Background: Efficiency
The Testo relies on measuring oxygen in the stack and also uses stack temperature, ambient temperature and other factors for its efficiency calculation. When oxygen in the stack climbs above 15%, or stays in the 16% to 18% range, it is a clear sign that the stove is not running very efficiently. On medium burn the most efficient of the 6 stoves, the Harman and Piazzetta, had 10% to 12% oxygen in the stack on their best days, and an overall average of 13% and 14% oxygen (Figure 8). The Enviro with the worst efficiency overall, averaging 18% at medium burn, 20% at high 17% at low (Figure 8).
By “efficiency,”we are referring to thermal efficiency, which is the ratio of heat delivered to the heated space to the heat content of the fuel burned, expressed as a percentage1. Efficiency is measured by what is known as the “stack loss” method, that tracks how much heat and chemical energy is lost through the chimney compared to what remains in the house.
Figure 8. Percent oxygen averages during the 30-day testing period measured by the Testo 320 gas analyzer on medium burn rates.
The Testo provided a European (LHV) calculation and does not conform to the EPA approved CSA B415 efficiency calculation method. Moreover, the Testo efficiency is a much rougher estimate since you cannot input exact moisture content and other parameters required by the EPA accepted B-415 calculation.
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Figure 9. Efficiency averages and trends for 30-day testing period of six popular pellet stoves
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The Testo efficiency and oxygen numbers do provide a reliable basis for ranking the stoves against each other over a month-long period2.
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Figure 10. Stack temperature averages during 30-day testing period, measured by the Testo 320 Gas Analyzer.
The trend lines in the efficiency graph show that 5 of the stoves lost an average of 5% in efficiency over the month trial. Each stove burned about 1 ton of pellets during the month. We are not aware of long-term aging studies on pellet stoves, and they are clearly needed. The most important take-away for consumers: regularly clean your stove, especially the burn pot. In addition, have a comprehensive professional cleaning done once a year or as recommended in the owner’s manual.
The other important take-away is that once manufacturers start disclosing verified efficiencies on the list of EPA certified stoves, choosing a higher efficiency model from a local dealer who can service it is likely to save you in fuel costs over the long term.
"Most manufacturers will not have to disclose their actual efficiencies until 2019 or 2020. Leading wood stove companies are starting to voluntarily disclose them, but pellet stove manufacturers are not. The EPA has some of the actual efficiencies, but manufacturers provided that data to the EPA as "Confidential Business Information." The Alliance for Green Heat filed a Freedom of Information Act request with the EPA, but to date the EPA is upholding the confidentiality of the efficiencies.
Stack temperature is a measurement of the gas temperatures in the middle of the flue, six feet above the floor. On a medium power setting, the Englander had the highest average stack temperature (433"°" F), while the Enviro had the lowest (298"°" F). On a low power setting, this pattern continued, with the Englander having the highest (382"°" F) and the Enviro insert the lowest (198"°" F) by more than 25"°" F.
Figure 11. Testo 320 Gas Analyzer in “sensor protection” mode after attempting a high power test on the Harman.
During the 3 tests on the high power setting, this pattern continued, with the England Stove having the highest average temperature (464"°" F) and the Enviro the lowest (327"°" F). Unfortunately, we were unable to complete 15-minute tests on the Harman at its high setting because when CO rose above 3,000 ppm, the Testo would go into shutdown mode, to protect its sensors (Figure 11). We believe the Harman would have had one of the higher average temperatures as it was the second highest temperature average (399"°" F) during the medium power burns and seemed to be radiating a great amount of heat in the screened in porch.
​1The Engineer’s Guide to Efficiency Requirements in Wood Burning Stoves, Intertek Lab, 2013, p. 3
2For a discussion of the measurement ranges and resolution/Accuracy and response times, see page 13 of the Testo manual: https://www.testo.com/resources/media/global_media/produkte/testo_320/320_IM_0970_3200_en.pdf.