To deliberately mislead consumers, appliance manufacturers quote their appliances' efficiency at Net cv (calorific value) or ncv. This enables them to quote a higher number, & so con purchasers into thinking their energy use will be less than it actually will be.
A manufacturer that states his appliance is, for example, 95% efficient (ncv) misleads a purchaser into thinking that only 5% of the fuel / energy is wasted into the flue.
Net cv is only a theoretical figure. It is thermodynamically impossibility to buy fuel / energy at Net cv, so by definition, all energy is supplied at Gross cv. The relationship between Net & Gross cv (in the UK for Natural Gas) is that Net cv is 1.11 x Gross cv.
e.g. An appliance that is quoted as 95% efficient at Net cv has a Gross efficiency of only 85.595%. In other words, a consumer who buys 100 units of fuel / energy, gets not 95 but only 85.595 units of useable heat.
The fact that manufacturers can quote efficiencies at Net cv means they can quote a figure greater than 100% for modern condensing boilers. (Could be approx 104% Net cv.) Hands up those who thought that at 104% efficiency, that boiler gave them 4% more heat than they purchased.
That boiler quoted at 104% Net cv efficiency is only 93.704% efficient and loses 6.296% of energy by case & flue losses.