How to read EPA wood stove certification data (g/hr, HHV, Step 2)

Updated July 23, 2026 · Reviewed against the current StoveAtlas dataset

Emission rate (grams per hour)

This is the headline certification number: grams of particulate matter — fine smoke and condensed organic compounds — released per hour of operation, averaged across a weighted set of burn rates in a standardized lab test. Lower is cleaner. The current New Source Performance Standards (NSPS) "Step 2" limits, in force since May 2020, cap new room heaters at 2.0 g/hr when tested with crib (dimensional lumber) fuel, or 2.5 g/hr when tested with cord wood. Many currently certified stoves test well below that ceiling — browse the heaters certified under 2 g/hr to see how far under the limit the cleanest designs run.

The g/hr figure is a lab result, not a guarantee: it assumes correctly seasoned test fuel and proper operation. A stove certified at 1.5 g/hr burning wet wood with the air choked down can emit several times its rated figure in the real world — see firewood moisture and seasoning.

HHV efficiency

Efficiency on the EPA database is expressed against the fuel's higher heating value (HHV) — the total energy in the wood, including the energy that goes into evaporating the fuel's own moisture. HHV is the more conservative of the two common efficiency bases; manufacturers sometimes quote a higher-looking LHV (lower heating value) number for the same stove, so make sure you're comparing figures on the same basis. Certified stoves on this site typically run 60–85% HHV efficiency. The former federal 25C tax credit used 75% HHV as its qualifying threshold — see the tax-credit guide for what that credit covered and its current (expired) status — which is why 75% became an informal marker of a "high efficiency" stove. Browse heaters at or above 75% HHV directly.

Test fuel: cord wood vs crib wood

The database records which test fuel produced a given certification. Crib wood is a standardized lab fuel — dimensional lumber assembled into open cribs — used in the traditional EPA test method; it's highly repeatable but burns somewhat differently than real firewood. Cord-wood testing burns actual split logs and better predicts in-home performance, which is why EPA and a number of states have pushed toward it. Because the two methods use different limits (2.0 g/hr crib vs 2.5 g/hr cord wood), don't directly rank a crib-tested 1.9 g/hr stove above a cord-wood-tested 2.2 g/hr stove without noting the test method — they aren't on identical scales.

2020 NSPS ("Step 2") compliance

The 2015 rule introduced initial ("Step 1") limits; the Step 2 limits that took effect in May 2020 roughly halved allowable emissions and, for the first time, brought forced-air furnaces and hydronic heaters under the same kind of caps as room heaters. Only Step 2-compliant models may be newly manufactured or sold in the United States today — the currently certified, in-production list reflects that. NSPS compliance governs manufacture and sale of new units, not the legality of operating an older, previously certified or uncertified stove already installed — though some states and local air districts separately restrict or offer change-out incentives for older units.

Certification numbers and covered models

Each certificate carries an EPA certification number and a list of model numbers it covers — often several trims or rebadges of the same underlying firebox tested as one design. The number appears on the appliance's permanent label and in the database record; installers, insurers, and some local permitting offices check it, and it's the quickest way to confirm a stove you're looking at second-hand is the same design as the certified record you're reading. Every model page on this site links back to its source EPA record so you can verify the raw data yourself.

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