An oxygen trim is an ‘add-on’ to the digital combustion controller. A probe is installed in the boiler flue exit and constantly samples the flue gases for the optimum oxygen content and ‘trims’ the air setting to ensure that the burner fires at optimum efficiency throughout the firing range.
It also allows for variations in the air-to-fuel ratio due to combustion air temperature, pressure and humidity changes, as well as minor variations in the chemical make up of the fuel gas. Another advantage is it takes into account inconsistencies of the fuel to air ratio control equipment such as valves.
With the fuel characteristics programmed in and fixed, the combustion air is constantly adjusted whatever the firing rate to ensure optimum combustion efficiency throughout the firing range.
Additional savings of approximately 2% can be achieved by utilising a trim system.
A CO trim works in a similar way to an oxygen trim but the probe is designed to trim the CO instead of oxygen, ensuring that optimum efficiency is maintained throughout the operation of the boiler.