I have a masters degree in fire protection engineering. Why then...do I keep setting off my smoke detectors when I start a fire in my wood stove?
(Note: Yes, there is a difference between smoke alarms and smoke detectors. I recently upgraded to a monitored smoke detection system because it helps me feel better when my dogs are home alone.)
Well it's mostly because I'm cheap. My husband and I love finding new ways to save money and installing a wood burning stove a few years ago was one of our best investments. My husband/lumberjack mostly gets wood for free and splits it himself. We were saving a ton...until lately. With the recent addition of twin toddlers in my household, I've found it less appealing to lug firewood through the house and open up a burning stove adjacent to the living room their playroom. Here is some data to prove my laziness during the last few years (yes, I track this stuff-nerd alert!) :
Luckily oil prices went down, so the spending hasn't been too horrible.
Anyway, I'm working from home today with the kids in daycare. It was a perfect morning to fire up the wood stove. I built my top-down fire and then filled my house with smoke. Luckily I was paying close attention, but if not, my smoke detection system would have made sure I was notified. Why did this happen? The stack effect.
It was around 37 degrees outside this morning and 68 degrees inside my house. The cold air inside of the chimney is denser (closely spaced molecules, less energy) than the air inside my house (more energy, more movement, wider spaced molecules). When I opened the wood stove door, the heavy cold air moved down the chimney into my living room, creating a downward draft (like when you open the freezer and the cold air drops to the floor). When I lit the fire in the wood stove, the rising heat and smoke were not enough to reverse the direction of the draft, causing the smoke to spill out of the stove and into my living room- setting off the smoke detectors. In more scientific terms, the pressure difference between my living room and chimney caused a stack effect. I did the math and found there was only a pressure difference of 0.003 in. W.G. Not much, but enough to cause a problem this morning.
The stack effect can have much bigger implications in tall buildings- especially ones in climates with extreme temperatures. If a 100 ft tall building in Texas is conditioned to 70 degrees but it's 100 degrees outside, there can be a reverse stack effect of negative 0.01 in. W.G which can cause the smoke from a fire to push downward instead of rising up, which is why smoke control and venting systems are so important in tall buildings.
So the lesson I've learned had to learn a few times... is to let the wood stove draft for a few minutes before starting a fire and burn a few pieces of newspaper to help warm up the chimney.
Want to learn more? Check out NFPA's resources on: