exchanger 150 referring now to fig1 b illustrated is an end view of the vestibule panel 111 with the burner assembly 140 of fig1 a removed and showing one insert 130 of course it is understood that for maximum efficiency each combustion tube 120 a 120 g would have a similar insert 130 in this view it can be seen that the insert 130 comprises three sides 131 133 and a tab 134 the insert 130 is coupled to and supported by the tab 134 that is coupled to the frame 110 in any conventional manner in one embodiment the tab 134 is captured between the vestibule panel 111 and the corbel plate 112 referring now to fig2 illustrated is a sectional view of the heat exchanger 150 along plane a a of fig1 b with the burner 113 a shown and an inset of the insert 130 a in relation to the combustion tube 120 a and primary combustion zone 231 in this particular view the insert is specifically insert 130 a in this sectional view it can be seen that the tab 134 is captured between the vestibule panel 111 and the corbel plate 112 thereby supporting the insert sides 131 133 side 132 not visible above the inside bottom and below the inside top of the combustion tube 120 a therefore only the tab 134 contacts any part of the frame 110 or.