What is the most efficient small wood stove?
A stove sized properly for your space is going to be the most efficient choice. Too large of a stove will overheat your space, and you'll need to choke it down or open windows. Too small of a stove will not keep you warm throughout the Winter. Learn more about how to choose the right stove size for a small space.
Once you've identified stoves that will fit your space, you can compare features to find the most efficient one.
How to Compare Wood Stove Efficiency
Wood stove efficiency can be difficult to compare across models. Not every stove has a published efficiency rating, and those that do are not all reporting the same measurement. For instance, North America uses the higher heat value system while Europe uses the lower heat value system, so the same stove will appear less efficient in North America vs. Europe. And a stove with high combustion efficiency doesn't necessarily have good heat transfer efficiency.
To simplify the selection process, you can look at the design of the stove for clues on how efficient of a heater it will be.
Combustion efficiency is a measure of how well a stove converts wood to heat, CO2, and water vapor. A stove that has efficient combustion will extract more heat out of the same fuel, produce less air pollution, and keep its flue system cleaner. Stoves with high combustion efficiency tend to have a few features in common.
Hotter fires are more efficient fires. One of the best ways to help a fire burn hotter is to surround the firebox with refractory insulation. Ceramic firebrick or refractory fiber insulation can reflect heat back into the firebox, which helps achieve higher temperatures and ensure more complete combustion.
Secondary Air Supply
A fire fed with air solely from the bottom (primary air) tends to suffer from incomplete combustion. Flammable gases like carbon monoxide can escape up the chimney, and partially burned particulates can form creosote inside the flue system. Incomplete combustion doesn't just cause poor fuel efficiency, but more air pollution and higher risk of chimney fires.
Wood stoves that add an additional air supply to the top of the fire (secondary air) can help ensure that enough oxygen is available for gases to finish burning completely. Often, the secondary air supply is preheated by the firebox, which also helps improve combustion efficiency.
In order for complete combustion to occur, the secondary air supply needs to mix with any combustible gases at a high temperature. Adding a baffle at the top of the firebox gives the flue gases extra time to burn completely before they escape into the flue system.
Heat Transfer Efficiency
Once a stove has converted fuel to heat, there's also the question of how much of that heat gets into the room, and how much goes up the chimney.
Stoves with relatively open air controls like older potbelly style stoves, some ammo box style stoves, or marine style stoves tend burn wood fast, and transfer relatively little heat into the living space. Fast-moving flue gases tend to escape the chimney before they get a chance to heat up the living space. That creates such a strong draft, that these type of stoves typically require the addition of a damper in the flue.
Stoves with airtight construction like Dwarf Stoves are easier to control with the built-in air controls, and slow flue gases down to give them a chance to radiate into your living space. Not only does an airtight stove give you longer burn times, but you'll get more usable heat with less fuel.
In addition to improving burn efficiency, a baffled firebox can also improve heat transfer efficiency. By creating a less direct path for hot flue gases to exit the stove, the baffle helps ensure more of the heat actually makes it into your living space.
Moderately Large Clearance Requirements
While a wood stove that can be installed with tight clearances may be easier to fit inside a tiny space, an ultra-insulated stove body can make for an inefficient heater. Larger required clearances are an indication of more heat radiating from the stove body, and therefore would tend to indicate better heat transfer into your space.
Very small clearances (6" or less) can be an indication of a stove that is highly efficient at converting fuel to heat, but then sends most of that heat up the chimney instead of into your living space.
The Dwarf's clearance requirements (16" to the sides and 18" to the backs), are a good balance between reasonable clearances and good heat transfer. For tight spaces, wall clearances can be reduced by up to 2/3. Read more about how to reduce stove clearances with heat shields.