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.