Resources » Glossary of Wood Heating Terms

Glossary of Wood Heating Terms

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Annual Fuel Usage Efficiency. Ratio of annual output of useful energy or heat to annual energy input to heater. Only heat leaving as hot water is deemed usage and calculated as part of space heating efficiency.

a thermostat that measures the temperature of water and opens or closes a circuit.

the solid residue of combustion. The chemical composition of an ash depends on the substance burned. Wood ash contains metal carbonates (e.g., potassium carbonate) and oxides formed from metals originally compounded in the wood.

American Society of Mechanical Engineers — Founded in 1880, ASME is a 120,000-member professional organization focused on technical, educational and research issues of the engineering and technology community. ASME sets internationally recognized industrial and manufacturing codes and standards that enhance public safety.

American Society of Test and Materials — one of the largest voluntary standards development organizations in the world and a trusted source for technical standards for materials, products, systems, and services. Known for their high technical quality and market relevancy, ASTM International standards have an important role in the information infrastructure that guides design, manufacturing and trade in the global economy.




device for measuring atmospheric pressure.

renewable energy made available from materials derived from biomass. May also be referred to as biofuel and includes solid biomass, liquid fuels (e.g. bioethanol and biodiesel) and biogases.

material derived from recently living organisms or organic material which has stored sunlight in the form of chemical energy. These include plants, trees, animals and their byproducts (e.g. manure and crop residues). Biomass is a renewable energy source, unlike other natural resources such as oil, natural gas, propane, coal, and nuclear fuels.

an enclosed vessel in which water is heated and circulated under pressure, either as hot water or as steam, for heating or power.

British Thermal Unit. Energy required to heat one pound of water from 59º F to 60º F. 1 BTU = .000293 KWh (Kilowatt Hour).

BTU Content

Fuel Oil (1 gallon) - 140,000 BTUs
Propane (1 gallon) - 91,800 BTUs
Propane (1 pound) - 21,650 BTUs
Butane (1 gallon) - 102,400 BTUs
Butane (1 pound) - 21,500 BTUs
Kilowatt Hour - 3,413 BTUs


the quantity of heat required to raise the temperature of 1 gram of pure water 1 degree Celsius (3,968 BTUs)

a chemical that accelerates chemical reaction: a substance that increases the rate of a chemical reaction without itself undergoing any change.

involving or causing an increase in the rate of a chemical reaction by the use of a catalyst.

a system that produces heat in a centralized location and distributes it throughout the structure. The most common central heating systems include forced air, radiant infloor and radiant baseboard.

a colorless, odorless, tasteless gas that is about one and one-half times as dense as air under ordinary conditions of temperature and pressure. It does not burn, and under normal conditions, is stable, inert and nontoxic. Although it is not a poison, it can cause death by suffocation if inhaled in large amounts.

Carbon dioxide occurs in nature both free and in combination. Because it is a product of combustion of carbonaceous fuels (e.g., coal, coke, fuel oil, gasoline, and cooking gas), there is usually more of it in city air than in country air. Some sources indicate that the natural balance of carbon dioxide in the atmosphere is growing from its stable level of 0.13% to a predicted 0.14% by the year 2000. It is anticipated that this extra carbon dioxide will fuel the greenhouse effect, warm the atmosphere, and further disrupt the natural carbon dioxide cycle (see Global Warming).

describes a process where fuel is completely gasified and burned, producing a minimum of polluting by-products such as smoke, creosote or ash.

a closed heating system is one that is closed off from the environment and its heating fluid is not changed except for maintenance. (See Open System)

Temperature of outside water entering water heating system; typically 40º F. during cold months in U.S.

a measurement (in percent) of how well heating equipment converts fuel into useable heat energy. Complete combustion efficiency (100%) would extract all the energy available in the fuel, though this not realistically achievable due to stack loss and boiler shell losses. Combustion efficiency calculations assume complete fuel combustion and the following three factors:

• The chemistry of the fuel (the proportions of hydrogen, carbon, oxygen and other compounds) and how much energy is chemically bound in the fuel.
• The net temperature of the stack gases or how much heat is not being used.
• The percentage of oxygen (O2) or carbon dioxide (CO2) by volume after the combustion process or how much O2 did the fuel completely burn.

water vapor condensing into water.

heat transfer across a surface, or transfer of heat through a material by passing from one molecule to another. One of the three types of heat transfer.

the transfer of heat that occurs due to the circulation of hot air. Another one of the three types of heat transfer.

a flammable, tar-like substance caused by unburned wood particulates mixing with moisture. Burning unseasoned wood and/or low firebox temperatures are the most common causes. A creosote build-up inside the chimney or stovepipe can result in unpleasant odors and unsightly discoloration and dripping on the stovepipe. In extreme cases, creosote represents a serious fire hazard for homeowners. Chimneys should be cleaned at least once per year to control to prevent an unsafe build-up.

