Wood-burning stoves are highly efficient, using anywhere from 70-90% of the energy in the firewood to heat the room.
This means stoves can have as little as 10% heat loss, although higher efficiency depends on several factors.
These factors include fuel quality, chimney height, appliance operation, and the combustion process deployed by the stove.
An efficient stove produces more heat for the room, so in theory, you could have a smaller stove with a lower heat output for the same effects.
This guide explores everything you need to know about stove efficiency, including how it is measured and how to improve it.
How is Wood Stove Efficiency Measured?
There are two ways to measure stove efficiency:
However, as the EPA points out, many manufacturers use combustion efficiency because it looks better as the higher of the two measurements.
While a higher combustion rating usually means higher overall efficiency, it can be as much as 20% lower than the overall efficiency.
This can skew your research when choosing a wood-burning stove, so it is crucial to determine which measurement the manufacturer uses.
Don't be afraid to seek clarity before committing and move on if you don’t get an answer.
The Efficiency of Modern Wood Stoves
Modern wood-burning stoves are between 70% and 90% efficient.
At the lower end of the scale, we have non-catalytic stoves with a single combustion source (firewood) and a fast burn time.
Non-catalytic stoves have come by leaps and bounds over the last two decades. Models made before 2010 were usually around 60% efficient, while models made in the last five years are between 70% and 80% efficient.
However, the most efficient stoves are catalytic stoves, which have a catalytic device that ignites the smoke and ash as a secondary fuel source.
Catalytic stoves are between 80% and 90% efficient, converting more fuel into heat for your room, and they also have a significantly longer burn time.
When choosing a stove, catalytic stoves usually offer the best efficiency ratings, but they are best for primary heating requirements (all-day burn). If you want occasional heat, a non-catalytic stove is cheaper and more than efficient enough.
We might see stoves between 95% and 99% efficient in the future, although this would probably require a third combustion process.
Factors Contributing to Wood Stove Efficiency
Wood-burning stoves provide an enclosed chamber (firebox) for fire, keeping waste gases inside to extend the burn time versus an open fire. You use vents to control airflow to the fire and, in turn, the fire temperature.
Catalytic stoves take an extra step, burning the smoke and ash from the fire and using the heat as a secondary combustion process.
The enclosed chamber – and the catalytic device in some stoves – boosts efficiency to 70%-90%, versus around 15% with an open fireplace.
Additionally, the thick steel casing absorbs and radiates heat long after the fire dies, helping prolong the heating effect.
Over the last few decades, stove manufacturers have also innovated, creating casings and controls that make stoves more efficient.
Some of these improvements include:
What Can Make a Wood Stove Run Inefficiently?
Although stove manufacturers provide efficiency ratings as percentages, you might not achieve anywhere near the numbers.
Most efficiency problems arise from using unseasoned wood, packing the firebox with too much or too little fuel, and starving the fire of oxygen.
Simply put, efficiency might be a percentage to manufacturers, but real-world efficiency depends on the proper use of the stove.
Here are the most significant factors that affect wood stove efficiency:
It can take several weeks or even months to learn the best settings for your stove, but this intimacy is all part of their charm.
How the Moisture Content of Wood Affects Burn Efficiency
Wood is only suitable for a stove when it has been dried to 20% moisture or less.
There is an obvious reason – water doesn't burn! Green and wet wood burn slowly at a low temperature and may even go out if wet enough.
Firewood with a moisture content below 20% burns more quickly and hotter, increasing the efficiency of your wood-burning stove.
When wood has a high moisture content, it has a much lower net energy density by a mass due to the weight of the water. So much energy is lost by water converting to steam that the stove temperature doesn't increase much.
There are more problems with burning green or wet wood, too, such as producing more emissions, smoke, and pollutants, which can produce a layer of creosote in the chimney and flue, which increases the risk of fire.
The moisture content of green wood is as high as 80%, and you will find it impossible to light unless you throw it on a raging fire.
The ideal moisture content is 15-20% - any lower, and the wood can be too dry and burn too aggressively and quickly for long-lasting heat.
You can buy kiln-dried or seasoned wood with the right moisture content. Kiln-dried logs usually burn hotter and longer because they are dried right through - seasoned wood usually has a little more moisture for a lower price.
Yellow Birch, Black Birch, Oak, and Elm produce excellent firewood. Buy locally to ensure the lowest carbon footprint and best deals.
Tips to Improve Wood Stove Efficiency
Making your wood-burning stove more efficient means converting more energy in the wood into usable heat for your room.
Here are several tips for doing just that:
Use kiln-dried/seasoned wood
Kiln-dried and seasoned wood burns hotter and faster because it has less water. It takes significantly less time to release heat with less energy required to evaporate water, which increases the efficiency of your stove.
The worst thing you can do is throw a damp log or green wood onto your stove because it won't burn hot and will also sizzle, pop, smolder, and smoke.
Perfect the art of fire-building
While the bottom-down fire method, where you put small wood at the bottom of the fire to start it, sounds logical, the opposite is better for a stove.
The best way to build a stove fire is the top-down method, where you place larger logs on the floor of the wood stove firebox, with smaller pieces of wood, kindling, and newspaper on top. This creates a cleaner, more efficient burn.
Because fire burns from the top of the stack, you don't have to worry about checking on the fire to ensure the large logs have caught fire.
Get a temperature gauge
Although you can feel the heat piping off your stove to adjust the fire ferocity, it's best to use a temperature gauge for fine-tuning the heat output.
The ideal stove temperature for comfort is between 500°F and 800°F (260°C to 426°C), with most cold winter nights calling for a temperature in the middle. A temperature above 650°F will maintain flames when you add new wood.
Play with your stove's settings
Key to your wood-burning stove's efficiency is a clean and steady burn. Playing with your stove's settings is the best way to figure out when to add more air, which vents to use, and how much wood to pack into the firebox.
It can take several seasons before you know your stove's best settings. We recommend reading your stove manual and manufacturer guide for best practices and adapting your techniques from these. Forums are also helpful.
Know when to add more fuel
Opening your firebox too early or late will impact stove efficiency. Knowing when to add more fuel ensures you keep your fire raging without a significant drop in temperature and prevents over-fueling, which can also lower the temperature.
As a rule of thumb, it's time to add more wood when the logs are mostly glowing red embers. You have left it a little too late when over 30% of the ambers are ash, and you will need to add smaller pieces of wood again before adding logs.
If you have a catalytic stove, you won't need to add more logs for several hours because the burn time is significantly longer, thanks to secondary combustion.