Backdraft!

Here’s a quick lesson in diesel furnace(*) physics:

(*: or whatever, a wood-burning stove, anything with fire)

The flame in the burner heats up the air, which causes it to rise up the flue and out the chimney.  This, combined with the wind passing over the chimney causing a suction effect on the flue, is called “draft”.  The draft works in combination with a positive pressure in the cabin, created by a combination of vents, wind traps, and the expansion effect of warmer air vs. colder air.  With a balance of positive pressure in the cabin and the draft, all smoke and carbon monoxide exits the cabin through the chimney.

The wind passing over the chimney, the suction effect mentioned above, is similar to blowing over the top of a glass bottle – the flute-like whistling that is made is the air oscillating inside the bottle, unable to equalize because there is no hole in the bottle for fresh air to fill the negative pressure created.

Interestingly, if you were to open, say, a hatch cover in a windstorm, it would be very possible to have this very same suction effect come into play.  If that happened, you could easily create a negative pressure within the cabin.

Now – hypothetically speaking – if one were to create a strong negative pressure in the cabin, a few interesting things might happen.  Firstly, we can probably assume that because the furnace was running, it must be cold outside – creating the suction effect would probably first suck all of the expanded, rising warm air out the cabin.  Secondly, because a negative pressure must be balanced, air would flow in rapidly to fill that void – and as we assume in the first point, that air would probably be cold.  Thirdly, that air would have to come from somewhere, and a convenient port of exit (and by definition entry) would be the flue, destroying the draft and creating a backdraft.

The dangerous part of a backdraft is that fuel from the furnace could potentially leave the furnace in an ignited state, and if the backdraft were strong enough, the influx of oxygen could cause an explosive fire.  With a diesel furnace however, this is quite unlikely, and what would probably happen is that the flame would be immediately extinguished, leaving the backdraft to fill the cabin with smelly, sooty diesel smoke.

Of course, this hypothetical situation could be easily avoided by simply using the barometric equalizer on the flue, in combination with a draft adjuster below the furnace.  One might not leave their warm, cozy bed in a heated cabin to peek outside, only to find oneself having to spend the next forty minutes with all the hatches open expelling smoke from the bedroom cabin and filling the cabin with cold November air.  One might not spend the next hour with the foul taste of diesel soot in one’s mouth, or shiver their way back to sleep without the aid of a furnace.

Well, there you have it.  A public service, by way of a basic lesson in physics.

Be glad you didn’t have to learn it at 3am in a windstorm.