The design fuel of the product is light oil or natural gas; The fuel is ejected from the burner, ignited by the electronic igniter, and combusted at a slight positive pressure in the furnace liner. The burnt high temperature flue gas enters the second and third return spiral smoke tubes by turning 180 ° from the combustion chamber, and enters the economizer by turning 180 ° from the upper part of the rear smoke box for heat exchange, and then is discharged into the atmosphere through the chimney.