It has been established to calculate the rated efficiency of a boiler based on the Lower Thermal Power, and no additional energy could be calculated from the cooling and condensation of the boiler exhaust. A gas boiler with a rated efficiency of 90%, when consuming 1m³ of fuel, produces (8,130 * 0.9) = 7.317Kcal.
The state-of-the-art boiler with condensing technology and controlled oxygen supply through the fan's variable speed, has a corresponding efficiency of up to 95%, so with the same 1m³ of natural gas it produces (8.130 * 0.95) = 7.723Kcal + 1.032Kcal (from cooling). and its condensation) = 8.755Kcal. Thus, a "conventional" yield is obtained: 8.755 / 8.130 = 1.08 or 108%
Condensing boilers were developed to exploit the latent heat of vaporization when condensing the produced water vapor into the exhaust gas (which comes from fuel hydrogen, fuel moisture and combustion air humidity). Gaseous fuels have the advantage of increased hydrogen content, while not containing sulfur.
In a condensing boiler we can exploit about 50% to 80% of the latent heat of vaporization of the water vapor, achieving a high efficiency of up to about 108% (with reference to the lower calorific value of the fuel).