Two ways to count the same fire
Burning wood makes water vapor, and that vapor holds energy that goes up the chimney. Higher heating value counts that energy in the total. Lower heating value leaves it out. Because HHV starts from a larger total, the same stove scores a lower percentage on HHV than on LHV.
Why the 75 percent line mattered
The federal 25C credit for biomass stoves required at least 75 percent HHV efficiency, which is why so many listings printed that figure from 2023. The credit ended for property placed in service after December 31, 2025, so a listing that still leads with tax credit language is out of date. The efficiency number itself remains useful.
How to compare two listings
Use a figure only when it names HHV or an EPA test. If one stove says 77 percent and the other says 82 percent with no basis, you cannot rank them. Efficiency also comes from a laboratory burn, so your wood and your chimney will move the real result.
What we do on this site
We print the percentage only as the listing states it, with the basis if it gives one. Where the listing says nothing we say that too. We do not convert LHV into HHV, because the conversion would be our number, not the maker’s.
Questions that keep arriving by post
Which is the honest figure, HHV or LHV?
Both are honest if they are labeled. HHV starts from a higher energy total, so its percentage comes out lower. The problem is a bare percentage with no basis at all.
My listing prints no efficiency at all. Is that a bad sign?
It means the listing does not tell you. Many low-cost stoves give BTU and heating area only. Ask the seller for the certified HHV figure before treating two stoves as equal.
Last reviewed November 1, 2026