I have a question about proper running ductwork in attic.
I have had someone tell me best to have ductwork in an attic next to and
even run between ceiling joist when possible and have the installation blown
in on top instead of having the ductwork hanging from several feet above the
joists. Said that even with insulation wrapped around the ductwork, the
heat/cold in the attic will eventually seep into the duct and will get a
blast of unheated/hot air out when first comes on. By having the duct as
close to the living space when possible and insulating as much on top of the
ductwork keeps the heat/cold loss to a minimum.
I can see where this may make sense, But is this done in any real
installations or would be praticle. I have seen somewhat both ways of being
done, not so sure if that was intended when I have seen the ductwork laying
between or across ceiling joists instead of being hung a few feet above ?
Any ideas as to do/not do it that way ?
If you ever have to work on or modify the ducts, it is nice to have it
hanging. (But WELL supported.)
With a heat pump and R-6 ducts in the attic, here in South Ccarolina,
the heat loss in the heating cycle is 5%. The heat gain in the summer
(cooling) is 10%. Source: ACCA Manual J, 7th edition.
Hang it up so it can be serviced. Also, if you bury it, depending on
the vapor barrier, sometimes the ducts can sweat, which can cause
problems as well.
(Paul, I know there is a snide remark coming, so let 'er rip.)
one of my rentals was like that. when i put central ac in, i had to
stack 2 feet of insulation around the intake up there to get it to cool
the house.also 2 power vents . ill say one thing for the setup, its
quieter than the intake in the wall.. lucas
Well, The house was one of the ones several years ago before Georgia got
stricter on builders that was the run of the mill redneck built, anyone with
a hammer was building houses, only needed to have a business permit and some
may not have had that. Everything done about as cheap as it could have been
and they called it custom built ?
About half of the ductwork was a flex type with a plastic covering that
completely disintegrated in most places and the insulation is starting to
come undone despite my attempt to rewrap and tape into place.
I am adding another room on, The Furnace/AC needs replacing so I am just
getting advice on how the new unit should be installed, I have done some
HVAC work in the past but not much on the duct work part.
Thanks for the info, any additional comments welcomed.
Use the foil /mylar covered flex to replace the old flex. Use at least
R-6 flex, R-8 If available. Flex duct should be sized at least 1 inch
bigger in diameter than sheet metal to pass the same amount of air.
Seal ALL connections with mastic and fiberglass mesh tape, NOT duct
The resistance to air flow is higher with flex due to corrugated inside
surface in flex VS smooth surface with sheet metal.
ACCA sells a ductulator (duct calculator circular slide rule with
scales for: Sheet metal, flex, duct board, lined sheet metal). Check
out the same air flow with the same static; the flex will be one size
bigger than sheet metal. The above ductulator is $45.00 to non members
Just bought one because I lost mine. Bought it at their online store.
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