Heater for a small workshop

I recently created a workshop in my garage for my knifemaking/leather working habit. I partitioned off a 12'x 14' area in one corner of the larger structure that has a concrete floor and 8' ceilings. I'm currently using a wall mounted propane heater rated at 20k BTU but really can't justify the expense of the propane. This isn't a space that needs heat for more than 8 hours/day, just to take the edge off so I can work in the shop. I regularly see 0-20deg F lows in the winter and would like to get the shop to 60-70deg in a reasonable time.

I have good fabricating skills and a decent shop, not afraid to take on a more challenging design if need be. I have some materials on hand that I hope can be useful.

A roughly 18gal stainless barrel 15"ID x24" tall shown located where i'd like to place the heater




Here i have another shot of the barrel along with a small rocket cooker i welded up out of 4" stainless pipe (the carpenter's square 24" shown for reference)




I'd love to be able to work with the barrel at least as it seems a nice fit in my small space and perhaps offering the flat top for a tea kettle....Can I use this for space heating a small (<200sqft) shop? What would you do?

Thanks,
Steve

It looks like you have a good start to a rocket mass heater, or heater (if you forgo the mass).

My suggestion is nothing on rocket mass heaters at the moment. My suggestion is on your mindset. The propane heat is really nice in that it is instant heat and a lot. If your not willing to pay for that luxury then realize you will have to change your behavior to compensate. For example, starting the rocket mass heater and letting it run for 30 minutes or so before you can get started on your project in some warmth. You may have to come back once or twice to feed during the warming period.

If you don't want to come back to feed once or twice during the warm up, perhaps a batch box or wood stove?

Brett, I hear you about the behavior changes and I'll say that I'm looking forward to them. I figured an hour or so to heat the place up. A batch box is definitely something to consider. I wondered about the effect of one on a 4" burner.
I
I assumed a 4" burner for that size barrel, didn't think a 6" would fit. I'm totally open to making a form and casting a base/ burner too. Do you know of any examples of successful smaller stoves?

I should give a little more context of the situation. The exhaust will be exiting the gable end wall (20' high) on the lee side of the building. Will I need to run stack all the way to the roofline? Building codes are optional here, lol! If it makes more sense
to add mass to cool the exhaust I'll go for it.

Steve, for what it's worth, see my "design in progress" here: https://permies.com/t/42529/rocket-stoves/FWIW-Rocket-stove-heater-concept#333627

It's essentially a batch rocket heater...wannabee. First, it's a 30 gal drum sitting on a Drolet camp stove, so pretty small. Second, "No Mas".....(no mass). And third, you can see the stack outside the window. I've made recent mods since that original posting that include increasing the exhaust port area in the bottom of the drum and increasing the exterior chimney stack an additional 4 ft. Still need to work on some modifications to the box stove itself (it can puff back a bit of smoke at times, but fortunately the shop as of this writing is still drafty and un-insulated) and at a later date, a more robust riser core inside the barrel. In the meantime, with the right combustibles in the box (not plutonium, just good dry kindling), the top-outside of the barrel gets up to 500 degrees F pretty quickly (the lid itself likely hotter). I do not have the outside stack going through the soffit as it should be; it has a wind-resistant cap and there is about a 12" distance from the top of that cap to the underside of the soffit. So far, runs have been no more than ~10 min. at that high temperature, around 300 - 400 degrees F for the rest of the time, and in use for about an hour each time as I'm still prototyping. I still need to line the box chamber with some firebrick and have not actually tried a true "batch" run of stuffing the box full for a longer period of time...... But the winter in northern Minnesota is still so very young, so 'miles to go before I sleep'.

If you can run the chimney vertically inside, you will get better draft as well as more of the heat being useful. Even if you have to go out the wall instead of through the roof, the higher the better. Unless you don't care about smokeback when the wind is blowing the wrong way, you should take the exhaust at least 2' higher than the peak. And if somebody in the future might want to use your chimney and hook up a conventional woodstove, you may save them from burning down the building by installing the chimney basically to code.

A 4" system is reported to be very tricky to get working well in a RMH, though rocket stoves can be scaled down considerably. If you have minimal mass and a good warm chimney, your chances are likely to be better. You might try building the combustion core with barrel on top and chimney exiting at the base, then if it works well, add a ring of masonry around the base to support the barrel higher. You can make a bell above the heat riser as tall as you want as long as you don't radiate away all the heat and have no draft left; play it by ear, see how much heat you have going up and add volume to taste.

I have read from people who have done it that small J-tube cores need the riser taller in proportion than larger systems, and the burn tunnel perhaps shorter.

Thanks for your time replying guys, it means alot. The relatively tight space in the shop and the potential limitations of the stuff I have on hand are making me realize i've got alot more mulling to do, Haha! I obviously don't want to go headfirst into a shaky design. Just seems like the tried and true RMH design may not be possible for me. We'll see, i'll fart around with it and be sure to post my results.

Steve