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.

An automatic water make-up device is usually found in a closed hot water system to add water when needed, whereas the open system, water is added manually. Also on a closed system there is a special valve between the cold water plumbing pipe and the boiler. The connection between is not direct because of the fluctuating pressure and being too high for the heating system. Therefore the use of a reducing valve is installed to control the right amount of pressure. Another caution would be to protect the system from impure water getting in. This can be controlled by having a back-flow preventer. Today automatic water make-up systems have this feature.

Safety Devices are also built into the boiler. One of these devices ensures that ignition has taken place. The oil burner or gas valve shuts down if proper ignition has not taken place. Also, there is a device for a high temperature limit. If the water within the system goes above a safe temperature (approximately 200 degrees), the system will shut down.

On a closed system a pressure relief valve is provided and will discharge water should the system exceed thirty pounds per square inch. With an open system this device is not needed as they are not pressurized and the excess water will flow to the expansion tank.

Distribution of heat throughout the home will depend on the space available and the features desired. Some types to choose from include:

Radiator: Is made of cast iron and most have a control valve at one end. With this valve you can control the water to the radiator, thus adjusting the heat that is produced. Normal operation of this valve is to leave it wide open and leaking water if turned is common.

Convectors: Convectors are made of either cast iron or copper tubing fitted with aluminum fins. They are a space saving of less than twelve inches high whereas radiators may be twenty-four to thirty-six inches high. Although you save in height space convectors have to be longer than radiators to produce the same amount of heat. Another drawback to convectors is that they heat up and cool down more quickly causing uneven heating.

Radiant Heating: Some hot water systems involve piping buried in floors or ceilings. The pipes are heated and then radiate heat to the room. Recently, flexible plastic tubing has been used were as previously galvanized steel, black steel or copper was used. The pipes are buried approximately three inches below the surface and are eight to sixteen inches apart.