all my expenses.

Suggestions and questions will be welcomed along the way as I've never built a bell system before!

Started cutting this weekend. Here is the opening for the barrel in the top of the tank and the opening for the exit flue at the far end at the bottom. Will be cutting steel for the core this week.

Here's a couple more pics as I get opportunity to move along with my 7" J-tube RMH.

The first shows the holes cut in the burn tube for the feed tube and base of the heat riser.

The second shows these two and the end cap in place for welding. The base of the heat riser is just short at this point as it will be inside the air jacket which will cool the burn tube.

More pics to follow...

Here's the next steps on this construction... I decided to add a supplementary air slot and channel just "downwind" of the feed tube before welding begins.

Here's the pics of this process:
-the opening cut,
-the opening in relation to the feed tube,
-the channel in place against the feed tube and
-the view down the feed tube and supplementary air channel.

The next steps will be adding the air cooling fins to the core and constructing the air jacket.

Progress has been slow on this project as I'm dependent on my son to do my welding and he has been very busy. But here are a couple of pics of my assembled "air-jacket" for cooling my core to prevent it from burning out. This will go inside the tank which will be my "bell" and the steel core (see pics above) will go inside of this.

Bruce, I'm trying to understand what it is that you are trying to accomplish.

Do you intend to insulate the J-tube? If this design gets up to temp, that metal is going to spall like crazy. However, I wonder how it will draft-- the riser looks terribly short for such a long burn tunnel. Will it vent through a mass? Do you have a sketch of the final intended layout?

Hi Chris, I'm seeking to build a 7" steel J tube rocket mass heater in a bell. No I do not plan to insulate the core but rather air-cool it! What you saw of the heat riser in a previous pic was just the base of it. The welding is finally done and I have some pics to follow of the completed core and the air jacket. This will al go inside a metal bell which will be enclosed in about 6" of cob.

Here's what the finished product will look like in operation (minus the cob mass around the bell!). I have assembled all the externals but am waiting for a thermocouple heat sensor which will read up to 1300C before I fully assemble the unit and actually install the heat riser within it. The heat riser will be lowered in 2 sections through the opening where the barrel is.

The first pic is a side view of the whole system. The unusual tank on the right is added just to further cool the exhaust gases and hopefully condense most of the water vapour and drain it away before it reaches the chimney. (Will see how it works!)

The second pic is from the front (feed area). Hope these pics help folks to visualize my very "unorthodox" air-cooled steel J-tube RMH!

As with my prototype used last winter (which worked very well) I will keep you updated with pics so you can see how much spalling or lack of it there really will be!

My next step will be cobbing the bell. This will be my first experience with cob. I've been experimenting and testing my materials the last few weeks. (More pics to follow in time of that process.)

If there isn't any insulation for the J tube, do you think it will throw off heat too early, reducing the burn temperature the way a normal wood stove works? If it does lower the temp to where the wood is not fully burnt, would there be a greater risk of creosote buildup when the remaining exhaust is cooled enough for creosote to form? Have your test burns of say 1 hour+ shown any issues with the feed tube getting too hot, where it might be a burn risk? Or will that all be totally covered in cob and not an issue?

Hi Mark,
Thanks for your thoughtful questions. I used a similar 6" steel J-Tube last winter and it burned fine without creosote.  My reason for this build in it's place is to eliminate the mass which took quite a chunk out our living room! Yes, the burn tube gets hot but like a hot burner on your stove, you just stay away from it! The cob will be 6" thick so will cover the air jacket but not the burn tube that extends out of it.

Hi Y'all, I have about half the height of the 6" of cob laid up around my 7" steel RMH in a bell.  I'm just wondering how those of you who have built with cob before have finished yours with a smooth or even shiny surface? Any pointers?  I'm finding that cobbing is a lot of work (especially when you do it alone) but it is simpler than I thought. Mine is hardening and drying "hard as rock" without any cracks for which I am thankful!

Any input from experienced "cobbers" would be greatly appreciated!

Here's a few recent pics of the cobbing process on my bell. In each layer of cob I have incorporated a lot of stones and in the top layers some large rocks which saves on mixing up so much cob!

