Residential wood heaters, which includes wood stoves and outdoor boilers, contribute significantly to particulate air pollution. EPA has regulated wood stove particulate emissions since 1988. Wood stove model lines that are in compliance with the rule are referred to as EPA-certified wood stoves.
Wood Heaters sold in the United are required to undergo emission testing to comply with U.S. Environmental Protection Agency (EPA) New Source Performance Standards (NSPS) guidelines and safety testing to comply with Consumer Products Safety Commission and insurance requirements. The safety testing requirements determine the clearance and installation requirements for a wood heater. This section of the Combustion Portal addresses EPA NSPS (New Source Performance Standards), but does not cover Consumer Products Safety Commission and insurance requirements.
At a federal level, air emissions from wood stoves and certain pellet stoves (only units with air to fuel ratio less than 35 to 1) are regulated by EPA through New Source Performance Standards (NSPS), which are rules aimed at manufacturers and retailers of these devices. The NSPS define a wood stove as an enclosed, wood burning appliance capable of and intended for space heating and domestic water heating that meets the following criteria:
- Air to fuel ratio less than 35 to 1;
- Usable firebox volume less than 20 cubic feet;
- Minimum burn rate less than 5 k/hr as as determined by Reference Method 28 test conducted by an EPA accredited laboratory and; and
- Maximum weight less than 800 kg.
The NSPS specify a maximum amount of air pollution that the units can generate. Testing must be performed by the manufacturers at laboratories using approved test methods for each model of stove sold in order to be certified for sale. The NSPS have been in effect for more than 20 years. In recent years, EPA has been working on revisions to the NSPS that are expected to significantly lower the amount of air pollution wood stoves generate. The new rules will also cover other residential devices that use solid biomass as fuel. For example, EPA anticipates new regulations for outdoor and indoor hydronic heaters and forced air furnaces.
In addition to federal regulations that apply to all wood stove manufacturers, some states have enacted more stringent regulations. Also, many local jurisdictions have ordinances and rules in place that impact installation and use of wood stoves.
New Source Performance Standards (NSPS)
The New Source Performance Standard for Residential Wood Heaters (40 CFR Part 60 Subpart AAA) were phased in over a several year period, starting in 1988. The initial rules (Phase 1) required that all wood stoves (excluding cook stoves and wood furnaces) release fewer than 8.5 grams of particulate matter per hour or 5.5 grams per hour for catalytic stoves, down from an average of around 60 grams per hour for conventional stoves at that time. These limits were tightened in 1990 (Phase 2), reducing the allowable emissions level for all wood stoves manufactured after this date to 7.5 grams of particulate matter per hour or 4.1 grams if the stove is equipped with a catalytic combustor. In 1992, the same regulations were applied to all stoves sold at retail, irrespective of manufacture date. As a result, any wood stove that you see in a showroom must have the EPA hang tag on it listing its emissions levels and certifying that it complies with the Phase 2 standards. It is illegal to sell a non-Phase 2 wood stove.
The following table summarizes emission limits for catalytic wood heaters and for non catalytic wood heaters and wood stoves equipped with catalyst.
|Type of Wood Stove
||Phase 1 Emission Limit
||Phase 2 Emission Limit
|Catalytic Wood Stoves
||5.5 grams per hour
||4.1 grams per hour
|Noncatalytic Wood Stoves
||8.5 grams per hour
||7.5 grams per hour
The New Source Performance Standards make a distinction between wood stoves with and without catalytic combustors ("cats" and "noncats" stoves). This refers to the two methods employed in new stoves to keep them running clean and efficiently. "Cats" use catalytic combustors, and "noncats" recirculate the smoke and reburn it. Normally, smoke isn't completely consumed in the burning process, because some wood gases require temperatures as high as 1,200°F to burn wood efficiently.
A catalytic combustor lowers this required temperature to 600°F, achieving a long, slow, controlled combustion that burns off the smoke that otherwise would leave the chimney as dirty, wasted fuel. The catalytic combustor needs minimal cleaning. If ash collects on the face of the combustor it can be cleaned with a soft brush. The internal "honeycomb" portion should never be cleaned with anything. It needs to be replaced after two to three years of normal use. Sluggish stove operation, creosote build-up and excessive smoke coming out of the chimney signal the need for a new combustor. EPA requirements at 40 CFR Part 60, Section 60.536(k) require manufacturers to provide consumers a two year catalyst warranty on all new catalytic wood stoves.
Noncatalytic (or recirculating) stoves use a heavily insulated firebox. This insulation keeps the heat in, creating a hot environment that encourages more complete combustion, with a secondary combustion chamber to burn off more gases and soot particles. "Noncats" don't need as much attention as "cats," primarily because they don't have a combustor to maintain.
EPA's certification process requires manufacturers to verify that each of their wood stove model lines meet a specific particulate emission limit by undergoing emission testing at an EPA accredited laboratory. Wood stove manufacturers must:
- Maintain a quality assurance program for production-line wood heaters
- Affix a permanent label to each wood heater that meets the applicable emission standard
- Attach temporary label that lists:
- Emission rate (using an EPA-approved test method)
- Heating range of the wood heater (for correctly sizing the wood heater)
- Overall efficiency
Please note that the NSPS defines a wood stove as an enclosed, wood burning appliance capable of and intended for space heating and domestic water heating that meets the following criteria:
- Air to Fuel Ratio less than 35 to 1
- Usable firebox volume less than 20 cubic feet
- Minimum burn rate less than 5 k/hr as as determined by Reference Method 28 test conducted by an EPA accredited laboratory
- Maximum weight less than 800 kg
Exemptions. Under the existing regulations, manufacturers of wood heaters not meeting the above criteria are exempt from the NSPS. Manufacturers may apply for an exemption by demonstrating that they do not meet any of the above listed criteria. Wood Heaters formally exempted by the EPA (see "EPA Exempt" wood stoves) must have affixed to them a permanent label that includes the statement: "EPA Exempt". EPA plans to propose revised wood stove regulations that will eliminate this exemption based on air to fuel ratio or burn rate. Furnaces, boilers, fireplaces, masonry heaters and cookstoves are also exempt from EPA NSPS regulations.
