OAK duct venting

[Spanky93]

Hello,

I am installing a RSF Focus 3600 and am at the point of running the duct for the OAK.

My run is a little over 25' so I am needing to bump it up from a 4 to a 6 inch vent (anything over 25' needs to be 5" per the manual). My question is what type of ducting can I use for this? The manual states "Use an insulated aluminum flexible duct rated at over 200° F". All the insulated flexible duct I have found (Lowes, HD, Supply House) is rated for 140° continuous and 200° intermittent.

I am not sure if I am overthinking this whole thing, but it will be inside a chase so I want to do it correctly the first time.

Thank you!

 

[stoveliker]

I'd run non-flexible duct instead.
A run this long is bound to collect dust and debris, and will be hard to clean out. In particular with the undulations from it being flexible.
It'd be overall sturdier for such a long length too.

Non-flex duct (i.e. just metal) can be insulated to ones hearts content.
Indeed do insulate it, as cold air will come in and you don't want condensation to drip from that duct (in joist bays or walls).