Hi all; Below zero outside , snowing and i'm sitting in the greenhouse, next to my rmh ,soaking up the warmth and dreaming of my next build. i have a 24 x 32 shop that is currently heated (or tries to ) with a double barrel stove. this works ok if the temps are moderate but is next to worthless more than 10 feet away in these temps. Nothing worse than your wrench freezing to your hand when you go to pick it up... SO..... Of course another rmh is called for !! Now only half of the shop floor has a slab ,the other side is still dirt. I have been thinking of pouring a portion of that side this summer. Plan A simply called for a brick encased rmh that will sit between the two work bays providing heat where i need it (next to the cold car) as well as a warm bench to lay tools on and (most important !) to sit and cogitate on (stare blankly at car ) I would insulate from the ground and place the rmh next to the current slab losing a little extra space on that side but gaining so much more !!! Now comes my BRILLIANT IDEA Plan B !!! or my disastrous mistake ! Pecs, everybody is talking about it. So i thought why not put it in the 12 x 10 slab that i want to pour this summer. Now i have read all the warnings about boom squish and don't care to experience that in person (no visits from a big al in an ambulance )! So i have been monitoring my temps in the rmh in the green house. The barrel and burn tunnel are to hot , the horizontal transition area is also to hot (close to 200 at times ) however once i enter the mass area, my temps top out at 130-140, less the farther from the transition area that i get. my idea is that i could place a container with a removable lid directly in the mass as i'm building it (a milk can came to mind first ) hard copper fittings would be mounted to the container that extend to the outer face where i would put a small circulating pump and connect to the pecs tubing and hopefully end up with a heated floor ! The milk can should have enough volume to supply 130 degree fluid (antifreeze ? ) to the pecs as well as providing a better medium than cob & rock for storing my heat ! Am I missing something , or does this seem like a valid idea ? I know next to nothing about pecs but have several friends that do so i can get information from them about its use. I realize that a portion of my heat will be going to the floor but i'm thinking that this may work. Any imput will be greatly appreciated . Tom

Thomas Rubino-
pecs=PEX
I like your ideas in general, but I'm still of the opinion that trying to heat water in the mass (post barrel) will not provide enough heat exchange to heat water to a usable temp for a thermal mass or to circulate in an underfloor pex system. I might do OK, but likely would take a long time to heat up the water mass or floor.
If the idea is to heat water for the thermal mass and not a cob I've still got this crazy idea of adding an open water jacket around the barrel and either circulating directly from that or using a coil of either pex or copper inside the barrel jacket as a heat exchanger for the circulating water.
A 6" system with a 30gal barrel then a 55gal barrel as the jacket going somewhere short of all the way up the 30gal barrel. No boom squish as the water could never go beyond boiling being in an open container. Just a note PEX will only handle up to ~180deg F.
Took a while to find this again but here's a nice plan for water heating. How to bulid a rocket stove water heater
I guess it's actually a bit like your milk can idea

Hi John; Thanks for the input ! The whole idea of plan "B" has been in the last 24 hours or so. I guess I have even more to learn than i thought, starting with spelling, lol. I do just want to heat the small slab to start with, and only when i am working in the shop on a daily basis. I work out of town and can be gone for a month at a time, so the whole system will drop to ambient temps when i am gone and need to be reheated when i start working at home again. I realize that is not the best way to use a rmh but it will be a big improvement over the current arrangement . max temp of 180 with the pex should be fine with my idea. I was not sure of the limits for temp , i did hear that 300' loop is another limit . I have much to learn, but as its not quite march yet, i have a good 3 months to figure this out.

Thomas, if i may!

There's the plan from 51 area!

Dig a bit more on that side you want to pour the new slab on, put a 4 inch layer of polyurethane foam on the ground, and on the perimeter. (don't know what you have over there, i know stiferite, efitoit, you could also use polysio) Lay the pipes, with spacers to get them a smidge above the foam. And pour your concrete. Good planning is required thought. You could even dig a bit more at one spot, and have the J tube mouth flush with the ground, so you could brush the dust in there to be burned (better in a woodworking workshop tho)

If you want to save a bit on concrete, you can fill under the pipes and on the sides with gravel or rubble. But you still need to insulate on the bottom and sides, to stop the loss of heat via conduction.

Hi max; Yes, I was planning on ground insulation for under the slab as well as under rmh itself. I was thinking that 2" would be enough but of course 4" would be better.Was thinking about using perlite with sand on top for under the rmh , so why not under the slab also? I should add that my real goal is the rmh in the shop! Adding the floor heating feature is like the icing on the cake ! If I build it as I envisioned and the slab never gets hot, then I empty the fluid & fill the milk can with rocks & sand or maybe just seal the pipes and leave it full of fluid! The slab will have the pex in it and I can try johns idea of an open tank around the barrel. The video of the "on demand" rocket stove water heater was fascinating , great idea for elk hunting camp! Tom

Thomas, well, here anything below four inch, slabwise isn't realy envisioned. But we build everything with concrete in france, so it's cheap.

