WOODSTOVE Buyer's Guide

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Cookstove
• EPA exempt, so efficiency tends to be low
• not good for serious space heating—great for cooking

EMISSIONS

Back in the late 1980s, the U.S. Environmental Protection Agency (EPA) established a mandatory smoke emission limit for woodstoves of 7.5 grams of smoke per hour (g/h). This regulation was necessary because older stoves were so smoky they were adding to air pollution problems in some regions. Today, all woodstoves and fireplace inserts must be tested and meet this limit, and many are certified for emissions down in the 1 to 4 g/h range.

EFFICIENCY

On average, the new stoves are about 30 percent more efficient than the old box, potbelly or step stoves of yesteryear, and even most of today's central heaters. That's 30 percent less cost if you buy firewood, or 30 percent less cutting, hauling and stacking if you cut your own. Although this higher efficiency is a byproduct of mandatory emission limits, it has made the EPA rules a winner for both the environment and stove users.

Unfortunately, none of the really useful performance ratings shown on stove brochures (such as efficiency, heat output, heating capacity and burn time) are standardized and regulated, so they can't always be trusted or compared one to the other. For example, the EPA does not require stove efficiencies to be reported, but for obscure reasons assigns default figures of 63 percent for noncats and 72 percent for cats. Some manufacturers have paid for independent efficiency tests and show the results on their promotional brochures. But because efficiency test methods have not been standardized and regulated, you can't be sure the figures are based on the same tests and calculations. On the other hand it appears that all EPA certified stoves are more than 60 percent efficient and some can deliver around 80 percent of the fuel's potential heat to the house. This is far better than the low-tech uncertified options, many of which are in the 50 percent range and lower. (An overall efficiency higher than 80 percent is not desirable because the resulting low exhaust temperature means weak draft and the risk of water vapor condensation, which damages the chimney.)

HEAT OUTPUT

Most manufacturers list a maximum heat output in British Thermal Units (BTUs). For popular stoves this falls in the 25,000 to 80,000 BTU range. But this figure can be misleading. First, the full output of a stove should not be used often since continuous high firing can do serious damage to the stove's innards. Second, the average medium-size house needs only 10,000 to 20,000 BTUs per hour of continuous heating power, even during cold weather. Finally, some manufacturers use the heat output rate from EPA testing, which uses softwood fuel, and others use the results of their own tests which can produce a considerably higher peak output depending on the fuel used. Still, a comparison of the maximum heat output figures shown on the chart will give you some idea of relative heating power among various models.

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