This system operates on the principle of circulation and recirculation. Water heated in a boiler is transmitted through pipes to radiators located throughout the house. At the radiators, the hot water gives up some of its heat. The cooler water then continues to flow back to the boiler where it is reheated and recirculated.
In this system, the boiler, distribution piping, and all the radiators are completely filled with water. When heated, water expands, increasing its overall volume. Consequently, all hot-water heating systems must be equipped with an expansion tank to store the increased water volume temporarily. When the system is shut and the circulating water cools, the volume decreases, drawing the water back from the expansion tank. Without such a tank, excessive pressures could be built up in the system that could rupture the distribution pipes and fittings.
The water circulating within the heating system operates under a pressure that normally ranges from 12 to 22 psi. Although the water is constantly recirculating and there is no need for additional water, an automatic water-feed device is provided with all systems as a precautionary measure. The automatic water-feed device is a pressure-reducing valve. The water supply to the boiler is taken from the house water supply. Since the house supply pressure is normally in a range from 30 to 60 psi, it must be reduced before being introduced into the boiler. The reducing valve is usually preset by the manufacturer to 12 psi.
There are two basic types of hot-water heating systems-gravity and forced. They are classified according to the means by which the water within the system circulates.