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.
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
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
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! <g> 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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