The amount of particulate matter (PM), commonly known as smoke is typically small in pellet stoves compared to wood stoves. The six stoves we tested are certified to meet the EPA particulate matter standard. We rarely observed any visible smoke after the first five minutes of start-up. Five of the stoves operated very cleanly on average, with the Quadra-Fire operating the cleanest of all.

To assess the cleanliness of emissions, we measured Carbon Monoxide (CO) in the stack, a common and easy way to test for combustion conditions. One notable result was that during the thirty day test, with the stoves operating almost twenty-four hours a day and without being cleaned regularly, four of the stoves– Harman, Englander, Ravelli and Enviro - maintained a relatively steady emissions throughout the thirty days, ending the month with virtually same average reading as they started. The Quadra-Fire did increase in emissions over the test period but it started far cleaner than the others and still ended as the cleanest stove.

The Harman operated cleanly at the lowest heat setting, but started to emit higher CO emissions on medium and had very high emissions when set at its highest heat setting during our simulated real world test regimen, similar to other studies. 1When the stove was tested for EPA certification by OMNI-Test labs in Portland, Oregon, from Nov. 11 – 13, 2014, it emitted 5.61 gram per hour at its highest heat setting. The vent cap, where the smoke exited from the Harman chimney, also built up the most soot during the thirty-day trial. If low emissions are a top priority for you and your family, we would not recommend this stove when burning at its high heat setting.

Figure 3. Carbon Monoxide (CO) levels with all the stoves burning at medium heat.
CO is a good proxy for the cleanliness of emissions from a steady state pellet stove. The straight lines are trend lines, showing the whether the stove was trending towards a cleaner or dirtier burn over the 30 day trial (Figure 3). Three of the stoves – the Englander, Enviro and Ravelli – had quite good and stable CO levels that slightly trended towards a cleaner burn. The Quadra-Fire started as the cleanest, and became dirtier. The Harman had by far the highest levels of CO, indicating a dirtier burn (Figure 3). Several data points are missing for the Harman because our test equipment went into shutdown mode because of extremely high CO levels (Figure 3). The Testo can read CO levels up to 5,000 ppm and starts its shutdown mode as it approaches 4,000 ppm. Other CO studies have found higher CO levels at a high power setting. The trend line for the Piazzetta is not shown because it only completed 3 of the 4 weeks of testing before it quit, which could produce an unrepresentative trend line (Figure 3).

One of the biggest environmental benefits of pellet stoves, compared to wood stoves, is that they can have consistent low emissions in the real world, whereas wood stoves depend on an experienced and attentive operator who use sseasoned wood. A pellet stove rated at 2 grams an hour could remain under 4 grams an hour in the real world for many years, assuming it is cleaned periodically. However, a wood stove rated at 3 grams an hour could easily emit 8 – 12 grams an hour in the real world,

Figure 4. Pellet stoves usually
emit whitish smoke for several
minutes upon start-up, but
typically do not emit any visible
smoke after start-up.
even if it is being operated relatively well and with relatively seasoned wood. This is because lab testing of wood stoves is not designed to predict how wood stoves operate in the real world. Lab testing of pellet stoves is able to be a better predictor of real world performance.

Pellet stoves should not leak any smoke or CO directly into a home, since consumers rarely open the door to the combustion chamber while the stove is lit. (The Harman is the only stove that suggests the user scrape the burn pot while the stove is lit.) Consumers regularly have to open the door on a woodstove to refuel it. Smoke can re-enter homes from the outside, and therefore lower emitting pellet stoves could have a smaller health impact.

Click here for more on cleanliness and how we tested the stoves.

1Houck, James E., et al. "Low emission and high efficiency residential pellet-fired Heaters." Proceedings of the Ninth Biennial Bioenergy Conference. October, Buffalo, New York. 1999.