My steel core burned out. I had my riser insulated with 2" of ceramic fibre blankets and it heated my uninsulated shop wonderfully but after two heating seasons the steel is disappearing. Mine was 6" square hss and 1/4 inch thick. It's now toast. Next core will be firebrick or ceramic board or a combination of both. My rocket was a simple heater with no mass. It was miserly on wood consumption and I loved how it worked but the guts need replacing. Will also be adding a pebble style bench to get max effect from the fuel.  Hope I can get it right!

Hi Doug, Steel cores should NEVER be insulated! That is primarily why they burn out. Rather they need to be air-cooled.  A steel core is a totally different "animal" from a refractory core.

I guess that I am still learning about these awesome heaters. I just ordered Erica and Ernies book and also Ianto Evans book on mass heaters. Maybe that'll take some of the guesswork out of what I want to do. I'm fiercely independent but will accept good knowledge and others expertise when I need to. Good luck with your build!

Well folks, this project is finally done!  I have painted the mass around my steel bell with "clay paint". (The same clay slurry which I used in my cob except the "paint" has been strained to remove coarse sand and pebbles.) My finish on the cob did not turn out as smooth as I thought it was. So I cheated and smoothed out the roughest spots with drywall compound before painting.

The pics show the first coat of paint and the final coat with the joint between my bell and the cooling tank in the exhaust line finally assembled permanently with silicone rather than plastic bags! My clay paint is almost a perfect match with the color of our wall!

The elbow on the top in the center is the exit from my core's air-cooling jacket. The air coming out of this usually runs over 300F and heats the house up quickly in the morning while the barrel does not give off much heat as it is mainly covered in cob.

Finally, the last pic shows the working end of the rocket with two accessories: -a draft stopper on top of the feed tube and - a spark arrestor at the entrance of the ash cleanout/auxiliary air intake.

The draft through this "beast" has been better than I had imagined! Better than my 6" system last winter, and this one has an even longer feed tube  (20.5 inches from top to middle of burn tube or 24 inches from bottom of burn tube to top of feed tube so 24 inch fuel is all contained). In fact on a windy day it draws way too fast and I have to restrict the draft with the draft stopper shown.

Since I have such a strong stream of hot air out the top of my core's  air-cooling jacket (usually averages around 300F) I figured that we could easily bake cakes, bread, cookies etc with that kind of heat. So I'm adapting the body of an old propane barbecue to set on top of my bell to utilize this heat before it heats the house! Also working on a circular cover as an experiment to see if we can bake a pizza on top of barrel!  Will post pics and let you know the results a little later.

Here's my new convection oven on top of my steel RMH's core air-cooling stack. It was built at no cost using an old BBQ body. So now it's saving us money on hydro (electricity for those who aren't Canadians!) Cooked our supper in it last night and hope to do some cookies later today. Please note that this is heated fresh room air and NOT smoke/exhaust from the combustion that passes through this unit.

Bruce, that is some awesome piece of equipment you've built. Thanks for sharing.

I think I've figured out your plumbing but want to make sure. The hot air vent stack is the galvanized pipe that exits the bell about in the center. Then the exhaust is the black pipe closer to the wall and it exits out through the wall via the 'T' while the other leg to the right goes to the condensation trap/tank. The BBQ body was placed over the hot air stack. I hope I got this correct.

You mentioned a 1300 degree capable thermocouple earlier; did you get it hooked up and gathering data?

Hi Duane,

Thanks!   Yes you have the "plumbing" figured out right!

Re the thermocouple: I had one on order hoping it would arrive before I had to assemble the whole thing and start heating the house. I planned to drill a hole in the side of the core just at the base of the heat riser and run the wire out through my air jacket. Well, it arrived the day I was putting things together and it was to long!  8 or 9" while my core is a 7" square!  So I had to send it back and therefore have no heat sensor installed at this point.

The first trial run of the "convection oven" went really well the other night. My wife mixed up a batch of raisin oatmeal cookie dough and I baked them on top of the rocket. The first tray was a little slow as the rocket was just heating up (temps in the convection oven were 250-300F) but the second tray baked well and much quicker when the temps were 325-350.  The cookies were great!

