A room in an old house is often heated by a radiator, a metal device made up of pipes that circulate steam or hot water.
If your house is heated with radiators, there will usually be one in each room, all connected to a central heating system. A radiator gets hot, and it sends that heat out into the room — in other words, it radiates heat. In fact, the word radiator originally meant "anything that radiates," including heat, light, or energy. By the 1850's, radiator had come to primarily mean "heater in a building or house."