Of the boiler types there are two definitive boiler categories of the most commonly available boilers for residential and commercial use. The first and most efficient boiler is the Condensing Boiler (for more information on how a condensing boiler or furnace works please see the Furnaces Page 2. The next type of boiler is referred to as the Conventional Boiler and among these boiler types there are three boiler sub-categories.
Commercial Fire Tube Boilers (Approximately 24 Million BTU/h Combined) hot water boilers, boiler preventive maintenance, gas boiler

    * Cast Iron Boilers - from the smallest residential boiler all the up to 10 million BTU/h boiler the cast iron boiler is the most durable. Cast iron boilers are commonly sectional boilers so the boiler can be torn down to get into tight spaces and then rebuilt. These boilers are most typically used in residential or light commercial heating applications and can be retrofitted with a domestic heat coil for domestic hot water use. For heavier commercial boiler use several of these boilers can be set up on a manifold piping system and offer boiler step-up sequencing to meet higher boiler heating demands when needed. Cast iron boilers can provide either steam or hot water heat in low pressure boiler applications. Boiler combustion efficiencies range from 75% to 93% depending on (whether it is steam boiler or hot water boiler) and boiler combustion controls, boiler flue dampers, sequencing of multi-stage boiler set-ups, frequency of boiler tune-ups and/or air and/or water pre-heaters (boiler economizers).
    * Water Tube Boilers - from 10 million BTU/h boilers all way up to 300 million BTU/h boilers these boilers are generally found in medium to large commercial/industrial use and can be either steam boiler or hot water boiler in low to high pressure boiler applications. These boilers can be either oil boiler, coal boiler, or gas fired boiler and pass hot flue gases around tubes filled with water. Boiler combustion efficiencies depend on several factors for these boilers including: whether it is steam boiler or hot water boiler, boiler combustion controls, boiler flue dampers, frequency of boiler tune-ups and/or air and/or water pre-heaters (boiler economizers).
    * Fire Tube Boilers - from .6 million BTU/h boiler up to 50 million BTU/h boiler these boilers use hot flue gases passing through tubes submerged in water generally found in medium boiler to large boiler commercial/industrial use and can be either steam boiler or hot water boilers in low to medium pressure boiler applications. Again as with the water tube boiler combustion efficiencies depend on several factors as noted above.

One other type of boiler not mentioned above is the electric boiler. One could say that an electric hot water heater is an electric boiler although there are electric boilers that can heat water to steam temperatures. Boilers can use many types of fuels (oil, gas, coal, wood, and electric power) to heat water (or other liquids) but the main focus of Boiler Types article was to categorize the mechanical aspects and differences of boilers.

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