Typical boiler types include: Fire-tube and Water-tube. Boilers are classified as "high-pressure" or "low-pressure" and "steam boilers" or "hot water boiler". Boilers that operate higher than 15 psig are called "high-pressure" boilers.
Steam and hot water boilers for heating systems are manufactured for a high or low pressure systems which can use oil, coal, gas, electricity and sometimes waste material for fuel. Some boilers have a dual burner to use gas or oil based on the availability.
- Low pressure & temperature boiler:
Low pressure boilers are rated for a working pressure of 15 psig (1 bar) for steam or 30 psig (2 bar) for water, with maximum temperature limitation of 250℉ (121°C).
These systems are in general used for space heating in residential and commercial buildings.
- Medium pressure & temperature boiler:
Medium pressure boilers are designed to work above 15psig (1 bar) for steam, or maximum 160 psig (11 bar) for water, with maximum temperature limitation of 350℉. (177°C) for water. These systems are used in large hot-water distribution systems like district heating, and in systems where process applications requires higher temperatures than achievable by low temperature systems.
- High pressure & temperature boiler:
High pressure boilers are designed to work with maximum 300 psig (20 bar) for water, with temperature exceeding 350℉. (177°C) for water. These systems are used in large distribution and process application systems.
All steam or water boilers are rated at the maximum working pressure determined by the ASME boiler code section (or other codes) under which it is constructed and tested.
Heating boilers are also classified as to the method of manufacture, i.e., by casting (casiron boilers) or fabrication (steel boilers). Those that are cast usually use iron, bronze, or brass in their construction. Those that are fabricated use steel, copper, or brass, with steel being the most common material.