One of the things Radiantech mentions in their manual is to leave the center of the floor uninsulated so that the earth acts like a large heat sink. If it had been my house, I would have done it. All I have to do now is figure out how to get the Munchkin's efficiency up to 100%.

 

It wouldn't be a bad idea to put a drain and air line fitting on there so that you could blow the water out of the system if you needed to kinda like you do for winter RV storage.

 

Glad it worked out for you, but what works for you may not work for everyone. You see contrary to popular belief heat DOES NOT rise, hot air rises. Heat moves from hot to cold, always has. Hot uninsulated slab, cold groud, you can guess where the heat goes. The thing about under slab insulation I will give you, but you still should insulate the footings, slab edge, and the first four feet of the floor to keep the heat from going outside and if you have a high water table or your slab is on rock you will have a problem. Most of the top hydronic guys (not Radiantech) in the industry design thier systems with under slab insulation (2" foam, not bubble foil mats).
As far as water heaters are concerned, yeah they work, and I have used them many times, but with the rising cost of fuel these days it makes no sense to use a water heater for anything but the smallest loads. In fact, you have probably paid for a high efficiency boiler already with the extra fuel cost with your water heater. One other thing about water heaters, do not run your radiant floor off your potable water heater. This is commonly refered to as an "open system" and some internet do it yourself radiant websites recommend this. Just google "Dave Yates- Legonella" and you'll see why it's a bad idea.
Gotta go to work now. Nick

 

As far as the tubing and pipes freezing, you're supposed to run a glycol and water mix in the system. If you run out of fuel, the circulator pumps will continue to run, making it more difficult for the system to freeze up. And if the power goes out, it'll take some time for the slab to reach ambient temperatures and even then the glycol/water mix should prevent freezing for awhile longer if you don't have a generator to run the pumps. And if you want to flush the system, you can do it through the fill/purge valves built into it.