Batch box workshop heater

I had a dream last night about a small batch box that fit inside a stack of two 55 gallon drums.  The total footprint of the heater was the drum diameter.  It didn't have hardly any mass so the heat all went right into the room.  This gave a quick, fast heat but it lasted as long as the wood was still in the firebox.  Perfect for a shop or garage that only needs heat for a half a day every once in a while.

The tricky part was connecting the door to the shape of the barrel.  In my dream, someone in Montana came up with a door that had a chunk of 55 gallon drum connected to it.  Like a mounting flange.  So you could buy the door from him, build the batch box, drop two drums on top of it and gasket/rivet the drums to the fancy door.

Since it was a dream, it probably wouldn't work in the real world.  But just in case it would be the best thing ever, I figured I'd post it here...

If anyone makes a million dollars off this idea, feel free to send me a bit :)

That dream sounds like a good idea Mike. Perhaps tonight you'll have part 2 and get more details.
Be sure to have a pen and paper at bedside on standby.
Keep us posted!

The dream has developed further.  Unfortunately I don't know all that much about batch boxes other than I love them dearly.

I sketched up the footprint of the heater in a 55 gallon drum.  It looks like a 4" system would fit perfectly, if I interpreted the batchbox dimension charts correctly....  On the sketch, each grid line is 1".

By nipping of the corners of the front of the box, the actual fire chamber would extend all the way to the door.  By putting jacking bolts at the other side of the barrel, it could be squeezed against the firebrick so that the barrel doesn't have to be bolted/epoxied/glued to the firebrick on the feed side.  But it might need some sort of gasket, especially at the top and bottom where there would be a shallow curved gap?

I'm imagining a second barrel could be sacrificed to make the curved door.  But doesn't that door need to be kinda heat tolerant?  If someone wanted to slap a chunk of barrel on there as a door, what kind of insulation would it need and how could that be attached?  Due to the curve I'm guessing windows are out of the question.

So maybe all you'd need to order for this would be a floor channel, stub and maybe door insulation?  

Like I said, I don't know nearly enough about this technology but I'd like to give it a go.  Please lemme know what I'm not thinking of with this :)

Hi Mike,
I don't see that the bolts are needed. If you sandwich some superwool between the bricks and the barrel (front and back), the friction should be enough to help hold the core in place. It also provides an expansion joint so that the dissimilar materials can shift independent of each other.

I think a door made of a part barrel as you suggested would shed the heat and not be a problem. You could also line it with superwool to help insulate the fire if desired. Some stainless bolts with a washer and nut could hold each corner of the wool in place as a simple way to secure it firmly to the inside of the door.

Are you going to insulate the core or just the heat riser?

Will your primary and secondary air be separate or combined?

Mike Haasl wrote:I had a dream last night about a small batch box that fit inside a stack of two 55 gallon drums.  The total footprint of the heater was the drum diameter.  It didn't have hardly any mass so the heat all went right into the room.  This gave a quick, fast heat but it lasted as long as the wood was still in the firebox.  Perfect for a shop or garage that only needs heat for a half a day every once in a while.

Hi Mike, your dream has been materialized before, last November to be precise. I've conducted a workshop building 7 of these double barrel heaters in two days. Most of the work was done although none of the 7 items was completed at the time.
To begin with, these were open-top barrels but the clamps wouldn't fit two top rims. So instead the bottoms were cut out because the bottom rims are thinner and two of those would fit in one clamp together.

What we did next was cutting and folding plate steel in a sort of U-shape. Each U-box was provided with a front that could shield the thickness of a split firebrick plus 1" superwool. A partially lid on the box provided for the barrel connection. The core was entirely made of split firebricks with superwool around it bar the front and the bottom. Height of the box was enough to house the core's height plus a space of 3/4" where the top seal would go. The whole of the core is built into this U-box, superwool along the sides, rear end and top.





The riser will be a 5-minutes item, resting on top of the core. The bottom end of the riser is just square and built out of split firebricks as well.

The interface of the U-box to the barrel is tricky, indeed. But it can be done by means of cutting out a smaller opening, bending and hammering a 1/2" flange around.



The opening can be made a bit wider so the core box slide in easily. By hammering the flanges back a good fit can be established.



This particular core boxes are all spot welded to the barrel, front and back. Of course, this could be done by drilling holes and pop rivet the assembly together.



No pictures af the final product, but I think you can get a clear picture of how it can be done. By the way, this is a 5" core, due to the split bricks it'll fit perfectly. As you can see the whole of the core is in the lower barrel, only the riser will be sticking out. A low mass core, insulated around and a 5 minutes riser to go with it. The top barrel can be disassembled for maintenance, exhaust is about 6" from floor level, wherever around the perimeter of the barrel.

Thanks Gerry and Peter!

Gerry, I'm guessing Peter answered your first question for me.  I guess I should insulate the core and the riser.  I don't know enough about it so I'll assume that's the way to go.

I need to learn more about the primary/secondary air options.  I thought the current designs have a channel along the floor with a stub in the back for secondary air.  I actually don't know where primary air comes in.  

My assumption was to build the fire brick part and then drop the barrel down over it.  So a loose fit while dropping the barrel into place followed by a way to pinch things in place is what made sense to me.  But I'm glad there are more options.

Peter, how did you fit the barrel around the fire brick assembly?  It looks like if it has to extend out into that metal box, you'd have to put the brickwork together inside the box and barrel.  

I'm glad split bricks worked, they're much easier for me to get...

When they were finished, how well did they work?

Mike Haasl wrote:Peter, how did you fit the barrel around the fire brick assembly?  It looks like if it has to extend out into that metal box, you'd have to put the brickwork together inside the box and barrel.

