Q: Greetings,
I'm just beginning to educate myself on modern-day wood stoves (been using a Fisher for decades) and I've seen stoves online for example, where
one has an efficiency rating of, say, 75% with emissions at 3.5 gm/hr., and another stove will have efficiency of 72% w/ emissions at 2.3. I can't
make sense of that. Can you shed some light on how these calculations are derived?
Thank you
Roger
 Hi Roger,
Thanks for the inquiry! A woodstove's heating efficiency rating doesn't correlate directly to its emissions rating. In fact, those numbers are arrived
at in two different tests:
Emissions testing is performed in EPA-approved test labs using the EPA's prescribed protocol. When testing for emissions, a nailed-together
"charge" of dimensional Pine is burned, and the particulate matter in the exhaust is measured throughout the duration of several fires at various
draft control settings. In this way, an average grams/hour particulate emissions rating is derived. Heating efficiency is not measured during EPA
emissions testing.
Heating Efficiency testing is performed using full loads of seasoned cordwood, and is designed to measure how much of the heat value contained in
the wood is extracted and delivered into the living space. When testing for heating efficiency, the following criteria are examined:
Extraction Efficiency: the load is weighed going in, and the particulate emissions and ashes are weighed after the fire to determine how effectively a
given firebox design breaks down the fuel to extract the available heat.
Heat Transfer Efficiency: this testing is performed in calorimeter rooms equipped with temperature sensors. Similar temperature sensors are
installed in the exhaust flue. The degree changes in the room and flue are monitored for the duration of the test fires to determine how much of the
heat extracted by the fire is delivered into the room, as compared to the heat lost up the flue.
Although a low emissions weight can improve a given stove's Extraction Efficiency score, the total amount of particulate emissions produced by
today's EPA approved woodstoves is so small that the affect of particulate emissions on the overall heating efficiency score is negligable. Thus,
even a model with an unusually low Emissions rating doesn't necessarily score a high Heating Efficiency rating.
Consider the two models you mention: the 1.2 gram/hr difference would lighten the total weight of emissions from the cleaner-burning stove by
about 10 grams at the end of an 8-hour test burn. Most likely, those 10 grams would be found in the ash remaining in the firebox, eliminating any
advantage in Extraction Efficiency. Even if the 10 grams weren't found in the ashes, a difference of 10 grams of particulates from a 40 lb. load of
wood would only amount to a .06% advantage in Extraction Efficiency, which would have virtually no effect on the overall Heating Efficiency score.
You can read more about woodstove efficiency testing from the
viewpoint of the test labs
here.
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