I think Super was just asking the question about mixing the water to prevent boiler shock from too cold of water coming back in. I have heard that before, but dont know if there is any truth to it. I have a small water heater inline that helps with thermal storage and probably would help with boiler shock, but my return line never gets that cold. I would imagine if you had a larger heat exchanger, or several, or a sidearm on your water heater, it could bring the temp down. A friend of mine has a sidearm on his water heater and he says that running the hot water in the house can drop the boiler temp in a hurry.