FLUE PROBLEMS All flues should be checked for blockages caused by falling soot, birds' nests or masonry debris; faults that will usually be revealed by sweeping. This should be done regularly, particularly when wood or solid fuel has been burned. Any holes in the chimney structure should be repaired, as air leakage into a flue will cool the gases, reducing the updraught, and can also cause the emission of toxic fumes. An efficient hearth must have a good air supply. A common cause of air starvation is excessive draught proofing (particularly when UPVC replacement windows have been fitted) and additional ventilation may therefore have to be fitted adjacent to the fire, or a door or window left open. Like all mechanical products the method of fixing cowls should remain rigid and may need to be checked occasionally whilst in use, especially in areas subject to high winds.