Wood-fired Efficiency

When considering the pros and cons of various wood and gas home-heating techniques, the ultimate indicator of efficiency is: ‘How much energy is being consumed to achieve a desired level of comfort?‘  Listed below are a number of key factors:

1 – Combustion efficiency: How thoroughly is the fuel being converted into heat?  With solid or liquid fuel-burning stoves and heaters this is measured primarily by CO (Carbon Monoxide) levels and/or PM (particulate matter of 2.5 microns and less) in flue exhaust gasses.  For slow-burning metal box stoves the CO and PM are best measured per hour of burn time.  Given the prevalence of such appliances, this is the common industry standard.  For masonry heaters (with a fixed rapid burn rate and long-flywheel heat distribution) the numbers are best measured per kilo of wood loaded into the fire chamber.   Because a masonry heater burns a full load of wood in an hour or two and then releases most of the heat slowly over the following 24 hours, measuring emissions per hour of burn time is a flawed parameter.

Slow-burn metal box stoves can achieve close to combustion 85% efficiency under ideal firing conditions.  Fast-burn masonry heaters are more likely to do so on a regular basis because they are less likely to crash into a smoulder due to operator errors.  Some masonry batch-box heaters are getting into the 90s, a few points below propane and LNG.   However, all of the wood-burning numbers get MUCH worse if the wood is damp.  Evaporating moisture requires  a tremendous amount of energy no matter what, and any fire is bound to run cooler until the wood is dry.  So the best thing anyone can do to optimize the combustion efficiency of their stove or heater is make sure the wood is as dry as possible.  Another important practice is loading the firebox to burn from the top down.

2 – Harvesting efficiency: How much of the heat generated is available for use within the dwelling?   Every liquid or solid fuel burning device surrenders a portion of its heat to flue exhaust for drafting function.  Metal box/pellet stoves with draft-enhancing electric fans can improve upon the heat harvest somewhat.  Masonry heaters do so with super hot gasses (usually over 800c) moving through heat-absorbing hollow earthen chambers and leaving 80c or so to float up the chimney.

3 – Utilization efficiencyHow much of the generated heat is warming our bodies?   Here is where masonry heaters have a distinct advantage.  The large radiating surface areas make them comfortable to touch.  Some have warm benches for gentle conduction of far-infra-red heat deep into our bones.  Given that we are ultimately generating heat to warm our bodies, the more directly we can connect with the heat source the less fuel we will consume.

4 – Operating efficiency: How much effort is required to run the device?  With electric, propane, or LNG one merely has to set a thermostat (or maybe flick a switch), pay the fuel company, and try not to think about the insanity of fracking.  With highly efficient wood-burning heaters now consuming 1/5th to 1/10th the solid fuel of the old metal box stoves… a little bit of exercise while connecting with the primordial elements can be good for the soul.

5 – Embodied energyHow much energy is required to manufacture the heater and provide the fuel supply?  This includes a proportional fraction of all the energy expended in infrastructure development, resource extraction, processing, transport and storage of materials involved.  A realistic assessment will also include the costs of ongoing and end-point waste disposal, and all environmental damages from the above.  As challenging as these elements may be to quantify, there can be little doubt that a simple, durable masonry heater constructed from minimally processed earthen materials, and fueled by locally-available, readily-renewable bio-mass… expends the least embodied energy and thereby has the smallest ecological footprint.