Next trial will be a pie!

Hi Bruce, another question for you. I am assuming that your 7" dimension is the internal width and depth of the main flame path box tubing. What I'd like to know is how long is the horizontal burn tunnel from front wall of the feed tube to back wall of the riser?

Thanks

Hi Duane,
Actually the 7" dimension is the external dimension of the steel tube. That's how steel tube is made.  Mine is 3/16" thick so the internal dimension is actually  6 5/8".

My burn tube is 36" long.  That may be a bit too long and I think if I did it over again, I'd likely make it shorter, maybe 24-30".

But I'd always make my riser as tall as possible (in the area of 60") as this is really the "engine" which drives a good draft.

If you're planning one similar to mine make sure you know the distance from the top of your bell tank to the ceiling and plan the lengths of your heat riser and air jacket exhaust sections accordingly as they will have to be lifted in from the top.

Also, please keep us posted on your build. I, for one, will be most interested to follow yours. Judging by the interest in my threads on this project, I think many others will be too.

Well, the fun continues with this one-of-a-kind steel, air-cooled RMH.....roasted our first chicken in the convection oven on top last night. Put the chicken approximately 4 pounds in at 3:30. It was browning nicely by 4:00 and was done, delicious and tender at 5:15.  I don't know how much it costs to run our electric oven for and hour and 45 minutes, but we saved whatever that number is!

Here's the before and after pics.

I like to see new designs tried.

would it be better to have chevron shaped cooling fins on top between the feed and burn tunnel?  or none at all, to allow free air movement over the top?

that way, air would be pulled in, and over the top and back again.
as it sits now, I can see the top having stagnant air, and not much cooling.

Brad Hengen wrote:I like to see new designs tried.

would it be better to have chevron shaped cooling fins on top between the feed and burn tunnel?  or none at all, to allow free air movement over the top?

that way, air would be pulled in, and over the top and back again.
as it sits now, I can see the top having stagnant air, and not much cooling.

Hi Brad, Well I like to try new designs and see how they work.

Good question about the fins on top of the burn tube. Here's the path my thoughts took while I was designing it....
(1) Should the cooling fins on top of the burn tunnel run across the top of the burn tunnel at 90 degrees or should they run parallel to it?   I answered that question by observing that if they were parallel to the burn tunnel, the outside fins on each side would stop cooler air from reaching the centre and thus defeat the whole purpose. So I went with the cross-wise arrangement.
(2) If the fins are as wide as the space from tunnel top to top of air-jacket (2") I will have stagnant air again for it will have no place to rise to. So I designed the whole arrangement with a 3" space between tunnel-top and air-jacket top so there would be an inch of space for heated air to rise up over the fins and flow freely through the jacket to it's own heat riser.

I can see clearly into the jacket and have only seen the metal glowing red on top of the burn tunnel once. That was when I was first experimenting with it and had the air-jacket heat riser completely closed off at the top!

A few times I have seen the sides of the burn tunnel glowing red but that is when I have failed to use the two small fans which I normally have in place blowing air on the side of the tunnel and into the air-jacket.

BTW, I just took a couple of pics of the interior of my core after 2 months of daily firing. There have been no scales of metal spalling in the ashes and no visible sign of any such spalling to date in the burn tunnel or the heat riser.    See pics below.   The first is the view into the burn tunnel showing the top of the tunnel from feed tube to heat riser.

The second is the view up into the heat riser. The scale that is seen is NOT metal spalling but simply soft ash which I was able to reach and scrape out just to be sure.

With my prototype 6" steel RMH used all last winter which only resulted in spalling on areas of the core which were not air-cooled and this year's 7" fully air-cooled core which has fins on the lower feed tube, entire length of burn tunnel and the lower third of the heat riser, I hope I have proven that steel cores can work quite well in an RMH AS LONG AS THEY ARE SUFFICIENTLY AIR-COOLED RATHER THAN INSULATED! More pics will follow at the end of this heating season and hopefully next year's too.