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Background: Heat output

For this rating, we combined maximum heat put and the heat turndown ratio for each stove. Maximum heat output was measured using the high power setting. We calculated the burn rate by measuring the time it took to burn a full hopper of pellets and also to burn a 10-pound test load. A combination of the data from these two methods allowed us to calculate a pounds per hour average for each stove.Once a pound per hour for the highest power setting was calculated, we multiplied by the BTU/pound output of the fuel (8,261) and an average efficiency (67%).

Burn rate x BTU/lb. x (efficiency/100) = BTU per hour

Calculated BTU output at high burn was almost 22,000 BTU/hr. for three of the stoves. This would be a slightly conservative if the stoves were higher than 67% efficiency. Or, BTU output would be higher with a high quality 100% softwood pellet that can have 9,000 BTUs per pound, but are not common in the marketplace. Our BTU outputs are based on pellets independently tested at 8,261 BTUs per pound, which is a bit higher than the average BTU content of most pellets.

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Figure 13. Infrared picture of Harman Accentra 52i insert.

The Piazzetta had the highest maximum heat output (21,973 BTU/hr.) and the Englander had the lowest (14,778 BTU/hr.). Three of the stoves had an output above 21,000 BTU/hr. (Enviro, Harman, Piazzetta), while 3 stoves were less than 18,000 BTU/hr. (Ravelli, Englander, Quadra-Fire).

The BTU numbers provided on the EPA list of certified stoves are provided by the test labs and do not have to use the actual efficiency of the stove. Labs can use the old 78% EPA default number, which is often 5 – 15 points higher than the actual efficiency, and they may be able to use very high estimated BTU output, rather than the BTUs of the pellets that the lab tested with.
 

The turndown ratio indicates the functional range of heat output of a combustion device. It is the ratio of maximum level of heat output to minimal level of heat output. A greater turndown ratio allows for more control of the stove heat range. For example, a stove with a maximum output of 4 pounds per hour and a minimum output of 2 pound per hour would have a turndown ratio of 2. The turndown ratio was measured 2 ways. First, each stove was filled to capacity (specified in hopper size section) and let burn completely at least two times on the highest power setting. High power burn times were recorded, and the hopper size was divided by the average of these times to reach a pounds per hour measurement. This method was repeated for low power setting burns. The high average was divided by the low average to calculate the turndown ratio. Next, each stove was given 10 pounds of pellets on the high setting and allowed to burn out completely. This 10-pound test was repeated on the lowest power setting. Burn times were recorded and divided from 10 pounds to calculate each pound per hour average. The turndown ratios were calculated using a combination of the full-hopper burn averages and the 10-pound test averages. The Harman had the largest turndown ratio (2.65), while the Englander had the smallest turndown ratio (1.41). The majority of the stoves had turndown ratios greater than 2, with the exception of the Englander (1.41) and the Piazzetta (1.74).

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