alternety... you are saying to pressurise with air while pouring... other info I have says to pressurise with water to keep the tubing from floating up before the concrete cures.

As for the open loop system... are you suggesting that you tee off the water supply to a water heater/boiler which the water is circulated thru? For some reason I was thinking that even when the heat is turned off that a properly designed system kept water circulating at all times so that it stayed fresh and was diverted either into the hotwater tank or hot water supply so as to avoid stagnation.... I need to look deeper into that. Thought the mixing valve took care of that.....