Types of Residential Boilers
Boilers are widely used in the United States and Europe to heat water for residential heating. Some boilers, known as combi boilers can also be used to provide hot water for bathing and cooking. While the primary purpose of all boilers is to transfer heat from gas, oil or electricity to water, different types of boilers work differently, and differ in their advantages and disadvantages.
Hot Water Boilers
Hot water boilers don't actually boil water -- they typically only heat it to between 140 - 180° F, and then distribute it through the building. These boilers typically circulate the heated water through the heating system with the help of an electrical a pump.
The heated water can be distributed through radiators, baseboard heating units, convectors, or even radiant heat systems embedded in the flooring or walls.
Other boilers heat water to (or past) the boiling point, and then distribute the resulting steam through the heating system. Because heating water to steam increases the pressure of the steam, steam boilers can typically circulate water without a pump.
Steam boilers heat water to a higher temperature than hot water boilers, in the simplest sense are therefore considered inherently less efficient. (It takes more energy to boil water than simply heat it, all else being constant.) However, a true apples-to-apples comparison requires that the electricity used to run the pump to distribute hot water through the heating system be included in the efficiency calculation.
High-efficiency versions of both hot water and steam boilers are available.