Thanks for all the input; it's been really helpful. Shanghaimom, I did a 'step test' with a 12x12 marble sample I had on hand, and you're right; it is so much colder than the tile. That has convinced me to install floor heat, even if it's only a secondary source. I had no idea that hydronic toekick heaters existed; the quotes I got all mentioned 'electric fan toekick heaters', and I didn't want to go that route. When discussing with my contractor last evening, he was very supportive of the hydronic solution; now I just have to make sure that the unit will fit under the 'door' side of my vanity (will need to access it for service, so it really shouldn't creep into the drawer side of the cabinet.) Fingers crossed! Mongo, thanks! I'll do my homework on the BTU calculations; thanks for pointing out what to look for. Lynneblack, thanks for the vent/fan/light combo idea. I've read on the boards that the Panasonic is a great unit, but one of the two items I'm keeping from the current bathroom is my Nutone melon vent fan/dimmable light.

To different animals to consider, Hydronic radiant versus baseboard, in terms of the water temp they circulate. With true hydronic radiant, you want to circulate water that is around 110-120 degrees. The goal is for the finished flooring surface to not exceed around 80-85 degrees. The problem with converting baseboard to radiant hydronic is that baseboard systems typically circulate 180-degree water. Running water that hot through a tight hydronic floor system can result in thermal expansion and contraction issues with the floor. For best performance, it usually results in having to put a the hydronic loop on it's own circuit separate from the the baseboard water. There are a few ways that can be accomplished. You might be able to get away with circulating 180-degree water if you did something like a below-floor suspended PEX system in the joist bays. Not "staple up" with transfer plates, but simply suspending the loop a few inches below the subfloor. For 180-degree temps, I'd prefer to use copper over PEX. But that's a whole new thread in and of itself. If the room is well insulated, or has no exterior walls, in-floor electric mats can provide enough BTUs to heat the room space. You'd need to consider the BTUs per sqft that those systems provide. An alternative would be in-floor electric for simple floor comfort warming, and something along the lines of a Runtal electric or hydronic Omnipanel for room heating. The Omnipanel can put out enough BTUs to heat a room.

Wianno - it's honed black Brazilian slate. John and David - thanks for the advice. I talked to the contractor last night and he said he thinks that if we reinstall one of our radiators, we might be better to go with electric and just use it as needed, rather than having an always-on hydronic system.. I will ask about this at the site John suggested, but any thoughts about this? Right now with the demo we have been down to one radiator, and even with a week of below zero temps last week, the kitchen stayed very comfortable.

Your HVAC expert who is designing the system should explain the requirements that the manufacturer has for their particular system. In general, in your situation, you will need lower temperature loops closer together rather than the traditional higher temperature loops further apart. The whole system has to be designed for the water temperature that you will need, as well as the PEX placement distance. Floor coverings also has to be taken into account in the plan. Rubber and carpet actually have insulative power (R values), and are not conductive to easily passing the heat through into the home. That has to be calculated when designing the system, and it will increase the lag time to temperature changes, which is already radiant's biggest drawback.