We are getting to the later planning stages in building our house and are planning on putting in one of those DIY radiant floor heating systems, open system (i.e. radiant and domestic heated by the same heater, unseparated, all tubing rated potable). For the future mother-in-law in the basement, we are planning on putting a couple European style radiators in there down the road. Has anyone heared of any such radiators that are rated potable? -Thanks!

Just say Know! Know what you are proposing, and know what the risks are.

Open systems are poor choices, and all panel radiators I am aware of are not suitable for open system installation. Add a heat exchanger, at least, if you must go this way. It's pretty unlikely that a water heater is your best choice for a full house system though. It happens, on very low load buildings, but it's not that common.

He may mean one of these new units that is basically an indirect with a built-in boiler. 100's of thousands of BTU/hr, and high efficiency.

I had a client use one for the exact same purpose(DHW/Radiant Floor/Open System). Gosh. I wonder if he's still alive?

He built a big house with ICF's, and he insisted on using R-50(walls) for the Heat Loss calculation. I'll bet he beats the sh!t out of that poor thing in the winter!

I am aware of those units. There are a couple of interest (the Phoenix comes to mind), but in general they are pretty overpriced and I would regard a mod/con a better choice in most comparisons. Can't run a water heater at 90 degrees, so there is a functional limit to how much "condensing" they can do where a mod/con can on a good system operate at 70 degrees and max out at temps that are still below the operating temperature of any DHW tank.

How big a deal is that? hard to say. But it is definitely nice.