[Porsche-O-Phile]

I've seen PEX used to do fairly long runs of heating water from heat pumps to area heaters - my understanding is that it affords very good insulation to the fluid inside - for this reason I don't think it would necessarily make a very good "radiator" to transfer the heat from the fluid to the slab (through the wall of the tubing). Maybe there's a way this is dealt with in those particular applications but I'm not familiar with it (would have to check). I've dealt with heated driveways which always used metal or copper piping and typically would introduce corrosion/failure loci at the penetration points (although there are probably ways to detail this to prevent it from decaying in those areas)

You might be right though - if you have a well-insulated living area and are relying on a heated slab, the cost might be less to run the associated water heater or heat pump than to run a furnace - it costs a lot (and is inefficient) to heat up a lot of air to blow around - but it is also easier to control space-by-space which is why it's typically done in large installations (VAVs, etc.)

I'd have to see a comparative analysis but my gut feeling is it'd probably be a toss-up with respect to operating cost versus a "conventional" forced air system which throws it back in the court of construction cost - probably a lot cheaper to do a ducted forced air system than a heated slab (especially if you're talking about more than one floor!)

If ya got the money though... I love the idea of radiant flooring. The quality of the heating is awesome.