I'm not an expert, but I doubt there would be a problem. Good plumbers will typically set tubs in a mortar bed, so there is that extra separation between the heat source and the tub. Besides, tubs see hot water temps all the time, acrylic FG units are used as steam shower enclosures, etc. So it's pretty apparent acrylic isn't a cold-only material. Yes, it would help to keep the tub water warm!

A couple inches, enough that it will help evaporate the leftover water in the cavity. The door will stay open at pretty much whatever angle you want to leave it at but a couple inches s/b plenty. My DW always forgets to leave it open and often when I go by and open it up (you can see water vapor on the door when it's wet inside) she'll go behind me and close it as she can't stand doors left open and doesn't consider why it may be open. She's getting better about it, I've even had guest walk by and shut it thinking someone must have forgotten to close it all the way. It's not a big deal, you can wipe it out or run a straight convection mode to dry it out.

oh well rub it in why don't you! :) We actually don't get a lot of pouring down rain where we live, but lots of drizzling. As far as the water issue, pumping is really not an option at this point. We have all the tubing and foamboard installed/stapled. Hubby says it will be ok... just wondered if anyone knew for sure. :)

The short story is we chose radiant because it worked the best with the floorplan / material choices we made. The long story is how we got there. We had the choice of a basement, crawl space, or slab. Basements are common in my area because of the required 3' footers - it doesn't cost much to go deeper and have a basement. Financially they make sense, but I don't care for them. They are cold, dark, and low ceilings feel oppressive. Crawl spaces are nice to allow access, but the floors were crazy cold in winter and the amount of humidity could also turn into a mold problem. We chose a slab foundation before we had land, and it worked out since the land we built on would not have supported a basement. Our slab has xps foam board under and along the edges for insulation. Our floor plan is completely custom that I did in autocad myself. All the common areas are on the south side and open. The bedrooms and bathrooms are on the north side. With an open floor plan (some of it 17' tall rooms) and concrete floors it makes forced air ducting more difficult. My experience with forced air is that it blows dust and curtains, feels like a cold draft across your neck when it first kicks on and forces the cold air out before the heated air, and is noisy enough to have to raise the TV or conversation volume when its on. Choosing radiant floor heat with separate zones for the common and private areas seemed like the perfect choice for heating. But we do use our air conditioner for 6-8 weeks and the floorplan layout was critical in being able to use a wall unit to comfortably cool the whole house. I was planning on finishing the concrete with coloring and waxing of some kind, but my husband became friends with a person who polishes commercial concrete floors and we had him polish our floors. It looks wonderful, and is easy to clean. The radiant heat is quiet and even heating with about a 2 hour lag time. I also thought it would be warm on my toes, but I learned that it's not - even in the middle of winter, and want to make sure others are aware of that so they are not disappointed. It's not cold, but its not warm either. Our upstairs is a quality pad and carpeted which feels nicer to my bare feet than the cool concrete. Each house and owner is different so I hope you find the best solution for your situation.