exterior wall 30 of the boiler section 14 provides a secondary heating surface the walls 28 30 are preferably constructed of steel or cast iron the water 15 is subjected to heat energy through radiation and convection as the hot combustion gases illustrated by the wavy arrows contact the interior wall 28 the exhaust gases 17 within the auxiliary heat transfer ducts 24 24 a provide supplemental heat energy by conduction through the exterior wall 30 the additional heat decreases the time for the boiler to reach optimum operating temperature a panel or jacket 32 fabricated with an insulating material 33 is placed over the ducts 24 24 a to reduce heat loss the heat retention also maintains the boiler operating temperature for longer duration of time after shutdown the exhaust gas discharge is controlled by a plurality of damper members as will further be discussed with reference to fig3 a flue damper member 34 is located within the main flue pipe 20 the damper member 34 is mounted for incremental rotational displacement for controlling passage of exhaust gases and may be manually set or preferably automatically controlled by a motor 36 the auxiliary heat transfer ducts 24 24 a contain a set of intake damper members 38 38 a preferably controlled by a motor 40 for incremental settings another set of adjustable vent damper members 42 42 a are preferably respectively controlled by a motor 44 44 a this arrangement provides independent operation of each damper member 42 42 a for compensatory venting in the event of unbalanced exhaust gas flow the motors 36 40 44 44 a are actuated by a control module 46 such as a microprocessor or equivalent type.