flexible ductwork on indoor furnace install

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NCredneck

New Member
Aug 29, 2011
82
RDU area NC
I have read several posts that say only metal ductwork should be used on wood furnace installations. I understand that the temps can get very high if the fan is not running due to a power failure or fan failure. Does this mean no flexible ductwork anywhere on the system? (ie. main trunk is metal but take offs are flex) At some point it would have to be safe wouldn't it due to air temp drop or metal duct acting as a heat sink? I can see the tie in from furnace to main trunk not being flex and maybe even a minimum distance of metal trunk but I gotta believe there are alot of add-ons that hit existing systems that would not have all metal duct or proper clearances from combustibles. So whats the code say in your state?
 
I'm not sure what code reads on the issue, but most manuals state that metal ducting to be used only. Looking at some of the data sheets on flexible ductwork, theres a rating of 0-200*f. I have personally seen our ductwork so hot that a drop of water would dance and sizzle during an outage. The design also plays a role where some ducting is setup for a gravity feed. As far as other installs, if something goes wrong and the insurance company finds either the wrong ducting or clearances they may drop the claim. Most if not all manuals state proper clearances and duct types to be used to avoid confusion and also for safety.
 
So let me get this straight, you can't run an ss flex liner down a chimney an hook it to an indoor furnace? Sorry for the newb question. Doesn't make any sense to not be able to use it since it's used for wood stoves.
 
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