Riser thickness

Howdy, I'm new here, even though I've been studying the content of this forum for a few weeks now. The knowledge here is fantastic.
I'm in the process of building a rocket shop heater. I made the heat tunnel and riser from .25" square steel tube. Now I see that this will just be a form for the castable fireclay and perlite. Before I make the outer form, how thick should I make the burn tunnel and riser?

Castable refractory cement would generally be about 1 1/2" thick, plus bulk insulating material around it (cob, clay/perlite, etc.). If you are mixing your own, it probably ought to be a good 2" thick, and the heat riser which will have no backup material needs to be 2" or more.

What dimensions are your metal parts? It is recommended that a first-time build be no less than 6" diameter, as smaller sizes are trickier to get right. Also, a system smaller than 6" may not have enough capacity to heat a space, depending obviously on the space's size, character and climate.

Your heavy steel "form" may be strong enough that its deformation under heat might damage the refractory around it. In any case, making a light wood or plywood inner form the right size is a minor job if you have any woodworking tools.

So far I have 3 3/4" id square tube for the j tube, welded to a 3/8" steel plate. I test fired it last night with a little wood and a waste oil drip. The temp was 15 degrees at the time. Fired great. A 15" diameter water tank will go over it. I haven't figured the exhaust size yet. My main concern right now is riser thickness.

Have you looked at kntryhart's videos on youtube? Specifically check out his "Rocket Heater Autopsy". He did pretty much exactly what you did for starters, and thought he needed an all-steel system to use waste oil. Someone else (here or on the donkey32.proboards.com forum, don't have links) discussed drip trays in the standard J-tube floor to keep the oil from soaking into the core. That would be easily replaceable if burnt out, and allow the good thermal properties needed for full combustion.

You have your steel J-tube welded to a heavy steel base, which will conduct a lot of heat away from the burn tunnel.

What is the size of the area you want to heat, and what climate do you live in? That is critical information for giving good advice.

The exhaust size would be the same as your combustion unit size, 4" diameter. That is not big enough with a J-tube source to heat any significant mass, only a small well-insulated room. It will give considerably less than half the heat of a 6" system.

I live in central Wisconsin. This will be to take the frost off some of the tools while building furniture in an insulated and drywalled 26x32 garage. This will at least be a good learning experience and all the materials were free.

Insulated is good, but a 4" system will only help a corner of a space that big. You want an 8" system to take care of a space that size. Do you want to keep the space warmish all the time for the tools, or just when you are working? How often do you use the space? If you want the tools to stay warm enough to avoid a lot of condensation, you really need a big thermal mass that can give a flywheel effect, as well as heating the space for working comfort.

I would suggest if you want to play with your test unit to throw some cheap insulation around the core (rockwool wired in place may work) and connect it and run as long as it lasts. Meanwhile, start designing a system to really take care of the shop. There are plenty of people here with experience doing just that.

That's a good idea Glenn. I use the garage almost every day and I've been heating it with a fuel oil salamander with the door partially open. Not really good for one's health. Maybe the rocket will at least help maintain the heat once the salamander has it up to temp.
I'll start looking for materials for a bigger system.