Several technologies, both primary and secondary measures, can be leveraged to reduce emissions from Solid Fuel Local Space heaters (SFLSH). In this report the focus is on log wood stoves. Primary and secondary measures to reduce harmful emissions and pollutants during combustion from wood stoves involve two distinct approaches. These measures address different stages of the combustion process and encompass a range of strategies to achieve emission reduction. Primary measures (both technical, including ACC, and non-technical) are also referred to as source control measures, while secondary measures are commonly known as post-combustion or end-of-pipe measures. Non-technical primary measures comprise improved user behavior, quick user guides, stricter emission limits, new test standards, user awareness regarding fuel quality being used, maintenance practices, community awareness programs, and so forth.
While primary measures focus on improving wood stove design and combustion efficiency, secondary measures are vital to further reduce harmful emissions. More specifically, primary measures refer to the design and engineering parts of the wood stove that aim to enhance combustion efficiency and prevent the formation of harmful compounds. These measures include improved combustion chambers, increased insulation, and better air control mechanisms for improved mixing of air and combustibles. While primary measures are essential for reducing emissions, they often have limitations in achieving ideal performance under real-world conditions. Secondary measures are additional techniques or technologies that can be employed alongside primary measures to further reduce harmful emissions. These measures primarily focus on post-combustion treatment and emission control; examples include catalytic converters, particulate matter filters, and chimney fans.
However, experts and most scientific literature recommend reducing emissions as much as possible by optimizing the actual combustion process first. Additional devices can still be necessary to convert or remove the problematic compounds after they have been formed, downstream to the combustion chamber. Such secondary emission reduction technologies can be built into the stove (e.g., a catalyst) or retrofitted in the chimney.
Secondary measures are critical in reducing harmful emissions from wood stoves for several reasons:
Complementary to primary measures: Secondary measures work with primary measures to optimize wood stoves’ performance. By addressing the limitations of primary measures, secondary measures can ensure a more efficient and cleaner combustion process.
Retrofitting existing stoves: Secondary measures offer the advantage of being applicable to existing wood stoves, allowing homeowners to upgrade their stoves without needing complete replacement, making it a cost-effective solution in some situations and an esthetic one in other cases (e.g., being able to keep antique stoves if they meet the emission limits after secondary measures have been installed).
Environmental impact: Wood stoves are a significant source of particulate matter, volatile organic compounds (VOCs), and carbon monoxide emissions. Secondary measures can help reduce remaining harmful pollutants to a certain degree, thus contributing to improved air quality and reduced environmental impact.
Using secondary measures, such as flue gas fans, catalytic converters, particulate matter filters, foamed ceramics, baghouse filters, or scrubbers, emissions of pollutants from wood stoves, such as carbon monoxide, VOCs, and particulate matter, can be significantly reduced. Such measures may provide a more cost-effective solution to homeowners than upgrading an existing stove. Authorities should encourage putting efficient secondary measures into practice to promote cleaner and more sustainable heating practices.
Wood stoves, also known as room heaters or chimney stoves—including cookers, fireplaces/inserts, slow heat release appliances, and tiled stoves—are widely used, popular appliances in Europe for renewable heat supply. The number of firewood room-heating appliances in Europe is currently about 65 million, with annual sales reaching around 2.4 million devices. Assuming an average product price of €2000, the annual sales volume amounts to about €4.8 billion (Reichert and Schmidl, 2023a).
One possible, albeit somewhat extreme, solution to the emissions issues is prohibiting sales and using wood stoves. Some European countries, such as Germany, Switzerland, and Italy, have implemented regional bans on firewood stoves. Paris also attempted to ban open fireplaces without success. However, such bans often face strong opposition from the population. Therefore, finding alternatives that meet the minimal emissions requirements while fulfilling people's basic and emotional needs related to manual firewood combustion would be ideal.
Some local areas in Germany now require the Blauer Engel Ecolabel certification (Blauer Engel, Kaminöfen für Holz, DE-UZ 212), currently one of the strictest certifications worldwide; currently, only five producers have products that are compliant. To achieve the stringent requirements of Blauer Engel, most appliances need to resort to ACC, catalyst, and ESP, making some of them quite expensive.
Since the beginning of 2022, the European Union has enforced general requirements regarding maximum emissions and minimum efficiency performance for stoves. Emission requirements will become even stricter in the coming years. Several manufacturers are exploring measures and technologies to achieve "zero-emissions" or “nearly zero-emissions” soon. Most larger stove producers already have some form of automation, from simple mechanical ones based on bi-metals, to fully automated solutions that include temperature, oxygen, and door-opening sensors. Some also have automated the ignition process, which can be initiated using a mobile phone.
Improving user behavior is also an important non-technical approach to enhancing the performance of stoves in real-life operation and reducing emissions. Even the most sophisticated ACC-operated stoves can be misused by using the wrong fuel, using fuel with too much moisture, or adding too much firewood.
If the wood stove sector does not significantly lower its appliances emissions within the next few years, a possible consequence might be the ban of wood burning, at least in larger European cities (ABC News, 2023; “Ban on wood stoves? Government has confirmed through its Environment Improvement Plan 2023 that there will not be a ban on wood burning stoves,” 2023; Carrington and editor, 2023; Purves, 2023).
Wood could be a winning candidate for space heating if the industry manages to reduce emissions down to accepted levels. In EU, an average household uses more energy on heating than on anything else. In 2023, space and water heating represented 77.6% (60% for space heating alone) of the total final energy consumed by the households (EU, State of the Union, 2025). Assuming the fuel needed to cover this 60% can be supplied sustainably (transport, biodiversity, forest maintenance, etc.), other sources, such as gas and electricity, can be partly phased out or redirected to more profitable uses.