swings the cleaning head 61 from the retracted position down and forwardly from the pivot 84 and into the air port 30 defining a locus indicated in fig2 by the dashed line 126 where the end of the cleaning member contacts and dislodges the excrescent material 32 the rod 63 is provided with an adjusting means such as a turnbuckle 128 for adjusting the length of the actuating rod 63 upon installation of the cleaning head assembly 60 in the plenum chamber 42 fig1 and 4 show the cleaning head 61 in a retracted position inside the plenum chamber 42 whereby the adjustable damper assembly 43 can be moved to a predetermined position for adjusting combustion air which flows from the duct via openings 130 131 in side walls 50 51 through the plenum chamber 42 to the opening 52 fig1 shows the damper assembly 43 in a near closed downward position for blocking all but a minimal flow of air through the plenum while fig2 illustrates the damper in a full open position with maximum air flow in the plenum 42 referring now to fig2 and 6 12 the adjustable damper assembly 43 is illustrated in greater detail comprising a mounting assembly 140 through which a damper actuating rod 142 is slidably and pivotally mounted through fulcrum tube 144 in a manner known in the art the damper actuating rod 142 is pivotally attached by way of a threaded t shaped connecting member 146 to arms 148 only one of which is shown in the figures of a slotted bell crank pivot member 150 which arms form a clevis between which pivot 152 extends for passing.