Jeff:
JJ> The previous homeowner did a DIY job of installing a new furnace using JJ> a twisted maze of flex duct to make all the supply and return runs in JJ> the basement.
(Oh-oh!)
JJ> Several heating contractors have advised us that flex is not intended JJ> for long runs and that this is the most flex they have ever JJ> seen. Also, there are no dampers and in some areas the flex is pinched JJ> and the insulation tearing.
I don't know about long run usage but pinching will cause decreased air flow (depending on the degree of the obstruction, of course). Additionally, the corrugations in the flex duct causes the air being pushed through to have 'turbalance' (for lack of a better word) ==> it bounces and catches on the sides, slowing down the airflow.
JJ> Couple of questions: JJ> - Is replacing flex with rigid sheet metal duct work generally JJ> worthwile?
I would say yes, as well as having the proper sized ducts. We had an addition put on a couple of years ago, requiring some modification to the ductwork. They noted I had installed a booster fan in the run to the kitchen. A few questions and they were also told the Dining Room was also a problem area. They replaced a section with larger ductwork, correcting those problems. (There is something called a "Manual J" used to calculate what is needed.)
JJ> - Is it better to use custom sheet metal fabricated to fit tightly JJ> between the joists or should we go with standard "round" ducts? JJ> More generally are there different qualities of sheet metal that JJ> we should be concerned about?
This system is a mix of round and rectangular -- also old and new; go with whatever the contractor decides. A rectangular duct using all the available room between the joists would have a larger capacity than a round duct fitting within the joists (and no sticking out). Yes, the stores with the 'industrial look' open ceiling use round ducts -- they're probably sturdier and more efficient than rectangular ducts but inthat application one doesn't have to be concerned with headroom.
As for the sheetmetal guage -- good question! You don't want something so thin it will pop/boom every time the air flow starts and stops. There is usually an slight bend in the shape of an "X" to keep the ductwork from flexing. Again, tell the contractor the possible noise is a concern and let him determine the proper guage. You might want to ensure there is a clause in the contract to this effect.
JJ> - One contractor talked about "vinyl clad duct insulation". Is that JJ> a good insulation solution?
No idea. I would think this is only a concern if the ductwork is passing through an uninsulated crawlspace or through an attic.
JJ> - What types of dampers should we be installing for best results?
Ones appropriate for the type of duct installed! They are not designed to stop all air flow. Once the mechanical ones are set generally best to leave them alone: the system is balanced. There are electronic dampers (I think only for rectangular ducts) but these for for zoned systems.
JJ> - Any other questions we should be asking regarding duct work?
I think you covered most of them!
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