HHV efficiency
HHV (higher heating value) efficiency is the share of the total energy in the wood — including the energy lost evaporating its moisture — that a stove delivers as useful heat. It is the conservative efficiency measure and the one the EPA database certifies; certified stoves typically run 60–85% HHV.
Manufacturers sometimes quote LHV (lower heating value) efficiency, which ignores the energy spent boiling off fuel moisture and therefore reads several points higher for the same stove. When comparing models, make sure both numbers are on the same basis — every efficiency figure on StoveAtlas is HHV, straight from the EPA certification record.
The now-expired federal 25C tax credit used 75% HHV as its qualifying threshold, which is why that number became the informal marker of a “high-efficiency” stove.
| Basis | Higher heating value — total fuel energy, moisture penalty included |
|---|---|
| Range across the certified fleet | 59–83% among the 158 currently certified room heaters; median 74% |
| Upper quartile | 78% and above; only about 13% of the fleet reaches 80% |
| LHV gap | LHV figures read roughly 5–8 points higher for the same appliance |
| Former 25C threshold | 75% HHV (credit ended for property placed in service after 31 Dec 2025) |
| Cost of fuel moisture | About 1,200 BTU per pound of water boiled out of the wood |
What the number is measuring
Oven-dry wood holds roughly 8,600 BTU per pound, and that figure barely varies between species — oak and pine differ in density, not in energy per pound. HHV efficiency compares the heat a stove actually delivers into the room against that full gross energy content of the fuel charged into it, including the portion that never becomes useful heat because it went into turning the wood’s own water into steam and pushing it up the flue.
Two losses dominate. Stack loss is the heat carried away by hot flue gas — unavoidable, because a flue that runs too cool condenses creosote. Combustion loss is the fuel energy that leaves as smoke, carbon monoxide, and unburned hydrocarbons instead of being oxidised in the firebox. A stove at 74% HHV is losing roughly a quarter of the log up the chimney between those two paths.
The figure comes from the same certification test that produces the emission rate, so it reflects a properly seasoned fuel charge and correct operation across the tested burn rates. It is not a steady-state boiler efficiency and it is not something you can reproduce with a stovepipe thermometer at home.
HHV vs LHV, and why brochures disagree with the database
Lower heating value subtracts the latent heat of the water vapour in the exhaust before doing the arithmetic — it asks how much of the recoverable energy you captured rather than how much of the total energy. For wood the ratio is about 0.93, so the same appliance that certifies at 75% HHV can honestly be advertised at roughly 80% LHV.
European test standards report on a net (LHV) basis, which is why a stove imported from the EU frequently shows a higher number in its own literature than in its EPA record. Neither figure is a lie; they answer different questions. The practical rule is to compare EPA record against EPA record and treat any efficiency claim without a stated basis as marketing.
Watch also for “combustion efficiency” — the fraction of the fuel that burned completely, typically in the high nineties — quoted as if it were overall efficiency. It says nothing about whether the resulting heat reached the room.
What a few extra points are actually worth
Fuel use scales inversely with efficiency, so the saving from an upgrade is smaller than the percentage-point gap suggests. Moving from 70% to 80% HHV cuts wood consumption by about 12.5% — on a four-cord season, half a cord, or roughly $100–$200 depending on local prices. Moving from a pre-certification stove around 50% to a modern 75% unit cuts it by a third, which is a real difference.
Efficiency and emissions are not the same axis and the certified data shows only a loose relationship between them. Several of the cleanest-burning stoves in the database sit near the fleet median on efficiency, and a few high-efficiency models are ordinary on particulates. If both matter, filter on both rather than assuming one implies the other.
There is a ceiling worth knowing about: a very high efficiency figure means cooler flue gas, and a cool flue in an exterior masonry chimney is where creosote condenses. High-efficiency appliances are the ones most sensitive to an oversized, cold, or short flue — an insulated liner sized to the stove collar matters more here, not less.
Frequently asked questions
What is a good HHV efficiency for a wood stove?
Across the 158 currently certified room heaters the median is 74% HHV, the range is 59–83%, and only about 13% reach 80%. Anything at 75% or above is genuinely above average — that was the threshold the former federal 25C credit used. Below about 68% you are in the bottom quarter of the certified fleet and burning noticeably more wood for the same heat.
Is HHV or LHV efficiency higher?
LHV is always the higher number, by roughly 5–8 percentage points for wood, because it excludes the energy spent evaporating the fuel’s moisture before calculating the ratio. A stove certified at 75% HHV is about 80% LHV. The EPA database publishes HHV, so compare EPA figures against EPA figures rather than against a brochure that may be quoting the net basis.
Does a more efficient stove burn cleaner?
Not reliably. Efficiency measures how much heat reaches the room; the emission rate measures how much particulate leaves the chimney. The certified data shows only a weak correlation — some of the lowest-emission stoves are mid-pack on efficiency and vice versa. If you care about both, check both columns rather than treating one as a proxy.
Why is the efficiency in the brochure higher than the EPA number?
Almost always because the brochure quotes LHV (net) efficiency or a combustion-efficiency figure rather than the HHV overall efficiency the EPA certifies. European-designed stoves are the usual case, since EU test standards report on the net basis. The EPA record is the number tested under a US protocol and is the one to use for comparison.
How much wood does a higher-efficiency stove actually save?
Fuel use is inversely proportional to efficiency, so 70% to 80% saves about 12.5% of your wood — roughly half a cord on a four-cord season. The larger jump is from an uncertified older stove at perhaps 50% to a modern 75% unit, which cuts consumption by about a third and pays back far faster than moving between two already-certified models.