If wet fuel is not dried sufficiently by the boiler
(because the fuel moisture content is outside the
fuel tolerance range of the boiler) incomplete
gasification and oxidation will occur and black
smoke will be produced. In addition, the tars
released at stage 2 will gradually coat the heat
exchanger surfaces resulting in reduced heat
exchange efficiency and the eventual failure of
the boiler. Tar accumulation is also one reason
why many manufacturers recommend minimum
running periods for their boilers to ensure that
combustion chambers and heat exchangers reach
full working temperature to drive off the heavy
volatiles deposited during the heat-up phase.
The energy used to evaporate the moisture is not
available to the appliance user.