Boiler Terminology

BTU - British Thermal Unit. The amount of energy required to raise one pound of water one degree Fahrenheit. 

1,000 BTU = 1 lb of steam

150 BTU = 1 sq. ft. of hot water

34.5 lbs steam/hr = 1 boiler horsepower

1 boiler hp = 140 sq. ft. steam radiation

240 BTU = 1 sq. ft. of steam

34,500 BTU = 1 boiler horsepower

1 gallon of #2 oil = 140,000 BTU

1 cubic foot LP gas = 2,550 BTU

1 cubic foot natural gas = 1,000 BTU

1 KWH = 3,413 BTU

1 therm. natural gas = 100,000 BTU


From The American Heritage™ Dictionary of the English Language, the relevant definitions of boiler and furnace are as follows: 

Boiler - An enclosed vessel in which water is heated and circulated, either as hot water or as steam, for heating or power. A container, such as a kettle, for boiling liquids.  

Hydronics - Proper term for the sciences and technologies associated with heating with hot water (versus steam). 

Furnace - An enclosure in which energy in a nonthermal form is converted to heat, especially such an enclosure in which heat is generated by the combustion of a suitable fuel. 

Heater - An apparatus that heats or provides heat. 

Burner - One that burns, especially: A device, as in a furnace, stove, or gas lamp, that is lighted to produce a flame. A device on a stovetop, such as a gas jet or electric element, that produces heat. A unit, such as a furnace, in which something is burned: an oil burner. An incinerator. 

In our context, a boiler is a piece of heating equipment that is used to heat water for use in a hot water-based heating system. Examples of hot water-based (hydronic) heating systems include under-floor radiant heat, baseboard hot water, radiator-based systems and systems that include fans pushing air through finned coils or radiators. A furnace is a piece of heating equipment that is used in a hot air-based heating system to heat the air that is circulated through the duct work. 

While most of the products manufactured by Alternate Heating Systems are technically boilers (because they burn a fuel that heats water), they can actually be made to operate as a furnace through the addition of a hot-air exchanger. The Outdoor Wood Furnace is technically a boiler, but the industry decided at some point to call the system a Water Furnace and this evolved into just calling the system an Outdoor Furnace. Our waste oil heaters are also technically boilers, but the more common phrase for this type of product is waste oil heater, so we couple that phrase with boiler to describe our waste oil incineration products. In incineration terminology, a burner is the part of the equipment where the fuel is actually burned, however, the word burner is sometimes also used in a more general sense to describe the overall apparatus in which the fuel is burned and heat is produced. An example of this use would be in the phrase 'waste oil burner', which is also a commonly used phrase to describe a device in which waste oil is burned. In the most technical sense, all of our boilers are also burners and/or heaters because a burner or heater is simply a device in which fuel is combusted and/or from which heat emanates. However, a boiler has the second component beyond the combustion of the fuel - that of heating a liquid (in our case water). Therefore, the word boiler includes the burner (or heater or furnace) component, but we use the words heater or furnace as well as boiler in order to fully explain the functionality of our products. However, when the word burner is used in our literature, we mean it in the first, more specific sense - the part of the heating equipment where the fuel is actually burned.

Another distinction between boilers and furnaces among our products is that a boiler is a pressurized system, and a furnace is a non-pressurized system. The advantage of a pressurized system is that oxygen is driven out of the water at around 7 - 8 pounds of pressure and above, making the water become non-corrosive (the oxygen is what causes oxidation, or corrosion, inside the pipes). Also, the boilers can run a higher temperature over 212 degrees without turning to steam thus creating more heat and utilizing the heating system to its fullest capacity.

The disadvantage of a pressurized system is the cost of the boiler itself due to the additional work required in constructing the boiler and the thicker steel that is required to withstand the pressure. Also, some areas have strict rules and regulations that mandate inclusion of ASME code.

A disadvantage of the non-pressurized hydronic heating system is that water constantly evaporates and leaves the system (hence the term "open system") causing the need to add more water. This adds to the sludge and mineral build-up in the system.Circulating the water through a non-pressurized system also requires more power than circulating water in a pressurized system.