Condensing Boilers and the Strange Case of Over-100% Efficiency Ratings
In a hurry? Skip straight to the numbers.
Open the HHV to LHV Calculator →A boiler efficiency rating of 107% sounds like a typo or a marketing exaggeration - it's neither. It's a real, achievable number, and understanding why requires knowing exactly what LHV assumes is permanently lost.
What LHV Assumes You'll Never Recover
As this calculator's own content explains, LHV excludes the latent heat of the water vapor formed during combustion, on the assumption that this water vapor exits the equipment as vapor, taking its latent heat energy with it, never condensing back to liquid to release that energy as usable heat. This was historically a reasonable assumption for most combustion equipment, since flue gas usually stays well above water's condensation temperature all the way to the stack exit.
Condensing Boilers Break That Assumption on Purpose
Condensing boiler technology is specifically designed to cool the flue gas, using an additional heat exchanger stage, down below the water dew point covered elsewhere in this category - deliberately causing the water vapor in the flue gas to condense back into liquid right inside the equipment, and capturing the latent heat released by that condensation as additional useful heat, rather than letting it escape up the stack as LHV assumes. This is exactly why condensing boilers achieve meaningfully higher real-world efficiency than traditional non-condensing designs burning the identical fuel.
Why the Efficiency Rating Can Exceed 100%
Efficiency ratings are calculated as useful heat output divided by fuel energy input - and if that fuel energy input figure is calculated using LHV (which, by definition, already excludes the latent heat as "unusable"), then a condensing boiler that actually recovers and uses part of that supposedly-unusable latent heat can show more useful output than the LHV-basis input figure accounted for in the first place, mathematically producing an efficiency rating that exceeds 100% on an LHV basis. This isn't a violation of thermodynamics - it's simply a mismatch between the (LHV) energy accounting convention used for the denominator and the real (HHV-inclusive) heat actually being captured and delivered.
| Efficiency reporting basis | Condensing boiler typical rating | Why |
|---|---|---|
| LHV basis | Can exceed 100% (e.g. 105-110%) | Recovers latent heat that LHV's denominator assumed was unusable |
| HHV basis | Always below 100% | HHV's denominator already accounts for the full latent heat potential |
Why This Causes Genuine Confusion in the HVAC Industry
North American residential and commercial HVAC efficiency ratings are traditionally reported on an HHV basis, where condensing equipment shows a high but still sub-100% efficiency figure (commonly in the 90-98% range) - while some European and industrial specifications default to LHV reporting, where the identical piece of condensing equipment can show a rating that appears to exceed 100%. A buyer or engineer comparing efficiency ratings across regions or specification standards without checking which basis each figure uses can end up comparing numbers that aren't actually describing the same underlying performance at all.
Applying This When Comparing Efficiency Ratings
Before comparing any two boiler efficiency figures - especially anything reported above 100% - confirm whether each is stated on an HHV or LHV basis using exactly the conversion this calculator performs; a seemingly impossible efficiency rating almost always has a simple, legitimate explanation once the reporting basis is identified.
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