EPA set pollution standards for most wood stoves in 1988, with stricter standards coming into force in 1990. These standards set the maximum amount of particulates (a measurement which coincides with pollution) that a wood stove can release into the atmosphere.
However, many solid fuel products are exempt from BOTH these standards and the tests which are needed to comply. Why? Because EPA tries to address the largest part of a problem, which therefore tends to solve the big picture. It can become very complex to create a single rule which applies to different sources of smoke...
So EPA settled on the regulation of "room heaters" burning solid fuel. To us, this means conventional stoves. Here are some of the common exemptions:
1. Central Heaters
2. Cookstoves
3. Large masonry stoves - over 800 KG (about 1800 lbs)
4. Stoves with giant fireboxes - over 20 cubic feet
5. Coal stoves
6. Stove which burn more than 12 lbs of fuel per hour
In the wording of the EPA:
"The EPA regulates wood heating appliances under the Clean Air Act’s New Source Performance Standard for Residential Wood Heaters at 40 CFR Part 60, Subpart AAA. The regulation applies to wood heating appliances that possess an air to fuel ratio of 35 to 1, a burn rate of 5kg per hour, a firebox volume no more than 20 cubic feet and with a total weight of less than 800 kilograms. Wood heating appliances that do not meet this criteria are exempt from the EPA wood stove regulations. Additionally, cookstoves, furnaces, outdoor wood boilers and fireplaces are also not regulated under the wood stove regulations."
Most of these exemptions are clear. However, there are a couple which may need further discussion.
Coal: Coal-only stoves are exempted, however a coal/wood stove must meet the wood EPA standards for clean burning (unless it meets one of the other exempt criteria).
Another confusing exemption is the 35:1 air to fuel ratio. What this means is that a stove which allows large quantities of air to enter the burning fire can be exempt. Another way to think of it is that the EPA rule applies only to "controlled combustion" (air limited) or "airtight" wood stoves.
In technical terms:
"Air-to-fuel ratio means the ratio of the mass of
dry combustion air introduced into the firebox to the mass
of dry fuel consumed (grams of dry air per gram of dry wood
burned)."
Although a 35:1 exempt stove will not burn as long as an EPA stove, they can often be choked down by using a stack
Damper and other similar methods with the result being satisfactory performance. However, all things being equal, most customer shopping for an efficient wood burning stove would be best purchasing a unit that passes the EPA emissions standard.
Stoves which are exempt do NOT need to be approved or listed as such by the EPA. However, the manufacturer should have data to support their reason for exemption and this should be confirmed by the testing lab when the stove is submitted for safety testing (ALL stoves need to be safety tested) . EPA does maintain a listing of certain exempt appliances, but this is on a voluntary basis.
These units may be "exempt" from federal standards that apply to the manufacturer of wood heaters, but some state or local air quality regulatory agencies have adopted regulations which may prohibit the installation of fireplaces and/or exempt units.
