Heating with Wood

When I was a boy, I remember going into the forrest with my father cutting wood.  We had an old Elan 12 HP.  It was the ugliest looking skidoo ever but it could pull a massive amount of wood.....and thats what he pulled...and pulled, and pulled.  Can't remember how many cords exactly but I remember we would cut it up and throw it in a pile....and it was the size of a small mountain.  When spring came, we would split each piece with an axe and then stack it.  I remember 3 or 4 tiers of wood stacked 4' high and the tiers were probably 25'-30' long. It would dry for the summer and then we would throw it aboard a truck and bring it to our house where we would then throw it into a shed and stack it again.   It was a lot of work.   It was dirty.  I swore that I would never touch wood again....until now.

I currently use a propane fireplace.  Its convenient....and that is where it ends.  Propane as an energy source is non-renewable, it's shipped in from outside of province and there have been times when the island supply has been depleted...not wood though...it's everywhere.  Sure would have been convenient to have had a wood stove during all of those rolling blackouts a few years ago.  Wood can be good!  Besides being a heat source, the hours of mesmerizing, hypnotic flames are enjoyable to watch.

Now lets talk energy.  A 2000 sq. ft. Passive House modelled with the climate data for St. John's would require a total yearly demand of 25.3kWh/sq. m.  So the total heat demand for the house would be about 5000 kWh.  This is an amazingly small number.  A cord of dry birch contains about 21.6 million BTU or 22.9 GJ (Giga Joules) which is equivalent to about 6400kWh of recoverable energy.  Many EPA stoves have an efficiency of about 80%.   So 1 cord of birch would have a combustion heat value of 80% x 6400 kWh = 5120 kWh which is almost equal to the 5000 kWh yearly demand.  One cord of wood is a pretty small amount.  In these homes, about 1/3 of the heat requirement is taken care of by solar gains.  In which case, I am expecting that the house could be heated with about 2/3 of a cord of wood.  If you include internal gains then that may make up to another 1/3 so you would only require 1/3 of a cord of wood to heat a house for a year!!!!  Now thats impressive!!!

The biggest issue with wood stove is finding one small enough to meet the peak load criteria of 13.3 W/sq. m.  At 13.3 W/sq. m., the total wattage required for this home will be about

13.3 W/sq. m. x 195 sq. m (ie 2000 sq. ft.) =  2.6 kW

In the world of wood stoves this is pretty small.  Even small stoves are usually around 4 kW.  You could burn the stove with less oxygen by closing off the damper but it leads to less efficient burns, more smoke and more wood consumption.  Pellet stoves are another option.  The burn is hot, but loads are small and the pellet use rate can be altered to meet the demand.  Pellets are convenient but almost all pellet stoves require electricity.    I have been considering a wood gasification boiler (www.walltherm.ca).  It has the benefit of being a boiler also. By adding reflective glass, and a insulation kit, the output of the stove is decreased to 2.5 kW.  A water jacket inside the stove soaks up about 12.5 kW of the energy exiting the flue which leads to an efficiency of 93%! This water is pumped in a circulation loop to a thermal storage tank.  So now your heat source can also provide your hot water....during heating season....more on this later.

As a renewable resource, wood is a good option if you are using a high efficiency stove.  Since the carbon that you release into the atmosphere was carbon that the tree soaked up from growing in the ground, the process is carbon neutral.  Its much more efficient than those electric baseboards heating your house connected to a grid thats operates at 30% efficiency.    Get a wood stove, pat yourself on the back...you just did something good for the planet!

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