A hot water system is a system that heats water and distributes it throughout the house by way of piping. Even though a boiler may be used to heat the water it is not actually boiled but heated to approximately 160 degrees. Some hot water systems may use water converted to steam to produce heat.
A Closed System is considered to be more modern. This system is under pressure in the boiler, piping and in the heat source within each room. The pressure in the system is a few pounds higher than what is required to force the water up to the highest level within the house. Approximately twelve to fifteen pounds per square inch of pressure is typically used. The air pressure builds as the system heats up and the water expands. The air then gets trapped in an expansion tank or cushion tank which is located near the boiler. As the water expands, it fills the expansion tank, compressing the air. The excessive air pressure build-up is prevented in the system as the water gets hotter. Closed systems usually have a circulating pump to force the water through the system.
Open Systems were an earlier form of hot water systems. These systems were not under pressure although there was an expansion tank. This tank was usually located above the highest radiator within the house usually found in a closet on the top floor. The expansion tank has very little water in it when the system is cold. A clear glass that covers the tank allows you to view the level of water as well as add water when needed. The tank fills up when the system heats up with an expansion overflow pipe to handle the extra volume of water. As the water drains out the overflow pipe it is drawn out to a roof or floor drain. Open systems normally do not have circulating pumps, the water is moved through the system by gravity.
Two major components of a boiler with the exception of electric boilers are a heat exchanger and a burner. A heat exchanger contains the burning fuel on one side and the water to be heated on the other. As the flame heats up the metal heats up the water. The heat exchangers are usually made of cast iron, steel or copper. A burner is required to burn fuel with in the heat exchanger. The most common fuels are natural gas and oil. The burners on hot water systems are very similar to warm-air furnaces.