Once the water is heated, it is kept hot with material called insulation.

VOA: special.2009.04.20

If you are living on means tested benefits AND you live in the hardest-to-heat homes you may be eligible for a free new central heating boiler, insulation for a hot water cylinder, loft, cavity wall and solid wall insulation, draught proofing for doors and windows or renewable energy technologies such as solar panels.

BBC: Battling bills - what help is there?

Assistance could be in the form of installation of draught proofing, loft insulation, hot water cylinder jacket or more effective heating controls and training on how best to use them.

BBC: NEWS | UK | Northern Ireland | Pilot scheme tackles fuel poverty

The 12, 700-square-foot Energy Star-certified house was crafted from low-toxic materials, withstood air tightness tests on the insulation, and has a water furnace geothermal system that provides hot water, radiant floor heat and air conditioning.

FORBES: Off-The-Grid Mansions