Forno FRHRE5 37-30 Consignes D'installation Et Mode D'emploi page 9

φ 6" for 30" & 10'' for 36", 48'' and 60''
6" to 8" for
the 30" & from 10" to 12" for the 36", 48" and 60".
e.g. – 1 roof cap, 2x90 elbows, 1x45 elbow =
30' + 30' + 9'=69' used, 31' available for
straight duct runs