CSA (Canadian Standards Association)
a not-for-profit membership-based association serving business, industry, government and consumers in Canada and the global marketplace. The organization works in Canada and around the world to develop standards that address real needs, such as enhancing public safety and health, advancing the quality of life, helping to preserve the environment, and facilitating trade.



A radiant heating system where the heating fluid flows directly from the heating unit to the tubes beneath the floor. (See Indirect System)

System used with an indoor heat source where intake air and combustion air are vented to the outside atmosphere.

A system of metal or plastic conduits used to distribute warm air from a central furnace throughout the house.


EPA (Environmental Protection Agency)
EPA leads the nation's environmental science, research, education and assessment efforts. It works to develop and enforce regulations that implement environmental laws enacted by Congress. It is also responsible for researching and setting national standards for a variety of environmental programs, and delegates to states and tribes the responsibility for issuing permits and for monitoring and enforcing compliance. Where national standards are not met, EPA can issue sanctions and take other steps to assist the states and tribes in reaching the desired levels of environmental quality. The EPA also conducts environmental research and sponsors voluntary partnerships and programs.


A system that uses fans and blowers to move heated air through ducts to heat the home.

A hydrocarbon deposit, such as petroleum, coal, or natural gas, derived from living matter of a previous geologic time and used for fuel. All fossil fuels produce carbon dioxide when burned and are a main cause of air pollution. (See greenhouse effect.)

The path followed by superheated gases as they exit the furnace.

The rate a liquid flows through a pipe or plumbing fixture, typically measured in gallons per minute or hour (GPM/GPH).

an enclosed space for the burning of fuel. There are many kinds of furnaces, the type depending upon the fuel and the use to which the heat produced within it is put. Most familiar are the furnaces used in the heating of buildings. In the hot-air furnace, fuel is burned within an inner wall and air, led into a space between the inner and the outer wall, is heated and is led away to the various rooms of the building. Hot-water (hydronic) furnaces, by which water is heated to be led through pipes to radiators, and furnaces that turn water to steam for heating purposes are common.


gallons per minute or gallons per hour -- a measure of the amount of water flowing through a fixture or pipe.

An increase in the average temperature of the earth's atmosphere, especially a sustained increase sufficient to cause climatic change. 

A greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. The greenhouse effect is a term used to describe the heating of the atmosphere owing to the presence of these gases. Without the presence of these gases, heat from the sun would return to space in the form of infrared radiation. Carbon dioxide and other gases absorb some of this radiation and prevent its release, thereby warming the Earth. This is an effect analogous to what happens in a greenhouse, where glass traps the infrared radiation and warms the air. The burning of fossil fuels adds carbon dioxide to the atmosphere,



A device that transfers heat from one fluid to another without a mixing of the fluids, such as air-to-water, water-to-water, or water-to-air.

A measure used to estimate energy requirements for heating. It is calculated by subtracting the average daily temperature in a given area from 65 degrees Farenheit. Yearly totals can be used to compare the severity of the winter in different regions.

AFUE percentage used to describe furnaces.

Description / Efficiency
Low-efficiency furnace / Less than 71%
Mid-efficiency furnace / 72% to 83%
High-efficiency furnace / 84% and higher

Of or relating to a heating or cooling system that transfers heat by circulating a fluid through a closed system of pipes.

Hydronic Heaters (also called outdoor wood heaters or outdoor wood boilers) are typically located outside the buildings they heat in small sheds. Typically, they burn wood to heat liquid (water or a water-antifreeze mixture) that is piped to occupied buildings such as homes, barns and greenhouseso to provide heat and hot water. Hydronic heaters may also be located indoors and they may use other biomass as fuel (such as corn or wood pellets).




A radiant heating system where the heat from the heating unit passes through a heat exchanger before circulating through the tubes under the floor. (See Direct System).

For over 100 years, Intertek has been recognized as the industry leading laboratory that provides companies around the world reliable testing, inspection and certification services. Intertek provides turn key emission, safety and emission services for hearth products (Woodstove, Fireplace, Pellet stove, Outdoor Cooking Appliances, Hydronic Heaters, Masonry Heater, and Gas Appliances.) Intertek has the same accreditations as Underwriters Laboratories, OMNI and other recognized independent testing agencies.




Also called an oxygen sensor, is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analyzed.


A masonry heater is a site-built or site-assembled solid-fueled heating device, consisting of a firebox, a large masonry mass, and a maze of heat exchange channels. It stores heat from rapidly-burning fires within its masonry structure, and slowly releases the heat into the home throughout the day. Masonry heaters currently do not require EPA certification. For more on masonry heaters visit the Masonry Heater Association.


Any gas found in the earth versus manufactured gas.

National Pipe Thread standard.



