Condensing boilers are able to recover heat from the boiler’s flue gasses, thereby leading to them having the potential to operate at higher efficiencies than standard boilers.
These efficiencies can only be achieved however, if the water temperatures that your heating and hot water systems are low enough. i.e. the return water temperature should be below 50°C. Above this temperature the boiler will not have any advantage over a standard boiler.
To maximise the efficiencies of a condensing boiler, your building’s boiler systems should be designed to operate at a 70° flow and 40° return temperature, in line with current CIBSE guidance.