Espar air heater ducting size

D2, S2 D2L, AS3 D2L, AS3 B2L uses only 60mm ducting. Cant increase the size.

D4,B4, M2 D4L, M2 B4L, AM3 D4l, AM3 B4L uses either 90mm or 75mm, however, the initial ducting has to be 90mm. Using 60mm on a 4kw heater will most likely result in heat reflection.