Since 1979, OMNI has developed specialty services for analyzing and assessing the impacts caused by biomass combustion processes. OMNI has provided these services to large and small public- and private-sector organizations. OMNI-Test Laboratories, Inc. was established as a full service hearth product (Woodstove, Fireplace, Pellet stove, Outdoor Cooking Appliances, Hydronic Heaters, Masonry Heater, and Gas Appliance) testing laboratory with emissions and safety measurement and analysis equipment. OMNI has the same accreditations as Underwriters Laboratories, Intertek and other recognized independent testing agencies.

An open heating system is one that is open to the environment or where the heating fluid is continuously replaced. For example, a domestic water heater is an open system because the heating fluid (fresh potable water) is constantly flowing through it. (See Closed System)


a renewable, densified biomass fuel that is typically made from sawdust. Pellets are extremely dense and are standardized in their size and shape (typically cylinders 1/4" or 5/16" in diameter and less than 1-1/2" in length) and contain less than 10% moisture content. There are currently two grades of wood pellets - standard and premium, with the primary difference being that standard pellets have up to 3% ash content and premium pellets are limited to 1% ash content.

a central heating appliance that uses pellets as it fuel. A typical pellet boiler installation provides automated operation and will include a pellet bin to store the pellets, a feed auger, a pellet boiler, an ash auger or ash bin to collect ash and the flue.

a sweet colorless, viscous, hygroscopic liquid used as an antifreeze, brake fluid, cosmetics and personal care items. The Food and Drug Administration (FDA) has determined propylene glycol to be "generally recognized as safe" for use in food, cosmetics, and medicines.

fit to drink. This is often assocaited with water or other drinkable liquids.


the transfer of heat by emitting waves of energy. Radiation is one of the three forms of heat transfer.

the deflection from a straight path undergone by an energy wave in passing from one medium (as air or gas) into another (such as glass or ceramic) in which its velocity is different.

sources that capture their energy from existing flows of energy, from on-going natural processes, such as sunshine, wind, flowing water, biological processes, and geothermal heat flows. Neither fossil fuels nor nuclear power are considered to be renewable.



visible gaseous product of incomplete combustion. Smoke varies with its source, but it usually comprises hot gas and suspended particles of carbon and tarry substances, or soot. Proper firing techniques and equipment can eliminate or greatly reduce the smoke produced by any fuel. Wood gives little smoke if burned when dry and if the fire is given a good supply of air. However, when wood smoke is visible, up to 60% of the energy in wood is being lost due to the incomplete combustion.

Amount of heat lost per hour (measured in %) while heater is in standby mode (no water being drawn). Loss can occur through jacket or piping. A rule of thumb average is 2 to 4 percent.



Difference between existing and desired water temperature. Number of degrees (ºF) water must be raised, whether from inlet or preheated water.

measure of heat. One (1) therm equals 100,000 BTUs.

the rate at which heat exchange surfaces transfer heat to the transfer medium (e.g., air to water or water or air). It is typically measured as the ratio of BTU output of hot water to BTU input of fuel. Types of heat movement that impact thermal efficiency:

• Conductive/Convective heating surfaces – also referred to as secondary or indirect heating surfaces including all surfaces exposed only to hot combustion gases.
• Radiant heating surfaces – also called direct or primary heating surfaces and consist of heat exchanger surfaces directly exposed to radiant heat from the flame. Radiant heat transfer is tremendously more effective than conductive/convective heat transfer and, contrary to commonly accepted belief, is where most of the heat transfer occurs in a wood boiler, wood furnace or forced air system.

a thermal resistor; a type of resistor whose resistance varies significantly with temperature.

a junction between two different metals that produces a voltage related to a temperature difference.


UL is an independent, not-for-profit product-safety testing and certification organization. UL has tested products for public safety for over a century. Each year more than 17 billion UL Marks are applied to products worldwide. (NOTE: UL is not affiliated with Greenwood Clean Energy.)



the term "boiler" typically refers to a device that converts water to steam for the purpose of heating or power generation. In the home heating industry, the term boiler typically refers to a device that produces hot water, not steam. So, a wood boiler is a device that burns wood to produce hot water for home heating. To eliminate this conflict in terminology, the Association for Testing and Materials now refers to wood boilers as "wood-fired hydronic furnaces" or "hydronic heaters."

a device that burns wood to heat air for use in forced-air heating systems. In certain regions of the United States, the term wood furnace and wood boiler mean the same thing and reference a wood boiler.

a device the burns wood to heat water for use in forced air or hydronic radiant heating systems.

the process that converts organic or carbon-based materials (e.g., wood/biomass) into combustible gases by reacting the solid fuel at high temperatures with a controlled (limited) amount of oxygen.

a central heating device that uses high temperatures and controlled amounts of oxygen to convert wood into thermal energy transported in water or other heat transfer fluid for purposes of heating.


Living areas separated into different spaces, with the temperature of each space controlled independently by a thermostat. Wood stoves and pellet stoves are zone heating appliances and are designed to be installed in the room in which they are heating. By contrast, wood furnace is a central heating appliance that is installed in a convenient location and connected to the existing central heating delivery system.

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