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Under conditions of incomplete combustion where some of the air is blocked due to lint or other cause, the combustible gas will continue to seek air for combustion and that can be in the exhaust. Increased exhaust suction due to low pressure on the side of the house can also move the hot gases through the exchanger to the exhaust before they can be cooled. Pvc can melt from overheat, exposing unburned gases to oxygen at the point of meltiing enough to leave an opening for air at that point. Water heaters will have air dampers that shut off the air supply to prevent feeding a fire during an overheat chamber condtion in 2004.