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this seems like overstating the efficencies gained from condensing. you dont *need* the return water to be cool, its just more efficient for normal operations compared to not using the exhaust heat. if you want water at 60deg and returns at 50deg, its still less energy than if it returns at 40deg.

water goes round in a circuit, one rad to the next. each themostat controls water coming off the circuit into that radiator. yes all the pipework would be heated, but also yes it's probably the cheapest way to run the heating. plus a wooly jumper.