Emission rate (g/hr)
The emission rate is the grams of particulate matter (smoke) a stove releases per hour of operation, measured over a standardized EPA lab test. The 2020 NSPS caps new room heaters at 2.0 g/hr (2.5 g/hr when tested with cord wood); the cleanest certified stoves test below 1 g/hr.
Particulates — fine soot and condensed organics — are the pollutant that makes wood smoke a health issue, so the certified g/hr figure is the headline number in the EPA database. It is a weighted average across burn rates, not a worst-case or best-case figure.
The lab number assumes dry test fuel and correct operation. A certified 1.5 g/hr stove burning unseasoned wood with the air choked down can emit many times its rating — fuel moisture and operation matter as much as the design.
| 2020 NSPS Step 2 limit | 2.0 g/hr crib wood; 2.5 g/hr cord wood (adjustable-rate room heaters) |
|---|---|
| 2015 Step 1 limit | 4.5 g/hr — roughly double the current ceiling |
| Currently certified room heaters | 0.3–2.4 g/hr; median about 1.5 g/hr |
| Share at or under 1.0 g/hr | About 24% of the 158 current room heaters |
| By technology (median) | 0.94 g/hr hybrid, 1.1 catalytic, 1.7 non-catalytic |
| Test burn categories | Four rates, from below 0.80 kg/hr to the stove’s maximum |
How the number is produced
The stove is installed in a lab on a scale, connected to a dilution tunnel, and burned through a series of runs at different air settings. Particulate is collected on filters from a measured sample of the diluted exhaust, the fuel consumed is weighed continuously, and each run yields grams of particulate per hour at that burn rate. The four categories span a low smolder (under 0.80 kg/hr of fuel), two intermediate rates, and the maximum the stove will run.
The certified figure is a weighted average of those runs, which is why it is neither the best nor the worst thing the stove can do. A stove that is very clean wide open and dirty at a smolder and a stove that is mediocre everywhere can land on the same certified number, and the record does not break the runs out for you.
Fuel is specified tightly, because it dominates the result. Crib tests use kiln-dried Douglas fir dimensional lumber at 16–20% moisture on a dry basis; cord-wood tests use real splits held to roughly 19–25% on a wet basis. Nothing about the test resembles a shoulder-season reload onto a thin bed of coals with wood that came off the stack yesterday.
Why your chimney and the lab disagree
Three things in normal use push real emissions above the certified figure. Wet fuel is the largest: water keeps the firebox below the roughly 1,000 °F that secondary combustion needs and below the temperature at which a catalyst stays lit, so the wood gas leaves as smoke instead of burning. Cold starts are the second — a stove produces most of its daily particulate in the first fifteen minutes, before the flue and firebox are hot. Choking the air down to stretch a load is the third.
Field studies consistently find in-home emissions well above certification values for exactly these reasons, and the gap widens the more a stove is run outside its designed operating band. The certification number is best read as a ranking of designs under controlled conditions, not a promise about your chimney.
The practical consequence is that fuel and operation are worth more than a fraction of a gram on the spec sheet. A 1.8 g/hr stove burning 18% wood with the air open until the load is charred will out-perform a 0.9 g/hr stove fed damp splits and shut down every night.
Reading g/hr against test fuel and appliance class
Two stoves are only directly comparable if they were certified on the same fuel. Because the cord-wood limit is 2.5 g/hr and the crib limit 2.0, a fair normalisation is to express each result as a fraction of its own ceiling: 1.9 g/hr crib is 95% of its limit, while 2.2 g/hr cord wood is 88% of its. Among currently certified room heaters, 119 were crib-tested and 31 cord-wood tested, so most head-to-head comparisons are crib against crib anyway.
Not every certified appliance has a g/hr number at all. Forced-air furnaces and hydronic heaters are regulated on a heat-output basis — pounds of particulate per million BTU delivered — which is why the emission column is blank for furnaces in the database. Comparing a boiler’s lb/MMBtu against a stove’s g/hr is meaningless without a conversion that depends on the appliance’s output.
Locally, the number can have teeth. Air districts with winter particulate problems run curtailment (“burn ban”) days, restrict installation of uncertified appliances, and fund change-out rebates that are usually tied to certified emission performance. Check the local air agency before assuming any stove can be installed anywhere.
Frequently asked questions
What is a good emission rate for a wood stove?
Under 1.5 g/hr puts a stove in the cleaner half of the currently certified fleet, and under 1.0 g/hr puts it in the cleanest quarter — about 24% of the 158 current room heaters clear that bar. The cleanest certified models run around 0.3–0.5 g/hr. Anything at 2.0–2.4 g/hr is legal but at the top of the certified range.
What is the EPA limit for wood stove emissions?
Since 15 May 2020, new adjustable-burn-rate room heaters must certify at or below 2.0 grams of particulate per hour on crib wood, or 2.5 g/hr if tested with cord wood. The previous Step 1 limit was 4.5 g/hr. The limits govern manufacture and sale of new appliances, not the operation of a stove already installed.
Do low-emission stoves put out less heat?
No — emissions and output are independent. The certified fleet includes sub-1 g/hr stoves with peak outputs above 60,000 BTU/h. Low emissions come from burning the smoke inside the firebox rather than releasing it, which if anything returns more heat to the room from the same log.
Why do wood furnaces not have a g/hr rating?
Forced-air furnaces and hydronic heaters are regulated under a different subpart of the NSPS that expresses emissions per unit of heat delivered — 0.15 lb/MMBtu for furnaces and 0.10 lb/MMBtu for hydronic heaters — rather than per hour. That makes sense for appliances whose output varies enormously, but it means their numbers do not line up with a room heater’s g/hr.
Can a certified stove still smoke?
Yes, easily. Wet wood, a cold start, an over-damped air control, or an oversized stove being run at a permanent smoulder will all produce visible smoke from a certified appliance. The certification demonstrates what the design can do with 20% fuel and correct operation; sustained visible smoke after the first fifteen minutes of a burn means something about the fuel, the air setting, or the draft is wrong.