The function of a boiler is the same as a furnace: to provide heat so that all parts of the home are comfortable. Boilers fulfill this function in a slightly different way than furnaces.
Hot water boilers, or what some people call hydronic heating systems, don’t really boil the water. They typically heat the water to a maximum of 200oF. Normal operating temperatures for many boilers are in the 120oF to 130oF range, depending on a number of factors, including outdoor temperature, design capacity, etc. There are boilers that do boil water into steam- but that is for a different blog.
Boilers work much like furnaces. Heat is generated by burning coal, oil, natural gas, or propane, for example. Heat can also be generated by electricity. There are electric boilers.
Furnaces use warm air to move heat from the source to various rooms of the house. Boilers use water instead of air to move heat through the house. Furnaces have heat exchanges, and so do boilers. The fire side of the heat exchangers for furnaces and boilers are virtually identical. In a furnace, however, the distribution medium is the house air, and that is what you find on the other side of the heat exchanger. Boilers have water on the other side of the heat exchanger.
Heat is transferred from the fire side of the heat exchanger, through the heat exchanger, into the water. This water is piped to the various rooms of the house, where it is released through radiators, baseboards, or convectors.
In some homes, radiant heating is used where the distribution pipes are embedded in floors or ceilings and heat is released along the entire length of these piping grids, buried in the finishes. In radiant systems, there are no radiators or convectors. There is also a lot less for the home inspector to look at!
Just as the air in a forced-air system is recirculated, so is the water in a boiler system. Cool water is brought to the boiler, heated, and sent to the radiators. The cool water is returned from the radiators through the piping system, to be warmed again by the heat exchanger.