New Source Performance Standards for Residential Wood Heaters - Revisions
As just mentioned, EPA is in the process of developing revisions to the residential wood heater new source performance standards. In addition to tightening the emission limits on currently regulated wood heaters to reflect improvements in best demonstrated technology, EPA anticipates new regulations for other residential devices that use solid biomass as fuel. For example, EPA anticipates new regulations for outdoor and indoor hydronic heaters (wood "boilers") and forced air furnaces. Hydronic Heaters (also called outdoor wood heaters or outdoor wood boilers) are typically located outside the buildings they heat in small sheds with short smokestacks. Typically, they burn wood to heat liquid (water or water-antifreeze) that is piped to provide heat and hot water to occupied buildings such as homes, barns and greenhouses. However, hydronic heaters may also be located indoors and they may use other biomass as fuel (such as corn or wood pellets).
EPA anticipates proposing the NSPS revisions and the new regulations sometime in 2011.
Need further Assistance with NSPS rules?
Rafael Sanchez; Sanchez.firstname.lastname@example.org, 202.564.7028
Energy Efficiency - Why Replace an Old Wood Stove with a More Efficient Appliance?
Replacing an old wood stove or fireplace with a more energy efficient appliance can save fuel, money and protect you and your family's health. Older stoves that were manufactured before 1990 burn wood inefficiently which wastes firewood, pollutes the air in your neighborhood and creates dust inside your home. Newer stoves can reduce smoke and dust, as well as cut heating expenses. There are many cleaner, energy saving options, ranging from gas to high-tech wood stoves certified by the EPA. To locate an EPA-certified wood stove, see the list of appliances. Read more about older and newer stoves in the Dirty Little Secrets brochure.
There are approximately 12 million wood stoves in homes today and 9 million of those are older, non EPA-certified stoves that are 50% less efficient than newer stoves.
Energy efficiency benefits of replacing old wood stoves and fireplaces:
- Saves money, fuel, time and resources
- 50% more energy efficient
- Uses 1/3 less wood for the same heat
- Cuts creosote build-up in chimneys that helps reduce the risk of fire
- Produces 70% less particle pollution indoors and out
Environmental benefits of replacing old wood stoves and fireplaces with new appliances:
- Reduces indoor and outdoor wood smoke pollution which has been linked to cancer, asthma and other serious health conditions. See health effects of wood smoke for more information.
- Improved combustion efficiency reduces CO2, methane and black carbon emissions.
- Saves billions in health benefits each year.
Federal Tax Credit
If you choose to burn wood and want to purchase a new, more efficient wood-burning stove, you may qualify for a federal tax credit. In December 2010, the Residential Energy Efficiency Tax Credit was amended and extended for another year. It provides 10 percent (up to $500) for the purchase of a qualified biomass-burning stove between January 1, 2011 and December 31, 2011 for any wood- or pellet-burning stove that meets the 75 percent efficiency rating qualifies.
Please note that the thermal efficiency rating included on the hang tag may either be a default value (i.e., 63% for non catalytic wood stoves, 73% for catalytic wood stoves and 78% for pellet stoves) or a "measured" value. On February 6, 2007, the EPA approved use of the CSA B415 test protocol as a means by which to determine efficiency ratings. The IRS sponsored wood stove tax credit program allows manufacturers to use a different method of determining efficiency. IRS allows laboratories to use greater flexibility in determining the thermal efficiency rating for tax credit purposes. To apply for a tax credit, use IRS Form 5695 (see instructions for Lines 3a through 3c).
State Initiatives and Laws
With the increasing use of wood stoves nationwide, there is growing concern about the health and environmental effects of wood smoke. The burning of wood is known to produce a complex mixture of particulate and gaseous emissions. It can be a local problem because wood smoke is typically emitted close to the ground and is highly dependent on wind characteristics to dilute or disperse it. In response to these issues, many northern U.S. states and some southern states have implemented strategies to reduce air pollution from wood stoves. These strategies range from publishing good practices to enacting regulations with emissions standards (e.g., WA) more stringent than the Phase 2 NSPS. Also, some states provide incentives such as rebates on purchases of more efficient stoves. Click on the state initials below to find more information for your state for wood stoves.
To report errors or updates on the above tables, please email George Cushnie.
Numerous jurisdictions have established legal requirements to reduce wood smoke. For example, some communities have restrictions on installing wood-burning appliances in new construction (see examples). The most common and least restrictive action is to limit use at those times when air quality is threatened. The appropriate agency issues an alert, similar to the widespread Ozone Action Day alerts. For more information, contact your local city or county government.
EPA's Burn Wise website. A partnership program of the U.S. Environmental Protection Agency that emphasizes the importance of burning the right wood, the right way, in the right wood-burning appliance.
Strategies for Reducing Residential Wood Smoke (2009). This document was written for state, local and tribal air pollution control officials to have a comprehensive list of strategies to help communities reduce wood smoke from residential heating. It provides education and outreach tools, information on regulatory approaches to reduce wood smoke, as well as voluntary programs to change out old, inefficient wood stoves and fireplaces.
EPA's Burn Wise FAQs. This page provides answers to questions EPA has received about wood-burning appliances and wood smoke.