Learn & Understand

HHV vs LHV: The Heating-Value Ambiguity That Makes Efficiency Numbers Argue

The companion calculator works backward from steam demand to a fuel firing rate using the fuel's heating value. That heating value hides a genuine ambiguity: most fuels have two of them, differing by a meaningful margin, and which one you use quietly changes both the fuel figure and any efficiency number attached to it. Mismatch them and two honest engineers can argue past each other forever.

Why a Fuel Has Two Heating Values

Burning a hydrocarbon fuel produces water vapor. That water carries a large amount of latent heat, and whether you count it defines the two heating values.

The gap between them is exactly the latent heat of the water formed in combustion, which for hydrogen-rich fuels like natural gas is a substantial fraction of the total. The fuel did not change; the accounting did.

Why This Muddies Efficiency

Efficiency is useful energy out divided by fuel energy in, and the fuel energy in depends on which heating value you used as the denominator. Use the lower value and the same boiler posts a higher efficiency number; use the higher value and it posts a lower one, purely because the denominator changed. This is why efficiency figures from different regions or vendors can seem to disagree when the boilers are identical, they are quoting against different heating-value bases. Comparing an LHV-based efficiency against an HHV-based one is comparing two different questions.

The Condensing Boiler Paradox

The most striking consequence is boilers that appear to exceed one hundred percent efficient. A condensing boiler is designed to cool its flue gas enough to condense the water vapor and recover that latent heat, heat that a conventional boiler throws away. If its efficiency is quoted against the lower heating value, which by definition assumed that latent heat was lost, then recovering it makes the boiler look better than one hundred percent. Nothing magical is happening and no energy is created; the boiler is simply capturing heat the LHV basis had already written off. Measured against the higher heating value, the same boiler sits sensibly below one hundred percent.

Fuel Choice Rides on the Same Idea

The heating-value distinction also shapes fuel selection. Hydrogen-rich fuels have the biggest gap between their two values, because they make the most water when burned, which means they have the most latent heat available to recover in a condensing system. This is part of why natural gas pairs so well with condensing technology, there is a lot of latent heat waiting in its exhaust.

Using the Fuel Requirement Well

Take the calculator's fuel firing rate as accurate only when the heating value you entered and the efficiency you assumed are on the same basis, both higher or both lower. When you compare boilers or read a vendor's efficiency claim, find out which heating value it uses before trusting the number, and do not be alarmed by a condensing boiler quoted above one hundred percent, it is recovering latent heat the lower heating value pretended was gone.