The inlet diameter for a Type O fan assembly can be either be a 4" inlet spigot (FHBTO-4) (https://www.reznor.co.uk/product/type-o-fan-assembly-100mm-4-inlet/) or a 6" inlet spigot (FHBTO-6) (https://www.reznor.co.uk/product/type-o-fan-assembly-150mm-6-inlet/. Please view the product pages for more information.
What is the inlet diameter for a Type'O fan assembly?
General technical information about Reznor's products