Depending on the pipe you want (i mean flue pipe) 4 inch isn't practical. I didn't think about puting PEx in the slab, or water heating or anything like this. But a KISS system.

If you're not keen on digging much, you could do a U shaped trench insulated with perlite on the bottom and sides, fit the flue tubes in there, make a layer of lightweight perlite concrete, like 2" inches all around at your 4 inch form finished ground, i still think foam insulation is cleverer. And pour a slab all over this.

Let me draw this.

So, i haven't drawn the elbows. But here's what i have in mind. You pour your slab, add gravell in the pool made by the insulation in yellow.

Hi Max; My slab will be 4"+ thick. I was thinking of the 2-4" of perlite /sand under the 4" slab with the pex sitting on top of the sand as i pour. Also I should clarify that the mass heater will be in the center of the shop running lengthwise , the slab that I am pouring will be the back 10' X 12' of the building, sort of behind where the rmh will start. My real question is will the fluid get hot enough as i have described ?

Well, sorry to have interfered then.

I don't know about heating liquids.

Max- I'm sure Tom appreciates your "interference". There is a definition of interference that means to prevent someone from going in a certain direction (redirection).
I've seen you post that link before, and you said you didn't know about heating liquids
It's looks like the same concept as the youtube link I posted.
I did a test run of my pocket rocket immersion heater tonight. I came to the conclusion it is more effective to heat an open barrel of water and circulate that rather than trying to use a heat exchanger pipe in the barrel. Well if the "pipe" is a 50ft garden hose anyway

Your sketchup drawing is the way the first RMH I saw was installed except it was a gravel floor instead of concrete. It worked very well in a 15x20 green house. My design was supposed to be a take on that albeit a poor one

Tom- I think it comes down to whether the exhaust duct might heat the floor as well as using PEX and circulating water. The water option involves a whole lot more designing, $ and work.

And Troubleshooting! hot water has a tendency to cause problems especially as we add more techie solutions to the problems, which yall are obviously aware of. You guys are are talking very interesting stuff though. In any radiant floor, insulation below the heated mass is super important for efficiency. I like the perlite idea, but wonder how economical it would be to do a whole floor. You would want to separate the sand from the perlite with something. I think R10 is a good minimum to shoot for under radiant floors on grade.

I know that 180 is a max design temp for PEX. I wonder how far past that one could push and what happens to the tubing if the main heat is well away from fittings and the loop has the proper expansion tank volume?

Hi Guys; Just an update on my idea. Work,life and a RMH core rebuild in the greenhouse has kept me from this project... but I haven't stopped thinking about it ! I have acquired A hydraulic tank from a piece of equipment as well as all the used hyd oil I need . I plan on building the hyd tank into my mass , using a small block chevy oil pump (run by a drill ) to circulate the oil thru the pexs and into my small slab. The mass of the rmh will run 30' between the two bays of the shop providing a nice warm spot to place parts as well as myself when working on the trucks. The small 10 x 12 slab with the pex may or may not get warm enough to help heat anything , but as long as the mass gets up to temp, i'll be happy. The tank of hydraulic oil has a flash point of over 400 degrees F so i won't have to worry about boiling or evaporation and even if it does not heat the slab it will still be a good heat holder in my mass.

Sounds like an interesting concept. I do wonder about a couple of things, though... What is the specific heat of hydraulic fluid? Will it hold enough heat to be useful, and will it transfer that heat at interfaces? Also, how will it react with the pex? Has it ever been tested for compatibility? It would be no fun to find that all your tubes in the slab and entering it are deteriorating. I have doubts that a drill would be a reliable long-term motive power source, but if the system works well, you could always put in a small continuous duty motor.

Hi Glenn; Not sure about your question of specific heat , I know from working on heavy equipment that the hydraulic fluid will give you a nasty burn when hot. I am hoping that in this location the oil will run apx 140-160 degree F , my tank has a temp gauge built in so i will know once it is all up and running. I'm told that the pexs is stable to 180 F . I will have to investigate if pex and oil are compatible, but my guess is that they are. The cordless drill idea is for experimental purposes , if it all works as i hope then at a later date I will hard wire a small motor in to replace the cordless. There has been some question about the ability of a 8" rmh having enough btu's to heat a mass as well as a slab ... I guess ultimately I will find out . My theory is that if it can't heat the slab and steals to heat much from the mass , then the heck with heating the floor , disconnect pump , leave tank full of oil and use it as part of my mass. Would I do this setup with oil inside a home ? No , hydraulic oil does have a smell when hot and makes quite the mess if it spills , but it already smells that way in my shop and small oil spills are the norm out there. My first rmh is in our greenhouse and it works so good that I just have to have one in the shop !