Updated: Jan. 2017
|Efficiency is measured with a "stack loss|
method," meaning a ratio of how much
heat stays in the room vs. goes up the chimney.
“If you want an assurance that you are buying a high efficiency stove, the best way is to buy from a company that publishes efficiencies using B415 and submits their numbers to the EPA,” said John Ackerly, President of Alliance for Green Heat. Currently, manufacturers are required to test for and post efficiency numbers for stoves certified after May 2015. But a large majority of stoves were certified and tested before then, and are not required to disclose their efficiency numbers. They can do so voluntarily, as Jotul, Blaze King, Kuma and others have.
Efficient stoves save money and time
New, EPA certified non-catalytic stoves are almost all within the 55 - 75% efficiency range, with most in the high 60s and low 70s. Catalytic stoves will likely be between 75% - 82%. “A 20% difference in fuel efficiency can add up to a lot of savings for consumers, whether you buy your firewood or cut it yourself,” said Ackerly.
With pellet stoves, the efficiency range is even greater and consumers stand to gain or lose a full 30% or even 40% in fuel efficiency depending on what model they buy. At least one pellet stove on the market gets only 33% efficiency and another 49%, but most are in the 60s and 70s, and some will be in the 80s. Industry leaders have conceded that most pellet stoves are not 70% efficient despite the EPA estimated default of 78% efficiency.
Consumers buying new pellet stoves have little access to reliable efficiency information and could easily come home with a stove that is between 55 - 65% efficient. Most pellet stove manufacturers either do not supply efficiency data or supply exaggerated data that that makes their stoves appear far more efficient than they actually are.
Most stove manufacturers do not disclose how they measure efficiency
The United States uses the higher heating value (HHV) for efficiency numbers for all appliances, whereas Europeans use the lower heating value (LHV). This makes it appear that European stoves and boilers are more efficient as a stove that is 75% HHV efficient would likely be around 83% LHV efficient. Many stove manufacturers use this discrepancy to their advantage by reporting efficiency using the LHV method, but do not disclose the method to the consumer.
This phenomenon arose partially because the IRS approved the use of LHV numbers to qualify stoves for the federal tax credit that expired in the end of 2011. To add to the confusion, the IRS did not say how efficiency should be measured, allowing industry to use many different methods. As a result, virtually every stove in America was deemed to be at least 75% LHV efficient by the manufacturers who made them. Most manufacturers continue to use LHV numbers and whichever efficiency calculation provides them with the highest number.
Virtually no company, agency or non-profit has openly and honestly discussed this issue and tried to help unravel the confusion around efficiency numbers. One of the few websites that has anything on it is www.combustionportal.org. It says “ On February 6, 2007, the EPA approved use of the CSA B415 test protocol as a means by which to determine efficiency ratings. The IRS sponsored wood stove tax credit program allows manufacturers to use a different method of determining efficiency. IRS allows laboratories to use greater flexibility in determining the thermal efficiency rating for tax credit purposes.”
The decision by the EPA to start posting actual efficiencies on their list of certified wood stoves is a step forward to help consumers identify real efficiency numbers.