A million properties across England could in future be heated by water from rivers, canals and the sea, the government says.
The Department for Energy calculates this is the potential of a technology known as the water source heat pump.
This relies on a heat exchanger, which uses a system akin to refrigeration to amplify warmth from pipes in the sea.
If the energy from the process comes from renewables, properties can be warmed with near-zero carbon emissions.
See this BBC report.