Efficiency
​Efficiency is important as it helps consumers calculate fuel savings between stoves as well as between fossil fuel alternatives and pellet stoves. Our test equipment, the Testo 320 combustion analyzer, calculates efficiency values differently from a test lab, therefore we are not releasing any exact efficiency percentages in our report. However, we can roughly assess the relative thermal efficiency of each of the 6 stoves compared to one another.
The most efficient stoves in our thirty-day testing regimen were the Harman and the Piazzetta. In our testing, the Ravelli, Quadra-Fire and Englander all had roughly the same efficiency, based on the Testo efficiency calculation. The Enviro had the lowest efficiency in our tests.
A key finding from our testing was that over a thirty-day period with very infrequent cleaning, efficiency dropped slightly in four out of six stoves based on medium burn rate testing. Most of the stoves showed about a five-point drop in efficiency except for the Enviro and Piazzetta, both of which went up slightly. More regular and more complete cleanings may have reduced the downward trend in efficiency shown by most of the stoves.
Our testing indicated that the six stoves had efficiencies within a fifteen-point spread. Other testing and pellet stove test data sets show a wide range of efficiencies, between 50 – 80%, with most falling between 60 and 75% using the EPA approved efficiency calculation (CSA B415.1), and an average between 67 – 70%. These data sets do not name the model and brand, but list the grams per hour and higher heating value (HHV) efficiency scores.
Based on the fifteen point range shown by existing data sets and our testing, if a home needed three tons of pellets, and spent $900, to heat with a 60% efficient stove, they would need 2.4 tons with a 75% efficient stove. The savings would be 20%, or $180.
Figure 3. Carbon Monoxide (CO) levels with all the stoves burning at medium heat.
All the stoves burned least efficiently at their lowest heat output setting, and we often found a 10-point efficiency difference between the low and high burn. If a stove has an average of 67% efficiency, it may run at 63% efficiency at its low level and 72% at its highest level. Using your stove on the lowest heat setting can still save you pellets, but you just won’t be getting as much heat out of each pellet as you get at medium or high burn rates. If your stove is controlled by a thermostat, it may be best to use the medium or high burn rates to achieve your desired room temperature, if the stove can be set to operate that way.
Our testing challenges the commonly held belief that pellet stoves tend to be more efficient than wood stoves. The very high levels of oxygen in the stack of the these six stoves may indicate that pellet stove manufacturers are sacrificing efficiency in order to provide enough air to keep the glass clean. Stoves that have an air-wash for the glass are actually leaking air in, usually beneath the glass surface. This friction across the glass from the air allows the glass to stay clearer. Our most efficient stove, the Piazzetta, also had the dirtiest glass, which may be a sign that they prioritize higher efficiency combustion over seeing the flame. Pellet stoves should have the capacity to perform at consistently higher efficiencies than non-catalytic wood stoves, and some may achieve that, but they were not among the popular stoves we tested.
We do not recommend using manufacturer efficiency claims as many of them appear to be exaggerated and are not based on independent third party calculations using the EPA approved method. Consumers will see many pellet stoves claiming efficiencies in the 80s, whereas most of those stoves are more likely to be in the 60s or low 70s. Only a few small pellet companies have been willing to release verified efficiencies to their consumers and they are not among the ones we tested. The EPA has also dropped their default efficiency of 78%.
Click here for more on efficiency and how we tested it.