In which situations do pellet stoves perform best? Do they emit air pollutants? The latest ADEME study answers these questions based on the observation of the use of efficient and recent pellet stoves in France during the winters of 2020-2021 and 2021-2022, and issues recommendations for the sector both possible innovations as well as the points of attention concerning the installation, the dimensioning of the ducts, the use of the stoves, the choice and the storage of the pellets as well as the maintenance of the appliances. These recommendations are based on the results of two measurement campaigns carried out, one continuously on energy performance and the other occasionally on emissions of atmospheric pollutants.
High energy performance, for low particle emissions
This new study reveals that the sizing of pellet stoves, the sizing of the ducts and the adjustments made during their commissioning are essential to ensure high energy performance of the installation and to limit particle emissions.
The real energy yields of the stoves are very good for more than two thirds of the appliances tested (around 85%, which is slightly lower than the "catalogue" yields but around 10 points better than the appliances with recent logs). For these same devices, pollutant emission levels are low, less than 25 g/GJ in solid particles, regardless of their operating speeds. The other devices (a third of those tested) have equally high performance at sustained speeds (more than 40% of nominal power) but much lower at lower speeds. To limit operation at reduced speed as much as possible, and this lower performance, it is therefore important to avoid oversizing the devices.
On average, the levels of solid particle emissions are approximately 30% higher than those communicated by the manufacturers and measured within the framework of standard laboratory tests. This discrepancy is explained by the use of realistic operating conditions. This confirms the benefit of developing test methods to promote tests that are closer to real conditions.
Appropriate practices to limit the impact of wood heating on the environment
Several recommendations have been made by ADEME to continue improving the performance of pellet stoves, in particular for manufacturers, installers and users.
The study recommends that manufacturers adapt the flow of extracted air (or excess air) to the quantity of pellets introduced into the combustion chamber at low speed; to restrict the operating range of the devices in order to avoid use at too low speeds (<30% of the nominal power); and to include a detailed description of the stove's possible settings in the instruction manual.
ADEME recommends that installers properly size the heating power of the device in line with the heating needs of the home to avoid using too low rates leading to degraded performance; perform a duct sizing calculation (draft neither too weak nor too strong); check the compatibility of the connected thermostats, as some external thermostats do not support all stove operating modes; make sure to analyze the need for the installation of auxiliary heating in order to provide sufficient energy to the dwelling and to avoid overheating of the living room (overconsumption); and to check the settings of the device (of the fan and of the frequency of cleaning of the crucible) during each maintenance.
Finally, ADEME reminds users of the importance of buying certified pellets and storing them in good conditions; adjust the correct combustion quality if necessary, when changing pellet brands or batches; to carry out the necessary annual or bi-annual maintenance, including chimney sweeping; and to favor the modulation mode when the need for heating is high (in this mode, when the set temperature is reached, the stove continues to operate but at a lower power), and the on/off mode when the need for heating is less, to avoid the lowest operating rates.
Wood heating in France
As an alternative to non-renewable energy sources, wood can replace the consumption of gas and electricity. It is the leading source of renewable energy consumed in France: 35% of renewable energy production in 2021. Nearly 7 million French households already heat with wood. But certain conditions must be respected because domestic wood heating remains the leading emitter of fine particles (PM2,5) in France, with 41% of emissions in 2020 (source: CITEPA). Most of these emissions come from old, inefficient log appliances and open fireplaces. A well-used high-performance stove emits up to 10 times less fine particles than an old appliance.
The number of homes heating with pellets is constantly increasing. In 2021, the sale of pellet stoves surpassed that of log stoves for the first time (according to the Domestic Wood Burning Appliances Market Monitor, Observ'ER, 2022). Despite their higher cost, these devices have many advantages:
- they are easy to use because pellet feeding and ignition are automatic and programmable;
- they have good battery life, up to five days;
- they have high energy efficiency;
- they emit less atmospheric pollutants than log appliances;
- they give the possibility of modulating the setpoint temperature.
- Campaigns conducted with private households equipped with recent pellet stoves (less than five years old), labeled Flamme Verte 7* and with a power of 7 kW (two appliances), 8 kW (14 appliances) and 9 kW (four appliances).
- The quantities measured continuously during an entire heating season (temperature, draft, electricity and pellet consumption) were measured from November 2020 to March 2021 in twenty houses: two built before 1975, 16 between 1976 and 2011 , two after 2012, with the same batch of pellets.
- Pollutant emission measurements were carried out on 19 of the 20 previous houses during the winter of 2021-2022, following the BeReal protocol (two days of testing).
- The twenty sites are divided into three climatic zones: zone H1 (from Auvergne-Rhône-Alpes, to the Grand-Est region, to the Hauts-de-France region and to the east of the Normandy region); zone H2 (the entire west coast extended inland to Poitiers and the Pyrenees); zone H3 (the Mediterranean coast).