A hot-water heating system (Fig. 175) operates by the movement of hot water from the boiler to radiators, where it gives off heat. The colder water, which has already given off its heat, returns to the boiler to become reheated. By the circulation of water, heat is conveyed from the boiler to the room. The movement of the water is due to the fact that hot water is lighter, or in other words its density is less, than cold water; hence it will rise and more cold water will come in to take its place. This movement will continue so long as there is a difference in temperature in the system. One sq. ft. of heating surface is required for 30 to 60 cu. ft. of space heated.