The superwool got in first, then the split firebricks, done with very thin layers of a refractory mortar. This'll work, has been done before with a three barrel tower. The top of the steel core box is mostly open, only closed where the barrel's wall is. So front bricks first and riser last. Because the superwool had a tight fit, everything was supported nicely.

Mike Haasl wrote:When they were finished, how well did they work?

What I've heard about those shop heaters they work quite well. I have to contact the guy who initiated the whole of the workshop. I'll ask for more pictures, possibly there are videos now.
Initially some were used as an open system, others were planned to have a door of some sort. The air supply was done through a smallish frame, vertically mounted steel plate in the middle, blinding the opening for 50%. The rest of the opening a spark screen, left and right of the middle plate. Simple, very efficient and very quick to shed heat.

Thanks Peter!  I reread your entire website and this post several times to try to understand this and I think I'm getting close.

First off, from your pics above, I didn't realize the steel box went into the barrel, I thought it just stuck out :)   Does it continue all the way across the barrel to provide some connection at the far end of the barrel?  If so, I'm wondering if there's enough CSA on either side of the U tray to let cooling gasses get past the firebox and down to the exit?

If I used split firebricks, does it need insulation around the firebox?

It sounds like that design doesn't have a P channel or any sort of secondary air tube.  It even sounds like the front is only partially covered.  That would make it a lot easier to build...

It would be wonderful if you were able to get more pictures and/or a video.

Thanks for all the help fleshing this out!

Mike Haasl wrote:First off, from your pics above, I didn't realize the steel box went into the barrel, I thought it just stuck out :)   Does it continue all the way across the barrel to provide some connection at the far end of the barrel?  If so, I'm wondering if there's enough CSA on either side of the U tray to let cooling gasses get past the firebox and down to the exit?

Yes, the u-box continues across the inside of the barrel and is tack welded to the rear wall. That's how all of the core is supported, the structure is the steel box. Bricks right against the inside of the box won't work, there won't be any play room in there. The superwool can be compressed a bit so there isn't a rigid wall around the core. At the place where the riser is, the wool is applied in a double layer left and right.

Yes, there is enough csa on either side, the gasses can past there easily and slows down again under the core. In fact, the space there act as a manifold, a very spacious one with that. Fine dust will settle at the barrel's floor, the riser is close to the barrel's wall which means the front of the barrel will warm up first.

The same sort of construction has been used for a 6" core in a three barrel tower and it worked beautifully. The core sticks out more, full firebricks inside the box but 1/2" superwool around it this time.

Mike Haasl wrote:If I used split firebricks, does it need insulation around the firebox?

Se above, the bricks are allowed to expand and contract thanks to the insulation plus it's a lot easier to assemble the core inside the box.

Mike Haasl wrote:It sounds like that design doesn't have a P channel or any sort of secondary air tube.  It even sounds like the front is only partially covered.  That would make it a lot easier to build...

Exactly what I had in mind while designing this shop heater. No glass, no secondary air channel which means it will act as if it would be an open system. It does need a proper chimney, though.

This door also works https://udsparts.com/lavalockr-quick-coaltm-55-gallon-uds-ugly-drum-smoker-access-door.html

Thanks Uncle Mud, that's a great option!

I've thought a bit more about this.  Looking at the Batchrocket.eu designs, if you wanted an open system without a door, couldn't the bricks just stick out like on Peter's older shop heater?  Then you can skip the welded box on the front all together?



Or is that just an option for cast refractory designs?

Mike Haasl wrote: Looking at the Batchrocket.eu designs, if you wanted an open system without a door, couldn't the bricks just stick out like on Peter's older shop heater?  Then you can skip the welded box on the front all together?

What you see in the picture is a cast core, supported by a brick column at the inside and one outside the barrels. The column inside is resting directly on the floor, so there's a slightly larger opening in the barrel's floor and the opening between steel and brick stuffed with superwool. Around the cast core and the steel of the barrel there's another stuffed gap. Both these seals aren't really permanent, the steel moves quite a bit with heating and cooling so regular (montly or so) inspection and maintenance is required. Despite being a barrel heater, it won't be movable at all, not even a tenth of an inch.

The one with the steel box inside will remain movable due to its construction. No rigid connection to the floor, the interface between box and barrel is welded or riveted, all steel is separated by a layer of superwool from the firebricks. There's no brick column inside the barrel so there's less mass to heat up. Most of the downsides of the old one are eliminated in the  construction of the newer shop heater.
All in my own opinion of course.

Ok, gotcha...  Thinking...  thinking...

So the critical part is to support the core/riser with a metal plate attached to the barrels.  If the bricks stuck out the front an inch, that doesn't sound like a problem for a system without a door...

And the sides of the U also help hold things together?  Probably so it's more moveable?  If you were building this in one spot and never going to move it, would you need sides on the U?

Sorry for all the questions :(

Mike Haasl wrote:And the sides of the U also help hold things together?  Probably so it's more moveable?  If you were building this in one spot and never going to move it, would you need sides on the U?

Yes.
That's one of the possibilities, yes.
Yes, definitely, so it all stays in one piece while it is in use.

When you say "the bricks are sticking out one inch", that would be in the middle of the firebox. Left and right, it would stick out two or three inches. For a 6" system, the core would stick out a lot more.

Thanks!  My sketch in the 3rd post from the top is for a 4" system so that's why I figured the bricks would barely stick out.  I did another one of a 5" and with the riser squished against the back wall of the barrel, the bricks came out about 2".  I didn't think having the riser that close to the barrel was a good idea so I reverted to a 4" system.

Assuming that is reasonable...  My shop has a low ceiling (6.5') and is about 20' by 15'.