HHV vs LHV efficiency Same stove and two honest numbers

Short version: compare stoves on the EPA's HHV figure. If a spec sheet says 87.5% or "85% efficient," look for the HHV number in the manual or the EPA database before you compare.

Where the Confusion Starts

“Can someone please explain the difference like I’m a 5 year old” asked one r/PelletStoveTalk member (LHV vs HHV?). Another, shopping for a 2,400 sq ft house, noted the Harman models “have only 75% efficiency. I see others like PelPro 130 claims 82% and I also found a model (forget name) at 87.5% efficiency” (How much does efficiency matter?). The PelPro PP130 is both of those last two numbers: 87.5% LHV on its product page and 81.9% HHV in its manual.

What HHV and LHV Mean

Burning wood or pellets produces water vapor, from moisture in the fuel and from the hydrogen in the wood itself. Turning that water into vapor takes energy. The USDA Forest Products Laboratory puts it simply: the higher heating value “is the amount of energy available from its complete combustion, including the energy from condensing the water vapor,” and the lower heating value “is the HHV minus the energy from condensing the water vapor” (USDA FPL Fuel Value Calculator).

A stove’s chimney carries that vapor away uncondensed, so its energy is never recovered. HHV efficiency counts it as a loss; LHV efficiency doesn’t count it at all. The result is a fixed gap on the same test.

The Same Test, Both Ways

Each HHV figure matches the stove’s EPA listing when rounded (the Woodstock 210a has since been replaced in the database by the 210b, listed at 76%), which is how you can tell the database is on the HHV basis. Testing follows CSA B415.1-10, which the federal rule requires for efficiency and heat output (40 CFR 60.534(e)).

Combustion vs Overall Efficiency

There’s a second trap. EPA’s Burn Wise guidance separates combustion efficiency (how completely the fire burns the fuel) from overall efficiency (how much of the fuel’s heat reaches the room). It warns that “an advertisement on a website or brochure may say ‘85% efficient’ and although not specified, it is the combustion efficiency. The overall efficiency may be 10% to 20% lower” (EPA Burn Wise). The EPA database lists overall efficiency.

A “best burn rate” figure is a third variation. EPA’s number is a weighted average of four burn rates. Blaze King’s test report for the Princess shows 83.9% at the lowest burn rate and 75.4% at the highest; the 82% in the listing is the weighted average across all four.

How Much Does Efficiency Matter?

Fuel use scales directly with it. Going from 70% to 80% HHV cuts the wood or pellets you burn for the same heat by about 12.5%. On a 100 million BTU season at USDA’s 20 million BTU per cord of seasoned firewood, that’s 7.1 cords at 70% against 6.3 cords at 80%. Try your own season and prices in the fuel calculator.

Across the EPA database, catalytic wood stoves have a median of 78% HHV, hybrids 79% and non-catalytic stoves 71%. Pellet stoves have a median of 77%. See catalytic vs non-catalytic for the breakdown.