The latest energy saving furnace and boiler technologies can also combine high efficiency house heating with hot water heating. If you use oil to heat your home, or if you use a gas-fired high-efficiency condensation boiler, you can also consider an indirect tank water heater which uses the boiler to heat your domestic water by circulating the hot water from the boiler through a heat exchanger in an insulated water storage tank. These heaters eliminate the energy lost up the flue from venting a gas-fired hot water heater. There is general agreement in the industry that energy savings are yielded even in the summer, particularly with a high-efficiency condensation boiler which effectively makes the hot water heater a zone that allows only that zoned area of the boiler to fire up. See our Article Heating Systems Explored

It is also possible to integrate water heating with hot air house heating through systems referred to as hydro air.   These systems essentially take the heated water from the water heater and pass it through a heat exchanger coil in an air handler through which air is blown and heated, and this heated air is then channeled into the duct system.   This system can also double as a cooling system by passing the air through a loop of chilled water in the air handler. 

The efficiency of a combination water and space heating system is indicated by it combined appliance efficiency rating (CAE). The higher the number, the more energy efficient. The CAE ratings vary from 0.59 to 0.90 and specify CAE 0.85 or higher.