“Efficiency” in our business means the ratio of the useful output energy to total input energy. Unfortunately, when evaluating combustion performance, there are two versions of the input energy because any hydrocarbon fuel has both “gross” and “net” calorific values (GCV and NCV).

To understand the difference, you have to appreciate that the products of combustion include water vapour, and that it takes energy (latent heat) to vaporise water whether it happens in a kettle or as part of the combustion process. In a condensing boiler you get that latent heat back. A fuel’s GCV counts all its chemical energy but its NCV disregards that fraction (10% in the case of natural gas) that will be absorbed as latent heat. So when you calculate efficiency on the basis of NCV you get a higher value than if you had used GCV, to the extent that you see condensing boilers advertised as having over 100% efficiency. That is actually true on an NCV basis, but only because there’s energy in the fuel that NCV ignores.

Why does this matter? Because when you look at a combustion test report from a maintenance contractor it may well be on an NCV basis, which somewhat flatters the performance. I prefer to use the GCV basis. Some combustion analysers also make an allowance for boiler standing losses in an effort to give a supposedly more realistic overall efficiency figure, but that just clouds the issue in my mind.

If you want to be sure you are getting results (a) in GCV terms and (b) without deductions for standing losses, you can take the raw measurements from a boiler test and feed them into this on-line calculator, which incidentally lets you try changing the input assumptions for a side-by-side estimate of the savings that would result.