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I've seen a number of systems like this in WI, and I really don't see an advantage. By using your dom tank coil, or with triangle tube the outer tank, you've increased the thermal mass of your primary loop by a few gallons at best. granted you will get some exchange within the domestic tank back to your smaller zones, but when those BTU's are gone you ask your boiler to go to high fire to replace them. The real advantage to a mod-con is the condensation at low return temperatures. Using the domestic tank not only depletes your ability to provide plenty of hot water,it costs more to run those small zones. Just install a 50 gallon tank in the primary and deplete that before asking the boiler to fire.