The pellet stove efficiency rating published by their manufacturers is a combined rating which includes combustion efficiency, electrical efficiency and heat transfer efficiency.Now pellet stoves mostly burn at over 98% combustion efficiency, and the electrical efficiency is about 99%. Let's analyse a typical pellet stove.
Assume, for the sake of argument, that the heat transfer effectiveness of a pellet stove is 60%. If you add up 98%, 99% and say 60%, you get 257. Divide that figure by three to get the overall efficiency and it comes out at 85.7%.
78% is the EPA's, (Environmental Protection Agency), assumed default figure for a pellet stove's overall performance. It's clear then, that the heat transfer efficiency of pellet stoves is often less than 60%.
Combustion Efficiency And BTU./Hr.
The published Btu. rating of a pellet stove relates to its combustion efficiency. This is a measure of the heat produced from burning fuel. It does not directly relate to heat available to the home as some of it disappears up the flue.
One pound of hardwood pellets will produce around 8,200 btu. Softwood pellets slightly more. As combustion efficiency is so good in pellet stoves, this figure is very close to the actual heat 'input' of the stove. So take a stove with a published rating of 40,000 btu./hr. Divide 40,000 by 8,200 and you see that at that output, the stove is burning 4.8 lbs. of pellets an hour.
This 'heat input rating' is the main figure we have for assessing the capability of a pellet stove.
EPA Approval And Pellet Stove Exemption
As well as assuming an efficiency of 78% for pellet stoves, the EPA also stipulates that they produce particulate emissions of less than 2.5 grams per hour to be approved. Tests for this must be carried out at an independent testing laboratory such as Omni.
To be exempt, a pellet stove must have an air to fuel combustion ratio of more than 35:1.
From the above heat transfer figures, it's clear that there must be a wide range of heat exchanger efficiencies in pellet stoves. To be effective a heat exchanger must have a large surface area, and the stove must direct hot air evenly over that area.
Broadly speaking, the heavier the heat exchanger, the more effective it is, so as a rule if comparing pellet stoves of a given heat input, the heavier stove is likely to perform the best. Two provisos to that: Some heat exchangers are made of aluminium which is lighter than steel, and don't compare a cast iron stove by weight with one fabricated from steel. Cast iron bodies are thicker and heavier than steel so compare like with like.
Bottom line? A pellet stove is only as good as the efficiency of its heat exchange system
Well,that's how the stove manufacturers and the EPA get their figures.
I'm indebted to Bevin McKinney who gave this more scientifically accurate assessment, and the relative importance of the three criteria.
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