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Using hot water bottles and electric blankets should work out cheaper than turning up the radiators.

The only heat source in the school is a small electric radiator in the second-floor office of the director, Mohammad Ibrahim Barakzai.

Mr Kashti was punched and kicked, suffering injuries to his face and head, and handcuffed to a radiator before being told he "would be killed" if he tried to escape.

And because they heat water to a lower temperature than boilers, they work better in well-insulated homes with larger radiators or underfloor heating.

That may include turning off radiators in unused rooms, switching lights off when they are not needed, and not leaving electrical appliances on standby.