A boiler is a closed vessel that heats a fluid, typically water, for various uses like central heating and hot water supply. A gas boiler works by heating gas, often from the mains or an LPG bottle, to warm water via a heat exchanger, then circulating this hot water to radiators and taps to heat rooms and provide hot water on demand or from a storage tank. A boiler is a closed vessel that heats a fluid, typically water, for various uses like central heating and hot water supply. A gas boiler works by heating gas, often from the mains or an LPG bottle, to warm water via a heat exchanger, then circulating this hot water to radiators and taps to heat rooms and provide hot water on demand or from a storage tank