Wood Stove Efficiency
Efficiency in a wood stove is important because it allows you to conserve fuel while getting the most heat into your space and producing the least amount of smoke and creosote. Installing the most efficient wood stove for your space can give you many years of clean-burning, trouble-free warmth. Download our Free Wood Stove Efficiency Guide to get you started on selecting the best features for your use.
What is the most efficient small wood stove?
The wood stove is only one component of an efficient installation. Choosing the most efficient wood stove will depend as much on the design and location of the structure as the design of the stove itself. First, use the BTU Calculator to choose the right size stove for your space, then 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 everywhere else uses the lower heat value system. The North American system does not account for the latent heat of water (energy required to boil off the water content of the wood before burning), but the Everywhere-Else system subtracts out the latent heat of water. As a result, the same stove reported with the North American system will appear less efficient than a stove reported with the Everywhere Else system.
To simplify the selection process, you can look at the design of the stove for clues on how efficient 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.
- Insulated Firebox – 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.
- Baffled Firebox – 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.
- Airtight Construction – 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.
- Baffled Firebox – In addition to improving burn efficiency, a baffled firebox can also improve heat transfer efficiency. By creating a less direct path for hot flue gasses 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 back), are a good balance between reasonable clearances and good heat transfer. For tight spaces, wall clearances can be reduced by up to ⅔ with heat shielding. Read more about how to reduce stove clearances with heat shields.
What is the efficiency rating of the Dwarf Wood Stove?
Because the efficiency percentage of a stove is less helpful than you might imagine due to a couple of factors, we haven’t tested the Dwarf’s burn efficiency in a lab, but the firebox design is an adaptation of an existing design that tested over 80% burn efficiency
Remember from above, there are two types of efficiency—burn efficiency (i.e. how well a stove converts fuel to heat), and heat transfer efficiency (i.e. how much of that heat ends up in the living space instead of up the chimney). The number reported on stoves is burn efficiency, which gives you a general measure of how clean the stove burns, but doesn’t tell you a whole lot about how much of that heat actually makes it into the room. Lots of “efficient” burning stoves send most of their heat up the chimney. A good way to tell if you’ve got one of those is if the manual requires a damper to be added in the chimney—lots of wasted heat creates a super strong draft, which requires an extra damper to control. Adding a damper is not required or recommended for a Dwarf stove.
The Dwarf has a baffled design and airtight controls, which helps slow down the combustion gases and actually get that heat into your living space, instead of sending it all up the chimney. And we’ve made upgrades to the air controls to give you much better control over your burn rate, so you can control primary, secondary, and tertiary air supplies separately. That means that once you’ve taken some time to master your stove’s controls, you’ll be able to get long, slow burn times without smoldering.”