Condensing boilers are highly efficient equipment increasingly used to heat water for heating or industrial use, characterized by taking advantage of the residual heat of the combustion gases, including the condensation latent heat of water vapor. The present work analyzes important aspects to optimize the design and operation of this type of equipment from the energy and exergy point of view, specifically, the effect that changes in fuel, water inlet temperature (20–70 °C), excess air used in combustion (5–100 %) and relative humidity of the air (10–100 %), have on their energy and exergy efficiencies. For this purpose, the energy and exergy balance equations for the reactive and heat exchange processes that occur within it were implemented and solved using the computational program engineering equation solver (EES). The model was validated by comparing its results with the efficiency curve of a commercial condensing boiler model. The results show an important effect of the fuel type on the operating ranges in condensing and non-condensing modes and on the energy efficiency values, finding that the technology is widely justified when used with natural gas, and not so much with the other fuels analyzed. Likewise, a favorable effect of the reduction of excess air for combustion on energy efficiency can be seen, which is why it is advisable to operate this equipment with the least possible amount of air that guarantees good combustion. On the other hand, exergy efficiency has the highest values using natural gas, and benefits from a higher water return temperature and lower excess air. The greatest irreversibilities are found in the main coil and the combustion chamber.