pressurized air f c into the rear portion of the cavity chamber or tunnel 65 formed in the heat exchanger 50 as depicted in figures the rear portion of the cavity chamber or tunnel 65 formed in the heal exchanger 50 is partially obscured by the lateral tubes 57 c traversing the tunnel 65 thus the second or rear vent 87 is configured so as to project the second flow of secondary pressurized air f c through gaps existing between adjacent lateral tubes 57 the injection of the second flow of secondary pressurized air f c provides a thermal air barrier that partially insulates the lateral tubes 57 c traversing the back of the heat exchanger 50 in addition the second flow of secondary pressurized air f c also absorbs the heat produced by the substantially horizontal combustion flow 69 generating a flow of heated flue gases 88 which exhausts up through the heat exchanger 50 during operation in one embodiment the second vent 87 may also be angled at a slightly upward angle with reference again to the figures and in particular to fig5 and 6 the air supply system of the present invention will be described in greater detail the air supply system of the present invention a forced air or pressurized system which is not susceptible to changes in ambient conditions such as wind direction or gustiness the air supply system of the present invention is comprised of primary and secondary air systems the primary air system supplies large volumes of pressurized air to the multiple burner assemblies 60 configured in the side of.