HEATING Castle Howard has long proved an expensive headache, requiring tens of thousands of litres of oil at a cost of almost £40,000 a year.

But now a groundbreaking green project has solved the problem by using water from the nearby lake to heat the stately home near Malton, where Brideshead Revisited was filmed.

A vast network of plastic pipes has been installed to absorb heat from water in the shallow lake, which is then passed through a compression system to make it warm enough to heat the house and supply its hot water.

The project by Ecovision, a Gloucestershire-based engineering company which has also installed a heating system at Prince Charles' Highgrove home, is one of the largest of its kind in Britain and is said to prove that natural heat from earth and water can be harnessed to warm buildings.

The £160,000 scheme, part funded by the Carbon Trust, involved temporarily draining the lake last May, removing silt from the bottom and then anchoring 56 plastic coils, each measuring 100 metres, to the bottom. The coils were then filled with diluted glycol anti-freeze, which absorbs heat from the lake.

The system works by pumping the water at a temperature of 10C up to a heat pump at the house via a buried pipe. A heat exchanger there extracts the heat from the glycol and passes it to an internal loop containing a liquid refrigerant. This is then compressed and turns into a hot gas, which is passed to a condenser which heats the water for the central heating system. Cooled water is later returned to the lake.

The project has already started to pay off, with Castle Howard's energy bill for last October, November and December totalling less than £1,500, compared with £16,200 for the same period in 2008.