If you’re purchasing a new stove, look for one certified by the U.S. Environmental Protection Agency (EPA) or a European model (their standards are more stringent). Scandinavian heaters that carry the Nordic Ecolabel are particularly good, since most have incorporated mechanical air-regulation devices to keep the fuel/air mix at optimal levels for improved efficiency and reduced emissions.
Wood heaters that carry the EPA certification will bear a paper sticker on the front of the appliance and a metal plate attached to the back or side that lists self-reported numbers for both emissions and combustion efficiency. To be certified, catalytic wood heaters cannot produce more than 4.1 grams of particulates per hour; noncatalytic models cannot produce more than 7.5 grams per hour. (In Washington state, these limits are lower: 2.5 grams per hour and 4.5 grams per hour, respectively.)
While the emission rate published by the EPA can be used to compare one model to another, combustion efficiency test methods have not been standardized and regulated. Independent, reliable data on wood-heater combustion efficiency—the ability to burn a given fuel completely and without pollutants—is difficult to find. When a piece of wood is burned, about 30% of the heat generated comes from the solids; 70% is contained in the gases. If the gases are not fully burned, waste heat and smoke (particulates) can result. Manufacturers self-report these numbers, and only a handful of wood heaters have third-party certification. Combustion efficiency numbers assume well-seasoned wood burning at maximum heat, with the air-inlet damper fully open. Damping down a wood heater can substantially reduce fire temperature and efficiency, while increasing particulate pollution.
However, in the next year or two, all wood heaters may be required to have third-party verification. Until then, there are five companies that have, at their own expense, obtained third-party verification for their wood heaters’ efficiency and combustion emissions (see “Wood Heater Particulate Emissions & Efficiency” table). Find a complete list of all wood heaters and their self-reported efficiency and emissions numbers at bit.ly/EPAWoodHeat.