There are two stages of combustion in wood, primary combustion and secondary combustion.
Primary combustion is the burning of the solid materials, i.e. the wood, what causes the heat in this stage is the breaking of the carbon bonds where the energy is stored. This is the second transfer of energy in the process of heating your house; the stored energy in the wood is being transferred to radiant energy when the carbon bonds break.
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Secondary combustion is the burning of gas fuels, which is where the flames of a fire come from.
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