inlet openings 70 of the blower wheels 19 which are vertically spaced apart in the illustrated embodiment the inlet openings 70 are circular and the exhaust tube section is aligned with the vertical diameters of such inlet openings as is typically the case the inlet opening 70 of each blower is coaxial with the rotational axis of the blower wheel along its length the exhaust tube section 54 is provided with exhaust outlets 73 and 74 the outlets 73 and 74 are respectively disposed in front of the inlet openings 70 of blowers 17 and 18 as is preferred the exhaust outlets direct the combustion products towards a lower region of the inlet openings of the respective blowers more particularly flow of combustion products is directed towards the lowermost quarter segment of the inlet opening with additional reference to fig5 and 6 the lower outlet 74 is formed by an opening 76 in the side wall of the exhaust tube section 54 and annexhaust vent nozzle 77 as shown the nozzle has parallel top and bottom walls 78 and 79 and outwardly flared side walls 80 and 81 the walls may be formed by plates welded to each other along adjacent edges and to the exhaust tube section around the perimeter of outlet opening 76 the nozzle 77 opens radially outwardly in a direction parallel to the rotational.