Cord wood vs crib wood test fuel
Crib wood is a standardized lab fuel — dimensional lumber nailed into open cribs — used in the traditional EPA test; cord wood testing burns real split logs, closer to how people actually use stoves. The 2020 NSPS allows either: 2.0 g/hr crib-tested or 2.5 g/hr cord-wood-tested.
Crib tests are highly repeatable but flatter than real life; cord-wood tests better predict in-home performance, which is why the EPA and several states have pushed toward them. The database records which fuel each certification used.
When comparing two stoves’ g/hr figures, check the test fuel: a 2.4 g/hr cord-wood result and a 1.9 g/hr crib result are not directly comparable numbers.
| Crib fuel | Kiln-dried Douglas fir dimensional lumber, spaced and stacked, 16–20% moisture (dry basis) |
|---|---|
| Cord-wood fuel | Real split rounds, specified at roughly 19–25% moisture (wet basis) |
| Applicable limits | 2.0 g/hr crib; 2.5 g/hr cord wood |
| Mix among currently certified room heaters | 119 crib, 31 cord wood, 7 pellet |
| Burn categories tested | Four rates, from under 0.80 kg/hr to the appliance maximum |
| Where it is recorded | The certified fuel type appears on every model page and in the EPA record |
Why a lab fuel exists at all
Certification only means something if two labs testing the same stove get the same answer, and if a stove tested in 2016 can be compared with one tested in 2026. Real firewood makes that impossible: density varies threefold between species, moisture varies with the weather, bark and knots change the burn, and how the operator stacks the load changes the air paths through it.
Crib fuel removes those variables. Identical kiln-dried Douglas fir pieces, cut to defined dimensions, spaced with nailed strips into an open lattice, at a controlled 16–20% moisture on a dry basis. Every run has the same geometry, the same air channels, and the same energy content. Loaded to a specified weight per cubic foot of firebox, the result is a test that repeats.
The price of that repeatability is realism. A crib burns from the outside of a regular lattice inward, at a rate the geometry largely dictates. A firebox packed with irregular splits burns unevenly, settles halfway through, and presents the air control with a very different problem.
Where the two methods diverge
Cord-wood testing burns real splits held to roughly 19–25% moisture on a wet basis, loaded the way a homeowner would. It is markedly harder to control: runs vary more, hitting the low burn-rate category reliably is difficult, and the same stove tested twice will not give identical numbers. The half-gram difference between the 2.0 g/hr crib limit and the 2.5 g/hr cord-wood limit is the regulatory acknowledgement of that spread — it is not a statement that cord-wood stoves are allowed to be dirtier.
The reason to prefer cord-wood results anyway is that they measure the fuel you will actually burn. Stoves optimised against a crib can perform less well on splits, and the well-documented gap between certified and in-home emissions is partly a fuel-geometry gap. A cord-wood certification is a closer prediction of your chimney.
It is also, at present, the minority path. Of the 158 currently certified room heaters, 119 were crib-tested, 31 cord-wood tested, and 7 are pellet appliances. So most head-to-head comparisons you will make are crib against crib, where the fuel question does not arise.
Comparing across test fuels without fooling yourself
There is no conversion factor between the two methods, and anyone offering one is guessing. The most defensible way to compare is to express each result as a share of the limit it was measured against. A 1.9 g/hr crib result is 95% of its 2.0 g/hr ceiling; a 2.2 g/hr cord-wood result is 88% of its 2.5 g/hr ceiling. On that basis the cord-wood stove is the better performer relative to what was asked of it, even though its raw number is higher.
That method is rough, but it beats ranking raw g/hr across fuels, which systematically flatters crib-tested appliances. Where the difference between two stoves is a tenth of a gram and the test fuels differ, the honest conclusion is that the certification does not separate them and you should decide on efficiency, output range, firebox size, or dealer support instead.
One thing the test fuel does not change is what you burn. A crib-certified stove is not a stove that wants lumber; it is a cord-wood stove that was measured with lumber. The fuel that determines your real emissions is the one on your stack, and getting it under 20% moisture moves the number far more than the choice between two similar certifications ever will.
Frequently asked questions
Is a cord-wood tested stove better?
Its number is a closer prediction of real-world performance, because it was measured on the fuel you actually burn. But it is not automatically a cleaner stove — the limits differ (2.5 g/hr cord wood vs 2.0 crib) precisely because cord-wood testing is less repeatable. Compare each result against its own limit rather than ranking raw g/hr figures across methods.
Why is the cord wood limit 2.5 g/hr instead of 2.0?
Because real split wood burns less predictably than a lab crib. Density, moisture, bark, and how the load settles all vary between runs, so cord-wood results spread more widely for the same appliance. The extra half gram is headroom for that variability, not permission to be dirtier.
What is crib wood?
A standardised lab fuel: kiln-dried Douglas fir dimensional lumber cut to defined sizes and nailed into an open lattice with spacers, held at 16–20% moisture on a dry basis, loaded to a specified weight per cubic foot of firebox. Its uniform geometry is what makes emission results repeatable between labs and across years.
How do I compare a crib-tested stove with a cord-wood tested one?
Express each as a fraction of its own limit. 1.9 g/hr crib is 95% of the 2.0 g/hr ceiling; 2.2 g/hr cord wood is 88% of the 2.5 g/hr ceiling, so the cord-wood stove looks better relative to what was asked of it. There is no exact conversion between the methods, so if the gap is small, decide on other attributes instead.
Does the test fuel affect the efficiency number?
Yes, though less dramatically than the emission number. HHV efficiency is measured during the same runs, so it reflects the same fuel geometry and moisture. As with emissions, the fairest comparison is between two records certified on the same fuel.