COMBUSTION EFFICIENCY
This is the percent of energy that is released as heat when fuel is burned. Camp fires burning dry wood usually have very high combustion efficiencies. Slow burning water stoves and pre EPA emissions compliant wood stoves have low combustion efficiencies. This is evidenced by smoky fires and the formation of thick shiny creosote.

HEAT TRANSFER EFFICIENCY
The degree to which heat provided within the appliance is transferred to the heating medium and not lost up the chimney. Heat transfer is affected by many factors including the surface area exposed to heat, the length of time that heat is exposed to the heat exchange surface and the availability of a heat transfer medium such as air or water to absorb the available heat. Fireplaces have poor heat transfer efficiency. Hot water radiators have good heat transfer efficiency. Water is a more effective heat transfer medium than air.

OVERALL EFFICIENCY
The amount of useable heat produced compared to the initial amount of energy available in the fuel. When purchasing a heater, this efficiency number is the number to focus on.

• Inefficient appliances and fireplaces produce overall efficiencies from 10-45%
• Typical boilers, furnaces and water stoves produce overall efficiencies from 40-60%
• Tarm boilers produce overall efficiencies up to 85%

Beware of wood burning boiler manufacturers claiming efficiencies over 85%. These efficiencies are probably untrue or are combustion efficiencies only.

Read on to learn more about emission standards.