Flue gas temperature in hot oil boilers is 80 ° C above the operating temperature. The recovery of this heat from the flue is possible by means of economizers. Boiler efficiency increases by 1% for every 20 ° C of reduction in flue gas temperature with economizers.

 
Economizers can drop to a certain temperature depending on the type of flue gas fuel. In natural gas boilers, this value is around 120 ° C. Depreciation times of economizers can vary depending on capacity and temperature.