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Heat Output and Turndown Ratio


Figure 6. This infrared photo
shows highest heat coming from
the windows and the heat
exchangers on the Harman.
Photo: Jessica Peterson.
For this rating, we tested for two different attributes - maximum heat output and turndown ratio -and combined them into one score.

For heat output, we tested for sheer volume of pellets that the stove could deliver to the burn pot in an hour at its highest heat setting. All other things being equal, the stove that can burn the most pellets the quickest will provide the greatest amount of heat.

The stoves that have the highest feed rate and can feed pellets into their burn pot the fastest are the Enviro, Harman and Piazzetta which all could burn about four pounds of pellets per hour. The pellets we used were independently tested at about 8,200 BTUs per pound, so if your stove could get 100% of the energy from those four pounds of pellets, you could get 32,800 BTUs per hour. However, most pellet stoves get in the 60 – 75% efficiency range, and we used an average of 67%, so the actual delivered BTUs are about 22,000 per hour.
* Harman and Quadra-Fire provide BTU input figures on their websites. Piazzetta claims their figure is BTU output. Ravelli and Enviro do not stipulate if they are quoting input or output BTU figures.

Our BTU output calculations compared the stoves against each other based on their tested feed rate (pounds of pellets delivered to the combustion chamber per hour) and an assumed average efficiency of 67%. Test labs report BTU ranges to the EPA that are then recorded on the list of EPA certified wood stoves, but test labs did not have to use actual efficiencies. Virtually every pellet stove reports similar BTU output at its lowest burn rate setting, falling somewhere between 8,000 and 12,000 BTUs per hour. But manufacturers, including four whose stoves we tested, somehow claim far higher BTU output on their promotional literature than the test labs reported. We do not recommend using BTU output claims in promotional literature, as it appears to be another area, like efficiency, where manufacturers can exaggerate numbers.

One reason test labs may report higher BTU output than we found is because they could have used the old EPA default efficiency of 78% for pellet stoves or even higher, instead of the actual efficiency of the stove, using the EPA approved method (CSA B415). They also may have been able to report numbers using European calculations (LHV) instead of those preferred in the US (HHV).1 The ability and willingness of manufacturers to claim far higher BTU output in their promotional literature than the EPA accredited labs reported points to a lack of industry or government guidelines for consumer advertising and may be perceived by consumers to be deceptive.

Can a pellet stove efficiently heat a home? Many consumers have years of experience heating a 1,500 – 2,500 square foot home with nothing more than their pellet stove. Of course, floor layout, insulation and region of the country are huge factors. A helpful guide of where a stove should be installed is typically within 25 feet of the common area of a home. Stoves in spare rooms or rooms above garages typically do not heat as well as a stove located within the insulated space near the center of the home. Sizing the stove is important because pellet stoves are not designed to be operated 24/7 at their highest heat setting. If your stove is undersized, it will likely have more performance issues than one that is correctly sized. Likewise, if you are just trying to heat an 800 – 1,200 square foot space, a smaller stove like the Ravelli or Englander will likely perform better.

Another consideration in stove sizing is recovery, or how long it takes to bring a cold home up to temperature. Smaller stoves will have a much longer recovery period than a large stove. A large stove with a good turndown ratio is ideal when considering the stove as a “whole home” heater. You get the best of both, quick recovery, but when demand is met, the stove can idle comfortably or if it has a thermostat, can modulate or even turn off and on as required.

Turn down ratio
We also tested for turndown ratio, or the difference between the lowest and highest heat setting that gives the consumer the widest range of heat output. If the stove delivers 2 pounds of pellets to the hopper at its highest setting and 1 pound at its lowest, it would have a turn down ratio of 2.

The best stove for turn down ratio is the Harman which can burn almost 4 pounds of pellets at high and only 1.5 at low. The Enviro, Quadra-Fire and Ravelli also had good turn down ratios. Both the Piazzetta and the Englander would not turn down very low, using over 2 pounds of pellets per hour at their lowest settings. The Englander had a very narrow range, giving the consumer little control over the amount of heat.

Length of burn
We tested for how long the stove could burn on a single load of pellets at the highest and lowest heat setting. The longest was the Quadra-Fire. Its large hopper enabled it to burn for nearly 50 hours continuously on its low setting. The shortest was the Ravelli, whose small hopper only allowed an 8-hour burn at its highest setting. On a thermostat setting, stoves could go far longer since they would only be burning part of the time.

Click here for more on heat output and how we tested.
1In Europe, pellet stoves typically have a range between 8,000 – 35,000, using LHV calculations. See “European Wood Heating Technology Survey,” NYSERDA, April 2010.