The boiler efficiency determines the amount of fuel that is consumed. The higher the efficiency, the less energy is needed to reach and maintain a certain temperature in an environment, for example in the house. High performance means efficiency and savings are achieved through efficiency.
Having said this, how can boiler efficiency be improved? The indication is given by the formula that defines the efficiency itself: R = energy transferred to the water / energy produced by the complete combustion of the fuel.
Let's start with the second expression, namely the energy produced by the complete combustion of the fuel. For the fuel to burn completely, the right amount of air is needed, no less and no more. In fact, if you use less air than necessary, part of the gas does not burn and does not transfer all the heat to the water it has to heat. If you use more, the excess air absorbs part of the heat which, consequently, does not go to the water to be heated.
It should be borne in mind that excess air is usually zero in gaseous fuels. The reason is that in gases the mixing of air and fuel is at the molecular level, which excludes the presence of excess air. Instead, it rises in nebulized liquid fuels or in very fine pulverized solid ones. There is a very high excess of air in large-sized solid fuels.
Turning to the other expression in the formula of boiler efficiency, that of the energy transferred to the water, inevitably it is lower than that of the heat produced by the combustion of the fuel because it is partly dispersed through the combustion chamber (passing through the walls) and partly in the fumes through the flue.
In order not to lose the heat that comes out with the fumes, some boilers use a heat exchanger (which is useful to ensure proper operation) which decreases the temperature of both the air used for combustion and the water that enters the boiler. The most modern and efficient boilers are condensing boilers which obtain high performances thanks to the recovery of the latent heat of the water vapor contained in the fumes.
To subtract energy from the water to be heated, by lowering the boiler efficiencyalso contribute to the encrustations of the internal pipes, the so-called tube bundle. The pipes must therefore be kept constantly clean.