Indirect Water Heaters
If you use a boiler for space heating it may be feasible to install an indirect water heater. An indirect water heater uses a separate boiler as the heat source. Hot water is circulated from the boiler through a heat exchanger inside a well-insulated water heater tank.
An indirect water heater is an excellent option because it eliminates the tremendous flue losses associated with gas-fired storage water heaters and the hassles and extra costs of tankless gas water heaters. When used with a modern, high-efficiency boiler, these energy savings hold true even in the summer when your boiler isn't needed for heat.
These systems can be purchased in an integrated form, incorporating the boiler and water heater with controls, or as separate components.