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Any central heating appliance that burns fuel requires air. Builders installing furnaces, boilers, or direct-fired water heaters must provide a source of air adequate for three purposes: • to provide oxygen for combustion of the fuel • to ventilate the space around the appliance, preventing it from overheating • to dilute flue gases and ensure their safe passage up the chimney Depriving an appliance of the air it needs can cause serious problems. A lack of sufficient combustion air can cause incomplete combustion, which in turn increases the production of poisonous carbon monoxide and other dangerous gases. Inadequate combustion air also causes excessive soot formation in oil-fired appliances, which significantly reduces efficiency. A lack of ventilation air, on the other hand, can