3. Total inlet pipe length is not to exceed total equivalent length of 15 ft. In case an outside intake riser (Fig. 1) is used, add in this intake length to the total inlet pipe length. (Allow 1 ft for each 90º elbow. 4. Use the provided 4" to 3" reducer to connect duct to burner intake (Riello burner only. Secure with silicone to burner intake. Use wraparound clamps at both the burner and the inlet termination to provide an easy means of disconnecting the combustion air intake from the burner for service and inspection purposes.