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Wood heating: efficiency and heat production

A stove that warms well is the definition of a device that allows wood to burn slowly and with minimal heat loss. To obtain this result, the ratio between the power of the appliance and the volume of the room to be heated must be taken into account, the reference ratio being 60 to 70 Watts / m2 for a ceiling height of 2.50 m.

Power of wood heating

The power of the device is conditioned by the volume to be heated, the design of the house and its insulation. Another important factor also comes into play, it is necessary to choose between the pleasure of taking advantage of the flame without constraint (open hearth, chimney) between the efficiency of the production of heat (closed hearth, insert).
Finally, it is also necessary to take into account the quality of the fuel (moisture content of the firewood) and the adjustment of the apparatus.

What is a stove that heats well?

It is a slow combustion device whose fire is controlled. A good fireplace or stove retains heat instead of letting it escape through the chimney. Technically, a slow combustion device converts combustion into heat energy. It burns the wood efficiently and diffuses the heat in the house.

What should be the power of my stove or fireplace?

It depends on:

  • The size of your house / room
  • The insulation of your house
  • The design of your house
  • What you prefer: visual pleasure or heat production

• The European regulation foresees 60 to 70 Watt per m² for a ceiling height of 2.50 m.

Performance and heat production of wood heating

The most important performance factors are:

  • The air capacity of the stove
  • The extent of the radiation surface
  • The type of wood and its moisture content
  • The insulation of the house
  • And the way the stove is set

An efficient stove will burn the wood slowly. A good adjustment of the air intake makes it possible to control the burning time.
The three phases of wood combustion:

  • Evaporation of water
  • Transformation into coal and gas
  • Coal combustion

There are many stoves that let unburned gases escape through the duct. These gases can represent 60% of the heat potential of wood. The presence of oxygen - at a temperature of 350° to 600° - makes it possible to burn these gases.
Some stoves have a secondary air inlet to burn the gases and particles, which would normally escaped unburned; these devices have a higher yield.
A slow combustion appliance usually has a primary air inlet. A secondary air inlet makes it possible to burn the gases and particles more efficiently at a lower temperature. This is called clean burning technology; it improves the yield by almost 40%.
The best yield will be obtained with a dry wood. Wet wood will generate less energy and will also cause creosote formation in the duct as a result of incomplete combustion.

  • 1 kg of wood equals about 3.8 kW (at 100% efficiency)
  • 1 kg of wood burned in an open fireplace generates about 420 W (12% yield)
  • 1 kg of wood burned in a traditional fireplace generates about 1.9 kW (50% efficiency)
  • 1 kg of wood burned in a clean burning fireplace generates about 2.8 kW (75% efficiency)

Wood heating: efficiency and heat production: wood

The amount of energy generated depends on the moisture content. The ideal humidity level is 20%, which corresponds to 2.8 kW per kg at a 75% efficiency level.

Wood heating: efficiency and heat production: heat

Dry wood generates more heat per kilo but reduces the burning time.

Wood heating: efficiency and heat production: heating

(photos / visuals: © Jøtul, except special mention)

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