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What is 'backdrafting' in a home?

Backdrafting occurs when combustion gases from an appliance enter the home instead of traveling out the chimney. This is a dangerouse situation that can potentially harm or kill the home's occupants.


One of the signs of backdrafting is melted plastic rings around the hot and cold water pipes protruding from the top of the water heater, on either side of the exhaust vent. This situation was identified at a recent home inspection (see first picture). Notice the bubbled appearance of the blue and red plastic rings?





There are multiple reasons this can occur. Further investigation revealed that the water heater vent connector (exhaust pipe from the water heater to the chimney) entered the chimney below the vent connector for the boiler (the home's heating system).


Two gas fired appliances can vent into the same flue, however, the 'smaller' appliance must be connected to the flue above the 'larger' appliance. In this case, the smaller appliance (water heater at 40,000 BTU) is connected to the flue below the larger appliance (boiler at 90,000 BTU). See picture 2, where the water heater vent connector is on the left, and the boiler's vent connector is on the right.





Connected in this way, and when both appliances are simultaneously exhausting hot combustion gases, the water heater's exhaust gases can not properly draft up the chimney as they are trapped below the boiler's hotter exhaust gases. The water heater's exhaust gases then backdraft, and exit into the basement space. The melted plastic rings are the visible result of the backdrafting.


Although this situation only occurs during the heating season, and only when both appliances are 'on', the situation is hazardous and should be corrected.

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