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WHY HAVE WOOD STOVE EFFICIENCY RATINGS CHANGED?

Q: I just helped my neighbor install a Pacific Energy Super 27 wood stove, the same model I put in my house a couple of years ago. We were looking through his manual, and noticed his Super 27 has an efficiency rating of 82.6%. This didn't sound right to me, so I went and got my manual, and the efficiency rating was only 69%! We took the magnifying glass and measuring stick to my 2008 model and his 2010 model, and they are absolutely identical. Can you tell me why the efficiency changed, when (as far as we can tell) the stove didn't?

Sweepy A: After you purchased your Super 27 and before your neighbor purchased his, the North American testing protocol for wood stoves changed. There have traditionally been two testing protocols used to determine wood stove efficiency: the High Heat Value protocol, and the Low Heat Value protocol. High Heat Value testing assumes that 100% of the heat value of the fuel is available for delivery to the room. Low Heat Value, or "real world" testing uses only the percentage of the heat value of the fuel that is actually available for delivery to the room.

Unlike the rest of the world, the US and Canada have traditionally chosen to use the theoretical fuel value of a load of wood (the High Heat Value) as a basis for determining wood stove efficiency. By this so-called North American protocol, a stove that burns a 10,000 btu load of wood and delivers 7,000 btu's of heat into the room is listed at 70% efficiency (7,000 / 10,000).

Test labs everywhere else on the planet take into account the fact that some 10% of those 10,000 btu's must be diverted up the chimney to establish an updraft and prevent creosote condensation, and that those btu's are not available for delivery to the room. Their reasoning is, since those btu's are not available, it isn't logical to penalize the wood stove for not delivering them. Thus, in the above example, a European test lab would adjust the fuel value of that same load of wood to around 9,000 btu's (the Low Heat Value) to more accurately reflect the efficiency of the stove itself. In Low Heat Value testing, our example stove would score 78% efficiency (7,000 / 9,000).

For many years, there had been a growing groundswell within the North American hearth product industry to adopt the European Low Heat Value protocol, as this would level the playing field between US/Canadian wood stoves and their European cousins, and make comparison shopping less confusing for the consumer. When the wood stove tax credit legislation was finalized on June 1st, 2009, those prayers appeared to be answered: the IRS specified that, for purposes of the tax credit, efficiency values be determined using the European Low Heat Value protocol.  North American wood stove manufacturers adjusted their published efficiency ratings to the new protocol, resulting in the same higher efficiency ratings enjoyed by their European competion.

December, 2015: The latest revision of the EPA's wood stove emissions legislation requires manufacturers to re-test their stoves for efficiency using the original North American High Heat Value standard, and submit their scores by 2020.  These scores will be published on the EPA List of Approved Stoves website and showroom hang tags, replacing the universal "average efficiency" numbers previously used.  This will finally level the playing field, as manufacturers outside the US and Canada will be required to test to the North American standard in order to get EPA approval to be sold in the US.

Sweepy While the High Heat Value efficiency standard appears to have won the battle so far as the EPA is concerned, we use the real-world Low Heat Value to compute the btu's available in a given species of wood. to read more, click here.

 

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