[djmcmath]

My dad has done this in two of his houses.

Pros: More efficient than most conventional heating options. Warm floors are great to put your feet down on when it's been a cold night. Very consistent temperatures due to the huge moderating water mass.

Cons: More expensive up front. Long temperature stabilization times due to the huge moderating water mass (e.g. if you leave for 2 weeks and shut the heat off, the thermostat is liable to cycle back and forth, badly underdamped for a couple of full days, before the temperatures settles in the desired region). The risk of leaks is present, and presents the potential for huge problems.

Dad's best luck has been with rubber tubing and an on-demand hot water heater. Set the tubing in the floor with staples, then pour light-weight concrete on top. On top of that, lay a subfloor and flooring material -- but be VERY CAREFUL not to hit the tubes with whatever fasteners you're using. If you're doing, say, hardwood, and you miscalculate the length of the nails, and inadvertently punch one through into a tube, you won't discover it for 6 months, at which point you've got 100 sqft of hardwood to replace.

So it's definitely best in a new construction-type project. I can't imagine seriously trying to make it work as a back-fit.

Dan