I have recently had the Biowin pellet boiler installed my home, leaving the oil boiler in place. The plan is to use the pellet boiler for DHW in the winter and the oil boiler for DHW in summer. I have a 40 gallon inderect Superstoe. Some of you have suggested getting a heat pump for DHW. I have looked into it a little and it seems there are heat pump water heaters that include the tank and what's called add-on heat pumps to augment an existing tank. Can someone please explain in more detail wht the difference is?