The Oxygen measurement at stack will show the boiler/ hot oil heater efficiency Excess air will be heated and lose go to the stack. Control the excess combustion air, the speed of induced draft fan control by the VSD (Variable speed drive) to get optimum air and highest efficiency, reduce the fuel consumption and electricity consume of ID Fan.
Density of air in day and night can be varies +-15oC. The night time we get the cool air which more air density. The systems can automatic control and always get boiler at high efficiency.