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WOODBURNING. Slow-burning – the dangers.

Wood is an excellent clean-burning, smokeless fuel when burned at high temperatures. It is a diabolical polluting fuel when burned at low temperatures. Worst of all it becomes a fire hazard when green or unseasoned logs are 'cooked' in a hot stove for hours at a time. In such conditions highly flammable tars and creosotes are produced that stick to the flues like glue and are almost impossible to remove.

There is a special temptation to under-run a stove when you want to wake up to a warm house – or if you are away all day and want to return to a home at a comfortable temperature. In theory a woodburning stove may be able to deliver what you want. In practice it is all to easy to set up the conditions shown here. As the hours pass, the temperature in the stove drop steadily and the production of smoke and tars increases. As when wet grass is dropped onto a low bonfire there just isn't enough heat to burn up the fuel and a pall of dense smoke is produced. By the time you return to the appliance it is barely warm, the glass is as black as pitch (it is pitch – literally!) and there is often an unpleasant, sour smell hanging around that catches your throat.

A stove's tolerance to being slow-burned depends on several factors. If your wood is bone dry and the chimney has been fitted with a low-mass liner back-filled with insulating material you may be able to turn down the heat for quite lengthy periods without incurring any penalty. But this will be much harder to do if the stove is too big for the job in the first place – or if it is equipped with a high-output central-heating boiler. (All boilers reduce the fire-box temperatures to a degree, but a worst-case scenario is a high-output boiler served by an out-of-control pump that circulates cold water through the appliance. As a matter of course a low-level thermostat should be fitted that disconnects the pump when the water temperature falls below 50º C.)

To sum up, a "dry" stove (i.e., no boiler fitted) burning fully seasoned logs in a warm (well insulated) chimney may tolerate fairly long, slow burns. A stove equipped with a central-heating boiler, fuelled on wet wood and connected to a cold chimney can never safely be run slow at all. If you are ever caught in this situation the safest bet is to switch to smokeless fuel without delay – assuming the stove is of 'multifuel' design and equipped to burn solid fuel.

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