A heating boiler comprises an outer housing with a cylindrical wall in the housing extending from one end thereof and defining therewithin a primary fire chamber. A cylindrical fire space wall portion is radially spaced outwardly of the cylindrical fire wall and extends along the length of the fire wall and has a secondary portion which extends axially beyond the length of the cylindrical wall and has a greater diameter than the cylindrical wall and defines therein a secondary fire chamber which communicates with the primary fire chamber. The housing also contains a wall spaced radially inwardly of its interior which defines a chamber water space between the wall and the fire space wall and also a hot water tank between the wall and the interior of the housing. A plurality of smoke tubes are arranged at spaced locations around the fire space wall and they extend through the chamber water space and permit passage of gases from the end of the secondary fire space remote from the primary space through the chamber water space and to a waste gas pipe leading out of the boiler. Tubular thin wall lining is located between the cylindrical wall defining the primary fire chamber and the fire space wall portion spaced radially outwardly thereof leaving a clearance between the cylindrical wall defining the primary fire chamber and the fire space wall which is bridged by the lining under certain operating temperature conditions.