Pellet stoves are promoted as a cleaner burning option. However, they are still significantly more polluting than a non-wood-burning form of heat.
For example, a 2022 study that compared pellet stoves with non-wood-burning boilers found that the pellet stoves had, by far, the highest emissions of sub-micron-sized particles.
Pellet stoves can emit large amounts of fine particles and high levels of CO and polycyclic aromatic hydrocarbons (PAHs), especially in the ignition phase when combustion efficiency is low.
Start-up emissions aren’t measured in certification testing, but are significant
Emissions tests for pellet stoves omit the start-up phase of operation. In the US, the testing protocol for EPA approval allows the pellet stove to be run for an hour before emissions are officially counted.
A review of the US EPA’s wood stove certification program conducted by the Northeast States for Coordinated Air Use Management (NESCAUM) found that, “on average, first-hour emissions were 175% higher than the [pellet stoves’] certification value. The first-hour values ranged from 14% to 576% higher (PDF).”
A European study found that when higher-emitting phases of the burn cycle were included, pellet stoves exceeded their certification limits.
Another European study similarly concluded that pellet stove emissions are higher in real life than their “technical standards” suggest, due to differences in how the stoves are used and differences in combustion conditions, as well as the exclusion of the start-up phase from testing.
An Italian study found that the carcinogenic and mutagenic polycyclic aromatic hydrocarbons (PAHs) emitted from pellet stoves during start-up “have a higher toxicological burden” than during steady-state burning and that the ignition phase, although brief, contributes substantially to the overall pollution output from pellet stoves.
More emissions testing issues
The NESCAUM report found that issues with pellet stove certification testing and test reports are widespread. “Of the 86 test reports reviewed, all had at least one element that triggered revocation criteria.”
Pellet impurities
The wood used in the manufacture of the pellets can also be an issue. In another NESCAUM report, researchers analyzed 23 wood chip samples and 132 wood pellet samples manufactured in the US and Canada and available for sale in eastern parts of the United States. They found that most of the pellets tested would meet US voluntary standards, but would not be likely to meet the higher standards for residential use in Europe.
Some pellet samples in their study had “unusually high concentrations of several heavy metals, including arsenic, copper and chromium.” Their analysis found a wide range of results, with many samples “higher than the ‘normal’ benchmark value.” Some samples had levels of heavy metals 4 to 7 times higher than the next highest sample.
Production of particulates high in cadmium and lead may be a universal property of burning wood biomass in pellet and wood stoves.
Toxins in pellet ashes
Researchers have found evidence that pellet stove ashes can be surprisingly toxic.
Cadmium and lead released into the room
For example, pellet stove ashes have been found to have “extremely high enrichment of the toxic elements cadmium and lead,” and there is evidence these particles are released into the room when the ashes are cleaned from the stove.
The authors of this study advise that pellet stove ashes should not be added to home garden soils “due to the high levels of cadmium and lead and potential for uptake by plants and surface contamination of root vegetables.”
Polycyclic aromatic hydrocarbons, too
Pellet stove ashes have also been found to contain polycyclic aromatic hydrocarbons (PAHs). Similar to the advice given in regards to cadmium and lead, other researchers have recommended that ashes from pellet stoves should not be used on plants destined for human and animal food due to toxicity concerns.
Climate pollutants—more methane than gas
Burning wood emits the potent climate pollutant methane (CH4), and pellet stoves are no exception. As one study noted:
Emissions of CH4 from pellet stoves were the lowest of any stove technology tested, but not trivial. Importantly, even the pellet stove CH4 emissions factor was approximately 7× higher than CH4 emissions from natural gas heating and 5× higher than oil heating. We recommend the inclusion of CH4 emissions from [residential wood burning] when assessing greenhouse gas emissions from residential heating.
Residential wood burning references
