Wood Heat 101
In Alberta, we really experience winter 1/3 of the year, or at least 1/3 of the year in actively heating our building. Edmonton has fewer degree days than Lethbridge, but if not for the Chinook winds and better solar access-there wouldn’t be that much of a difference. Said another way, Edmonton has 5120 degree days, Lethbridge has 4500 which requires more heating for Edmonton. However the daily low temperatures in January and the daily highs in July are virtually the same.
After the North American Ice Storm of 1998, many saw the need for have a wood fired back up heat source. The ice storm took out power lines and some part of Quebec went up to three months without power. No power=no heat as furnaces need power to run.
There are two concerns with Wood Burning Appliances:
The micro particulates that accompany the combustion of solids. While new wood burning appliances pollute less, they still pollute. The flip side is any combustion of any fuel will pollute.
The environmental concern with the release of carbons through the combustion are true however these also get released with the combustion of natural and propane gas in furnaces. Consider this thought though-carbon is released from wood whether it burns or decays. So waste wood, either from construction or dead trees in the forest would release the same amount of carbon if you burned than if you just let it rot.
The other true clean energy that we have used is photovoltaic panel system driving an electric boiler system. Not everyone can afford this system however.
How Wood Burning Stoves Work
Heat transfer in 3 ways, conduction, convection and radiation. While you can work the system with a fan of varying descriptions, Stoves work primarily on radiation(direct line of sight) and conduction(one molecule rubbing against another one, rubbing against another one-and so on). Also, heat moves from hot to cold. Therefore wood burning appliances also function better centred in a space.
There is a combustion chamber, fed by fresh air near the bottom to allow hot waste air to exit the flue. Totally simple, see the schematic below:
A good wood burning stove is in the 70-80% efficiency range, and fit within the EPA guidelines for emissions. This certainly falls short of the high efficiency furnaces and boilers which occupy the 95% efficiency threshold. A positive for wood burning stoves is there are no moving parts to break down, although do require sweeping. The problem in not in the combustion however. It’s in the flue, consider the below image:
The temperature readings above suggest that the lack of efficiency is due greatly to the flue exhausting an abundance of heat. For our American friends the fahrenheit equivalents are 510f and 345f respectively. This suggests the problem is the flue. If you have read any of our previous posts, we often clammer-on about how modern day people sit too highly on the technological laurels. This had been figured out centuries prior by other cold-weather cultures. Generally there are wood burning appliances that are can ‘masonry heaters.’ Masonry Heaters scrub the heat off the waste air and slowly radiate the heat throughout the day. Each culture had their own solution. Check out more about masonry heaters in Wood Heat 102.