I'm looking at installing radiant heating behind lath and plaster walls, I have to open up the exterior of the home and installing between the framing would be easier than under floor. Is this possible, and what should I expect for problems? this home is in Minnesota and was built in 1873.
Yes, it is possible, but there are a couple precautions to take note of.
First, you don't want to put the tubing higher up the wall than 4'. This is to help avoid, say, putting a nail through the tubing when hanging a picture. Second, the water temperature through the tubing cannot be higher than 120 degrees. Over this temperature, anything gypsum based (plaster, drywall, etc) will crumble.
Our PEX+ & PEX-AL installation manual (located here) has some good information on the different install techniques.