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.