Heating With Wood – Wood Stove Efficiency

When it comes to heating with wood, there is a question I usually get from wood heat aficionados, “How efficient are your stoves?” My answer is usually, “Just as efficient as I can make them.”

For most of us, our concern for wood stove efficiency is commensurate with the price of wood that goes into the stove. At least that’s the way it is with me – to some extent. My wood sources are free of charge, so I’m not concerned so much with efficiency, but I do take measures to increase the efficiency of my stoves. I might as well get as much return on investment as I reasonably can.

Generally speaking, the more you pay for a stove, the higher it’s efficiency will be. Modern stoves that cost $2,000 or more tend to be well built with air injection and hot gas scavenging capability that reduce emissions and maximize the heat you get from the wood. They might be as high as 70% efficient.

Less costly stoves in the $300 range are little more than cast iron boxes with a door to toss in wood, and a chimney at the other end that gets attached to the flue. Efficiency might be 40% or less.

Efficiency of older stoves can be improved in several ways:

  • Use a fan to rob hot air off the surface of the stove. I blow air on the face and top of my fireplace insert in the kitchen. It pushes hot air off of the stove and down the hall, greatly improving the efficiency of the stove over simple radiation and natural convection.
  • Use a stove pipe heat exchanger to rob heat off the exhaust and blow it into the room. These products are thermostatically controlled to operate when stack gases are hot, and turn off when stack gases cool down.
  • Install a baffle in “straight through” stoves that requires the flame to take a longer route before leaving via the flue. The longer the flame stays inside the stove, the more heat is given up to the stove instead of going up the stack.
  • Improve combustion efficiency with enhanced air intake. This results in cleaner combustion and higher internal temperatures.
  • Rob heat off of the stove surfaces in the form of a heat exchanger to use the energy somewhere else in the house such as domestic hot water or hydronic heating.

Even the simple act of blowing air across a stove can force combustion air out of the stove and into the room, so do this with great care. Having a smoke and carbon monoxide detector in the house is always a good idea.

Please note that any modifications to wood stoves will void warranties, and may create hazards in terms of carbon monoxide poisoning, release of sparks, and overheating of the stove. Modifications are ill-advised unless you are very familiar with the combustion process, and are willing to do extensive testing to ensure that your modifications aren’t going to create a health or fire hazard.

Also, if you’re going to incorporate hydronic heating or domestic water heating, you’ll need to take care to install expansion tanks, pressure relief valves, and otherwise make certain that failure of the system doesn’t cause a flood in your house.

Lastly, check your insurance policy. It might not cover any damage or injury covered by a wood burning appliance that has been modified.