Bill they do sell risers for rebar…
Failing that get a cinder block and bash it into pieces. Sit your 1st layer of wire onto it it will give you a uniform 1.5 inch space from something of the same hardness…
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Bill as David said I would also recommend staking it with rebar. If not your forms will float up. Leaving that foam larger than the slab is a great idea. I would also recommend insulating inside your forms and leaving the form boards in place, treated of course. Your chickens will eat that foam if left exposed. They love it.
Congrats on the new chick. It is my understanding that if left with the flock they will kill it. Once it reaches equal size it can be reunited . I have no experience wit this just something I was told or read. Did you just leave one marked egg or are you expecting more?
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Bill - Gotcha. I didn’t realize your batteries were also in the generator shed.
My plan is to dig a storm shelter into the side of the hill, and put my batteries in there before winter.
Our winters are nothing like yours, but I have seen how much difference temp. makes.
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In my experience, rather than run the foam horizontally out beyond the slab, we have dug a trench a couple feet down and put the foam vertically around the slab area.
also good idea to leave a foot deep trench around perimeter and fill with concrete to minimize rodents digging under the slab.
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Bill, I did about the same thing with my work shop; gravel, foam and pex.
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Jeff, how did it work? How long to warm up? How long did it maintain temps? How far under the concrete? Feed me… lol
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Here’s a video of my set up for the generator shed with in floor heat.
I have the shed facing South for the possibility of more solar panels in the future.
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Off to a good start. I don’t know much about -25F but the form needs to be fastened down before concrete is poured. Maybe it is, I just didn’t see it.
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Hi Bill, ten slabs each 14’ wide and 6’ long. So, the building g is about 14’ wide by about 60’ long. Each slab has 100’ of pex. The top cantilevered past that. First floor about 11 or 12’ high and the second or so level is above that.
With my job I could never fire steady so I gave up on heating it. It would take about a week to warm it up to about the mid 60’s, the slab that is. In the middle of winter it would cool down in about two days.
I loved having a bit of heat in the shop in the middle of winter. Still had to dress up a bit but for working it was great. I just could not keep it going with my job. Not to mention that old age has caught up with me - slowing down. All my spare time was spent keeping it alive. It was nice turned out to be a big waste of time but when I retire from the factory it would be just the ticket.
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Help me understand Carl why the form needs to be fastened down, also, how?
So @Jeff this is an 8’x8’ with about 80’ of 1/2" PEX. Would you recommend to start circulating the coolant in October or November to start building up the heat? I only anticipate having to run the generator about once a day for 2 hours.
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Hi Bill, I’m not Carl but I guess he ment the frame will kind of float on the poured in concrete. Same goes for any loose stretches of the pex. I think I remember you mentioning putting another net on top, that would solve that problem. Holding down the frame will be little harder. If you somehow could nail the nets to the frame that might help holding it down when concrete is poured in.
I love the idea of capturing the waste heat this way and floor heating is really nice. I did it the same way when I built my shop, only I used leca blocks for a frame and I heat with my boiler/tank. I also use coolant in the pex via a heatex in the house. That way I’m not forced to keep shop temps up if I don’t feel like it.
I think you will discover you won’t need no equipment to regulate flow. Just get rid of any restricting thermostate in the genny so you don’t overheat the pex and you’ll be fine. The thermal mass of the concrete will take care of everything.
Edit: Don’t strike any matches in that building or the H2 from the charging batteries will make it fly 
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On the hangout Friday a couple weeks ago we talked about taking too much heat off the diesel engine right away and not letting it get up to design running temperature. That is why I suggested putting a tee in the radiator hose after the thermostat and having a separate pump for the pex. A little more complicated but better for the engine maybe.
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Bill in addition to the forms floating up as the concrete is poured the weight of it will bow your forms out and the joints in the corners will also fail. I personally would not run the pex so close to the perimeter of the slab either. Keeping it away from the edge will help to slow the heat lose.
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Thanks for your responses all. I too was worried about bulging of the sides. I was fortunate enough to have Dom M’s phone number and him coaching me through this process.
I’m guessing he wasn’t worried about the forms floating because I told him I was doing it with my mixer 2 bags at a time. So now that makes sense to me. It may take me 2 or 3 days to do this part because of my bad back. So with all the concern, I will make sure that some of the spikes he had me put in are covered in concrete the first day
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Bill,
I will throw in my 2 cents worth. First, yes the chickens will eat the styrofoam! I don’t get what they see in it, but I had a small styrofoam cooler in my garage, and they ate the whole thing. 
Second, you might need an (insulated) expansion tank for the heated coolant, it will need someplace to go as it warms. Thirdly, I think you might want more elbow room around your generator and batteries, but that is up to you.
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Looks like all the concerns are covered. Forms do float, and bow, and “un-nail” themselves. Pouring by hand minimizes most of that. Good luck.
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Bill, I poured my whole floor with a HF mixer, the old fashion way. Would have been a bit cheap to have a truck roll it in but too big of a job for me and my pex might have been messed up.
If using pure water you might have a problem. Like Don said do not cool the engine below operating temp. How about engine exhaust?
I would start using it as soon as it starts to get cool/cold and your running the engine. Put 4" of foam around the out side and protect it from the sun.
Now we need you to start to make charcoal and store the waste heat in your slab… 
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Another reason to not hook the pex up to the water pump before the thermostat is if you run it in the warm months - do you still want to heat the slab? You could divert the hot water thru a heat exchanger to other uses like shower or preheat domestic hot water or wood chunk dryer or ? ? ?.
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I plan to do something similar with engines waist heat but will use a large storage tank. The coolant will flow through a coil in the tank. Other coils placed in the tank will pull the heat of as needed for other uses such as domestic water heat or space heating. doing it this way I can add heat to the tank from other sources such as a solar drain back system or a wood boiler.
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