What about using SALT, that might solve your freezing problems, from my experience and the description of the system you should be fine. If you want the system to last from 5-10 years, you are going to have other problems before the salt corrodes anything. The copper should be fine, however I don't know what the coils are made of so that's the only things i might worry about. Pool pumps are typically plastic and most of them are designed for both fresh and salt water. Anti-freeze is evil! It sounds like you want to help the environment, so stay away from it. Do it the way nature does, There is lots of liquid water in the artic circles- because its salty.

Thanks for the heads up and suggestions!! --I did realize that the mat was more...but thought it might be worth it in a DIY install...might be just too much more to consider when trying to cut costs. --We have all the time in the world to get this done since we've been living in this house only using one bathroom for five years...so it's nothing new. We could certainly thinset the cable or mat and then wait to cure...no problem. --We don't need the radiant heat to heat the bathroom--it's actually the toastiest room in the house, having a nice big window with southern exposure and a big beast of a cast-iron radiator (another one of my reasons to maybe nix the idea of radiant heat). --The exterior of the house has blown-in insulation and we do have storm windows (although the windows themselves are original and I plan to keep them that way...I LOVE the wavy glass...much to the chagrin of friends/family who think everyone should get replacement windows! Not me!) --We have been upgrading the electrical and this bathroom was gutted down to the studs, so it will be all new wiring. Kitchen was down to the studs too, so little by little, we're changing out all the wiring. Walls are all original plaster (except new bath and some that were changed out in the kitchen). --It's not really a "master bath," per se. The downstairs of this house has one full bath and the upstairs has one full bath--at the back of the house next to the sixth bedroom--actually as far away from the master bedroom as you could be in this big 'ole house! So it kind of has to be an "everyone" bathroom. When indoor plumbing was introduced to this house, they kept it all in the back right corner of the house--for the kitchen and up/down baths. We someday *might* make the designated nursery (a 10'X10' room off the master bedroom & the hallway) into a real master bath...but that would take running plumbing to the front of the house, which my husband thinks would be very difficult and cost-prohibitive. --I don't think the layout of this bathroom is tricky at all--it's just an 8'X6' rectangle (not counting the tub area). The only "tricky" area is that we will not be tiling under the cast-iron radiator--it's got sort-of a painting piece of wood that it sits on, slightly higher than the adjoinging floor...maybe 2'X1'. That's the only tricky area. :) Sarah

Thanks to you all for the input. I have something of an update. The tech came out and checked the well and heat pump and found both working as they should, but was surprised at the high heat level in our registers. He finally decided that our vents were not large enough for the system and were in effect holding the heat in the heatpump (i.e., the airflow is not sufficient. You have to remember this is a retrofit in a 100 year old house.) He recommends, and I think he is right, that we find a way to get more vents working. Another part of the problem is that the vents are located in the center of the house, and the cold air returns are at the perimeters. So the outside walls never really warm up (and remember they are stucco exterior over tile block with plaster interior). The thermostat is centrally located, senses correct temperature in the center of the house, and shuts down the system. But those outside walls are still cold, and in a few minutes the chill reaches the thermostat and on comes the system again. In fact, it runs about 2/3 of the time in colder conditions. It appears the heat pump is probably responsibly for about 60 percent of the electrical draw and the well pump the remainder. The tech set the fan to run longer and turned the heat up a couple of degrees, and that appears to be helping; the run-time is comparable (about 14 on, 7 off) and the comfort level is higher, so he's accomplished something, it appears. I'm supposed to hear from another of the company Monday about the vent question. We don't discharge the water into the well casing; we have a "dry well" (just a small sump) that receives the discharge. We sit over an acquifer that exchanges completely every day and pump from 80 feet (58' static level) and our water temperature is about 48 F. The water we discharge has a long was to go to reach the well pocket, so that doesn't seem to be a problem. The well serves the irrigation system as well as the heat pump.We may yet use well water for part of our household use, might save $10/month to do so. But we're still on city water, the minimum charge for which is about $15.00/month. I think too that we could save an addition $20 or thereabouts once we get the desuperheater hooked up, since we're paying $36ish for our only gas appliance, our water heater. At least if we do that it'll be perfectly clear where the money's going! Thanks again to you all for your help. Stay tuned ... :-)M

Check your initial post titled "Carrier Infinity Hybrid - Final Quote". I ran the numbers for you. Good luck.