New Pole Barn Build - Can Concrete Floor Be The Mass

It's looking like this might be the year I get my pole barn - something in the 30' x 40' range.  Since this has not been built yet I have the option to include something in the floor for heat and I'm wondering if anyone has run the output of a RMH under a concrete floor?  I don't need to heat this up to short-sleeve temps, just warm it up to make it comfortable to work.

I'm thinking something like digging a 16" deep trench through the middle of the floor and suspending an 8" diameter pipe in it and then pouring the concrete floor.  I would build the rocket stove part at one end and the exhaust would go out the other.  The stove barrel would provide quick heat and sending the exhaust through a tube in the floor would warm the floor up.

I'm sure it cannot be as simple as I'm describing, so I'm looking for someone that has done this already.

Thanks,
Jerry

I don't know how many have done it, but I'm sure it's doable.

I would put insulation under/around the trench before filling it, or you would be mostly be trying to heat the earth.

Yes it has been done. Yes it can be done. No, I don't recommend it. The thermal mass in a RMH takes a certain amount of the heat out of the exhaust  to store it in the building for later. Too little and you're just heating the outdoors, too much and there isn't enough heat left in the exhaust for your final chimney to draft and the whole thing stops working. The heat sink effect of a bell or bench is much easier to predict and much less variable than the heat sink effect of a concrete floor and all the tons of earth it is thermally coupled to. For example, the cob floor of a cottage I built with about 20' of exhaust pipe running through it makes an excellent heat sink most of the season, gently warming our feet and the room without taking up a lot of precious space like a bench would, BUT if the floor gets too cold (like we go away for the weekend, or it gets down to 10 below), the chimney stops working because the exhaust temperature drops to about 50 degrees. The quick fix has been to install a duct fan on a switch at the top of the chimney. It helps, but the real fix is that we are cutting up the cob floor and sending the exhaust straight to the chimney about 5 feet from the RMH so it doesn't have a chance to lose all of its heat. In a building as big as you are talking about I think the problem would be much worse. You could run the pipe through a chase of soil say 2' deep by 3' wide that was insulated from the ground but that sounds like a lot of work compared to a bench or a bell. Keep in mind I have seen benches and bells have all the heat sucked out of them too by an uninsulated concrete floor. Good luck.
--Mud

What is the recommended path to heating a pole barn/workshop with some kind of rocket technology?  Intermittent use and not well insulated - just something to take the chill off.

This is the workshop version of our cottage rocket. It produces about 30k btu. An 8" would produce about twice that. It is purposefully made without added mass so it warms up fast. We installed one in Paul Wheaton's lab shop and reports are good. Several similar ones are also doing well. We are going into production as we speak.

I just finished an 8" batch box (Peter berg's specs) in my 40x56' uninsulated pole barn in central Illinois and am very happy with it- 2 stacked barrels for instant heat and one large bell, single skin brick. with your dimensions I would recommend an 8" batch box-- not a top feed. I switched to the batch box in 2011 with my 34x30' shop: the Ianto Evans type top feed just didn't allow enough btus to be burned and heat up that shop... I tried for over a year.

Regarding floor heating, other commenters pretty much covered the issues and I also would say that heating the floor is possible but would require isolating the mass so it doesn't bleed off heat and also figuring out a smaller floor area to heat: your pole barn would have way too much floor space to heat. as one person said, you only have a finite amount of heat to bleed off before your exhaust loses it's pumping capacity.

You might want to look at Peter Berg's shop batch heater which has 3 stacked barrels; if you start with 2 or even 3 you could later try taking the top barrel and running 50' of 1/2" copper around the top section inside the top barrel, screwed/bolted to the sides of the barrel. Then run an antifreeze solution powered by a small pump through a pex grid on maybe 200 sq.' of floor. Any liquid heat transfer system needs to be designed and built with safety features including pressure relief valves, no air pockets, etc. so if you've never done this I would get some help from someone who has. One advantage of adding the coil later is that you could start and see/feel how the stove performs and then modify it.

Regarding ducting, I no longer use ducting and instead have switched to bells/chambers. Bells are superior in every way, in my opinion. I would not recommend using ducts to bleed off heat into storage areas. Also, you will need to be careful about dropping your exhaust too low, especially in a duct as it will stall if you go below a certain level.

That's a very interesting question regarding under a floor venting of a RMH. It could be done with careful design and construction. You could have a bypass to go straight to a chimney and/or go under the floor.
Insulation, insulation, and did I say insulation. Without insulation under the concrete and trench you're conducting into the ground.
Personally, I don't know why anyone would go to the trouble of building a RMH and not heat LOTS of hot water with it. Then pump the water through PEX tubing in the concrete slab. If you're off grid its a challenge to pump without a decent solar or hydro energy system. But the pumps are small and low power. And I assume you want hot water. It's not that much harder in my opinion to add hydronic heating. Once i lived with radiant floor heat I haven't wanted to go back. The insulated concrete will stay warm for much longer than heated air will.
But if it is for intermittent use you also want what I call a "flash" heater. A stove that flashes off heat quickly when you need it. A exposed barrel or two as mentioned by previous responders.
Be careful.
That's great you are building this year. Take care.

Here are some pictures of the heater in the pole barn. a few specs:
8" flue
chimney is about 20' tall, black 8" stove pipe sits on an 8" elbow on floor of bell, then switches to 8" heating duct at about 8' (thinner gauge, galvanized), 12" heating duct starts at top of bell and sealed with cob to for a semi air tight insulating space like a double wall insulated chimney, total cost, around $200; temp at eye level is around 160F (uncomfortably warm to touch but doesn't burn)
T primer out of chimney for cold starts, works well;
bell is around 2.5'x 5'x3' tall, all single skin standard brick setting on 8'' blocks and perlite cob floor, top is bricks set on steel t posts, covered with screen and cob;
firebox is peter berg's 8" dimensions, standard firebrick except ceiling which is 1" ceramic fiberboard. I will redo this in the summer and make some refractory slabs to set on the ceiling to protect the soft fiberboard.
p channel is two 1.5" steel tubes with flaps on end to create turbulence;
riser is 4' tall, 8" diameter, octagon shape made of 1" ceramic fiberboard.
Almost no smoke if I start the fire right, takes about 5 minutes to rocket. The one picture of the chimney is steam on day 1. The single skin bell works great in my barn since it heats up faster and retains some heat to give off overnight but I would only need that to keep engines sortof warm on really cold nights.

Overall it's a real good heater. My thanks to Peter Berg and Matt Walker for sharing their work on the batch heaters!