The boiler is the part of the system that heats the water to be distributed. The key elements of a boiler include the burner, combustion chamber, heat exchanger, exhaust stack, and controls. Boiler accessories including the flue gas economizer are also commonly used as an effective method to recover heat from a boiler.
There are two general types of boilers: ”fire-tube” and ”water-tube”. Boilers are classified as “high-pressure” or “low-pressure” and “steam boiler” or “hot water boiler.” Boilers that operate higher than 15 psig are called “high-pressure” boilers. A hot water boiler, strictly speaking, is not a boiler.