If you have spent time at woodheat.org, you know that we don't think outdoor air supplies for woodburning appliances are a good idea, for two main reasons. First, laboratory testing has shown that they are not effective at reducing smoke spillage into the house. And second, if an air supply is connected directly to the air intake of a stove or fireplace, the flow direction can reverse if wind causes a greater level of negative pressure at the outdoor weatherhood than the chimney is producing in draft. This may seem like an unlikely event, but several such events have been reported to us. The video on this page is the first visual proof of combustion gases flowing through a combustion air duct to outdoors.


  • The video is exactly as we received it from the homeowner. There is no sound track
  • This is a Regency Warmhearth EPA certified factory-built fireplace vented straight up two floors and out the roof
  • You can see the short outdoor air duct to the left of the fireplace
  • You can see flame curling up into the exhaust outlet just behind the glass
  • More disturbing, however, are flames curling up into the combustion air inlet slot in front of the exhaust outlet
  • At 1:36 and 3:24 the flames go out when suction at the weatherhood equals chimney draft and flow stops
  • The confused homeowner opens the door at 5:45 and gets a puff of smoke
  • At 7:15 the view switches to the air intake outdoor weatherhood, which has been removed by the homeowner
  • It is very windy outside, as you can see by the movement of the hedge and tree beyond the corner of the house
  • At 8:20 you see a significant amount of smoke from the air duct