Wood stove efficiency varies wildly from stove to stove and yet is poorly understood in terms of what it means for heating your home with wood. Open fireplaces typically operate at about 10% efficiency, basic wood stoves between around 30% and 50% efficiency and modern high performance stoves can reach as high as 90% efficiency.
If you are heating with wood the difference between using a 30% efficient and an 90% efficient stove can mean you burn 3 times as much wood over the course of a year. Not only is this wasteful it is loads of unnecessary extra work; cutting, splitting, stacking, seasoning and then moving your firewood before burning it. Life is simply too short!
What Is Stove Efficiency?
Simply, wood stove efficiency is a measure of how much of the chemical energy in your firewood ends up in your living room, compared to going up the chimney. To measure it you need to very carefully control the conditions and in practice this is done in environmental testing laboratories.
The chemical energy stored in wood is actually quite easy to find. Most types of wood have rating in BTU’s per cord. There is some variation between different tree species but basically each kilo of dry wood contains pretty much the same amount of energy. To find the energy going into the stove you weigh the wood before you burn it.
Calculating the heat transfer into the room is harder – you can get a reasonable figure by measuring the temperature of the air in different places, air speeds due to convection and so forth. there are some relatively complex bits of fluid dynamics involved but it can be done. The stove efficiency is therefore the percentage of energy transferred to the room compared to the total amount of wood burned.
These figures are impossible to calculate meaningfully in the home, which is why it is so important to know the rated efficiency as provided by the manufacturer. Generally older style “airtight stoves” are less efficient and cheap imports tend to be very low efficiency, sometimes as low as 20 or 30%.
What Affects Wood-burning Stove Efficiency?
These factors fall into two main categories.
- Those which effect combustion, how well the wood burns
- Those which effect how well heat is transferred from the stove to the room it is in
We’ll look at both of these separately, and talk about ways that you can improve the efficiency of your existing stove.
Combustion Efficiency
Efficiency of combustion concerns how much of the possible energy in the fuel wood is actually released in the stove. As a general rule hotter fires give more complete combustion and so tend to be more efficient. We can manipulate this thorough wood stove design where we insulate the main stove box to keep as much heat IN the main firebox as possible. This seems counter intuitive when we are trying to get as much of that heat as possible out of the stove!
The hotter the stove is the more the chemical structure of the wood breaks down to smaller and smaller molecules as it burns. The very smallest of these are carbon monoxide and hydrogen. If we have a cooler stove the wood breaks down far less and the larger molecules do not burn properly and pass up the chimney as smoke.
High temperature is only the first step – to make the most of these small, ready to burn, molecules we need to add extra oxygen. The best of the modern stoves add preheated air to a separate part of the firebox where the combustion gases are burned. This is usually referred to as a secondary burn. If we choke off the air these molecules go straight up the chimney without releasing their heat energy in the stove – this is both polluting and wasteful. I’ve heard it described as “turning down the heating by blowing out the gas but leaving the tap running”. The best modern stoves have this secondary air carefully regulated, sometimes beyond the control of the stove operator. This is to help the stove meet air quality regulations but the added benefit is the boost in efficiency!
Turbulence within the stove helps mix the gases so that all parts of the flame have sufficient air to burn. Open wood fires tend to have “lazy” flames and the same can be seen in some wood stoves if you turn the air supply down. Designing the air inlets so that they create turbulent high velocity jets of air yields a nice clean burn.
Heat Transfer
Given that we need the firebox as hot as possible the question remains how to get as much heat as possible from the stove before the hot gases go up the chimney. In an existing stove there is nothing we can usually do about the stove internal features – we might be able to direct the flue pipes around the indoor spaces a little and gain more heat that way. Sometimes we can install a heat exchanger in the chimney pipe, above the stove itself.
The best modern stoves have this heat exchange component built in to the design, so that they extract as much heat as possible from the gases before they reach the chimney.
External to the stove we sometimes improve efficiency marginally by installing a wood stove fan. These increase the flow of air over the surface of the stove, increasing heat transfer to the room. The improvement is small and given the price tag you may find a new stove a better investment!
Are High Efficiency Stoves A Good Investment?
Stoves with efficiencies approaching 90% cost considerably more to purchase because the interior design tends to be a lot more complex and the quality of workmanship tends be be higher. You may consider this a good investment if you do a lot, or all, of you heating with firewood. Firewood costs around $200 per cord (varies considerably with location and type of wood) and upgrading from 30% efficiency to 90% may save you many cords worth of wood over a heating season.
If you are buying a stove for using a few evenings per year then a cheaper stove is probably adequate.