My whole house hydronic radiant system is supplied by 2 large hot water heaters in my basement.  During the colder winter months, my bathroom which has 2 outside walls, neer seems to get warm enough, and actually is downright cold on really cold nights and days.  I have tried everyting I could think of to remedy this--my installer suggested I insulate the joists in the crawl space under the bathroom, which I have done to no effect.  It just occurred to me that I keep the hot water heaters at aboout 120 degrees.  Should the water temp be higher?  I can put it up to about 160 degrees if that would help my situation.  Any thoughts?  What is the correct temperature I should be aiming for?  Thanks for any assistance.

Bathrooms typically have a very low ratio of floor area to wall area. How many square feet of radiant floor do you have, vs square feet of window, and square feet of exterior wall (and what insulation is in the wall?)

Are the crawlspace walls, foundation sill, and rim joist insulated? Sills & rim joists are notoriously air-leaky, and could be thermally-by-passing the joist insulation. (This is less of an issue if the insulation is snugged up right against the subfloor, but not totally eliminated.)

Is the bathroom on a separate micro- zone, or is it's radiation controlled by a larger-zone thermostat? If part of a bigger zone, unless it's heat-loss characteritics and square footage of radiant floor are in balance with the rest of the zone it will run at a different temp than the rest.

Is this a mini-slab? Warmboard? Staple-up? Suspended tube or... ??? If this is a suspended tube or light-duty staple-up without heat spreader plates, you can double (or more) the amount of heat you get out out of the tubing and into the floor by retrofitting extruded aluminum plates screwed the sub-floor, with a compression-fit groove for the tubing (hopefully a standard PEX.)

With Seattle's relatively low heat loads you should be able to get enough heat in there with 120F water unless it has large & leaky single-pane windows, no wall insulation, and only 12-15 square feet of radiant floor for a 100 square foot room. But if the bathroom is just an adjunct to a larger zone it may need more radiation to better balance it. A heated towel rack type radiator is a common solution to the minimum floor area problem often found in bathrooms.

If you try all of those things that Dana recommends to no avail, and you decide that you want to add some heat output to the bathroom (to avoid raising your water temperature above 120), you might consider looking at radiators (Buderus, Myson) rather than towel racks.

I have the same problem - too little floor area (tubs, cabinets, etc.) in both of my bathrooms - and in the end, I elected to heat some tiled walls to keep my water temperatures down. But on the way, I looked at both towel racks and radiators. I didn't want to go electric (since I had already paid for hot water, and the house was "being built") but I found little information on hydronic towel heaters, though there are a few out there, probably made to work with high temperature water.

But, there's plenty of information on using radiators with lower temperature water; for example, you could get 1300 BTU's / hour out of a 2 foot by 2 foot by 2.5 inch Buderus radiator with 120 degree water. Larger sizes would lead to more heat, and I think that if you can manage the plumbing, they make for good add-on options.

Jeff

Just throwing this out there, our house has 3 inches of foam and since they put the showers in first they never foamed behind them. I will kick myself forever for not making them foam behind them. The worst is the kids bath in the northwest corner. I think the insulation is fine but if air is going to leak in, it comes in there, not so good in winter.

I ended up putting that bathroom on it's own zone and it runs all the time which helps a lot.