My concrete contractor advised tying the tubing to 6" wire mesh with the mesh directly on the 2" rigid blue foam. The mesh is there specifically to accomodate the tubing layout. but may add some strength to the concrete slab even though it is on the bottom.
Once the tubing was in place, the rebar was layed out on top of the tubing and then the slab poured. My slab is 6" thick so this still leaves the rebar in the lower one-third of the slab. The rebar must be low in the floor to add any strength.
Putting the rebar down first and tying the tubing to it is perfectly acceptable, but remember that the concrete guys are going to pull the rebar up as they pour the slab. My concern was that if the floor was only 4" thick that they might inadvertantly get the tubing so high in the slab that it could be hit when saw cutting the relief slots.
Let me suggest an excellent book on all things hydronic:
Modern Hydronic Heating by John Siegenthaler. This is a very expensive book at $120.00, but you will know how to take EVERYTHING into consideration. All you have to do is avoid one mistake and the book will become free. This book goes into everything from calculating the actual heat load and necessary btus to heat the space to designing a cost-effective system that will actually work. I have no interest in either the author or the book, but I do think it is a true resource of valuable info for those considering a hydronic system.
One thing that should always be held in the front of the mind is that tubing runs should never exceed 300' regardless of the size of the tubing. I skipped a few steps and have one run that is about 400' so the amount of fluid pumped in that section will be effectively less than the other sections. This can be compensated for, but at added expense and a slight-loss of effiency.
On a side note, I think I have decided that it is actually more efficient to use an electric boiler rather than a propane boiler. I was getting seriously concerned over the rising cost of propane this winter as it went over $3/gal. I had based all my costs on $2/gal propane, and suddenly it hit me that with a heated slab I can store a lot of heat in it at night time and therefore buy electricity at an off-peak rate.
Some quick math: 1 gal of propane = 92,000 btus. 1 KWh = 3413 btus. So it takes about 27 KWh to generate the same amount of heat. Off-peak electric rates are 5 cents/KWh times 27 = $1.35 vs. $3.00 for propane to get the same amount of heat! When you calculate the amount of heat actually available from the propane boiler the savings are even greater. A condensing boiler can be 95% efficient, but a non-condensing boiler is only about 80% efficient. At 80% efficiency there are only 73600 btus available from a gallon of propane (but you still have to pay the full $3.00/gal!), so to be comparable, it requires 21.5KWh to equal the available energy from a gallon of propane. Even if you bought electricity at the going rate (here locally) of 12.5 cents/KWh, that equals $2.69 of electricty per gallon of propane. It's still a deal! And, I doubt any of us think that petroleum product pricess are going to stabilize anytime soon.
