There are three main types of residential heating systems: Hot Water; Forced Hot Air; and Steam. The source of hot water and steam systems is from a boiler. The boiler literally boils water and then circulates it through either baseboards or steam radiators. With baseboards or steam, areas can be zoned to different parts of the house which are fed through a different circulator or zone valve for each designated area. Each zone will have its own thermostat. A steam system is circulated through cast-iron radiators or convectors. You will find many older homes were built with steam systems.
Another system, called Radiant Heat, also uses a boiler as the heating source and the hot water produced is circulated through a series of tubes which are built beneath the surface of the floor, usually under ceramic tile. Heat then radiates from the floor and heats the area. Radiant heat may have multiple zones.
The source of a hot-air system is the furnace. There is no hot water involved here. The furnace incorporates a heat exchanger which is heated by the flame produced by the burner. A blower then blows, or forces the hot air through only one zone of duct work. Usually, there are no multiple zones with forced hot air systems unless the home is very large. In that case there will be two independent furnaces with separate duct work.
There are advantages and disadvantages for each system. Forced hot air systems produce dry heat and this condition may cause some people, who have respiratory conditions, discomfort. Built-in humidifiers are used to alleviate this problem. On the other hand, these systems are more efficient in that unlike boilers, the furnace does not maintain an internal minimum water temperature. Since water systems boil water and then circulate the hot water, the boiler must maintain an internal temperature. Therefore, energy is used to maintain this temperature. If you have a boiler, check the dial on the front - it's called an aquastat, it will read anywhere from 170 to 220 degrees, depending on how its adjusted.
There are now new, more efficient model boilers that are designed not to maintain internal temperatures. These boilers are called "three pass boilers." They are manufactured usually in Europe; notable brands are Biasi and Buderus. Most American made boilers are one-pass systems.
Domestic hot water is produced either by a separate water heater with its own burner or is produced internally from the boiler through a coil. With a separate water heater, a flame is produced by either an oil-fired or gas fired burner. Water heaters can be sized anywhere from 30 to 100 gallon capacity.
Another source to heat water is from an Indirect Hot Water Heater. They must be connected to the boiler with a separate circulator. Water supplied by the boiler is maintained hot through an internal heat exchanger, usually a steal tank within the indirect hot water heater itself. Indirect hot water heaters do not have burners.
Since a furnace uses a heat exchanger rather than water as a heat source, domestic hot water may only be provided through a separate oil or gas fired water heater (electric water heaters are also available), and not an indirect hot water heater.
Depending on the age and condition of an boiler, a new, three-pass boiler can save you up to 50 percent in your heating bills.
