Round metal ductwork is versatile, long-lasting and easy to work with. Learn the best methods for cutting and installing it from a master tin bender.
By the DIY experts of The Family Handyman Magazine:July/August 2012
Whether you’re adding
new heat runs in a
basement or changing the
layout of an existing HVAC
system, you’ll probably be
working with round metal duct
pipe. We invited Bob Schmahl
to give us a few pointers. Bob’s
been a tin bender for more
than 40 years. He insists he
still doesn’t know everything
about ductwork, but we
weren’t convinced. These tips
should help make your next job run that much smoother.
Adding heat runs in a basement may change
the airflow in the ductwork going to other rooms. Each register
should have its own damper that can be accessed
for adjustment. If those dampers can’t be
accessed from below, you’ll want to install them
close enough to the register so that you can
reach them through the register opening. Bob
likes 4 x 10-in. boots (not shown)—you can
easily fit your hand in them to adjust
dampers, and there are more
grate cover options for that size.
If you have to disassemble existing ductwork
fittings, there’s no need to peel
off the old foil tape first. Instead,
just score the tape at the seam
with a utility knife and remove the
screws right through the tape.
When it comes time to retape, just
clean off the dust and apply new tape right over the old.
There’s no question that flexible duct is
easier to install than metal ductwork, but
consider this: Flexible duct can degrade
over time. It collects dust and is almost
impossible to clean. Flexible duct needs
to be larger than pipes to allow the same
amount of airflow. The most common
problem Bob has seen: “People get careless and turn corners too sharp, which creates kinks that severely restrict airflow.”
When assembling pipe, start at one end
and work the seam together like a zipper. Use
one hand to keep the two edges close and the
other to apply downward pressure. Use your leg, a
workbench or the ground to support the back side
of the pipe. If you make a mistake and have to dismantle
a pipe, slam it down flat on the ground, seam side up. It should pop right apart.
Aviator snips work fine to cut holes in a trunk line, but
only if there’s enough space. If you’re dealing
with close quarters and you own a right-angle
drill or attachment, you may
want to invest in a sheet metal
hole cutter. Otherwise you
might have to take
down the trunk line.
You can buy hole cutters
online for about
$65. Malco is one
When you’re installing a pipe between two fixed parts, it’s
impossible to slip in the piece using the crimped ends and
still get the required 1-1/2-in. overlaps at both ends. Overlap
one side as you normally would and create a butt joint on
the other. Use a draw band connector to complete the butt
If your supplier doesn’t carry draw band connectors, make your own by
cutting a piece of pipe to overlap the ends, and then screw
and tape the band into place. If you’re working with 6-in. pipe, you’ll need to use 7-in. pipe for the bands.
Caulk (don’t tape) the
the trunk line and a
before you connect
pipes to it. That way,
you’ll be able to turn
the take-off out of the
way to caulk above it. Regular silicone is fine.
If the ducts are going to be concealed, all seams need to be
taped or caulked. Here’s Bob’s trick for taping a seam on a
pipe that’s installed close to the subfloor: Cut a piece to
length. Peel off part of the backing. Slide the backing up and
over the pipe. Finally, pull down on the backing, which will
pull the tape along with it. Inspectors will want to know
you’ve used an approved tape, so buy the stuff with writing on it, or keep the roll on-site until inspection.
When cutting pipe, Bob likes to
mark the size he needs on each
side of the open seam with a
marker. Flat metal is easier to
cut than curved, so he uses his
knee to support and flatten the
pipe while he opens it up. Then
you just sight on the far mark
while you make the cut. It’ll be
straight and perfect every time.
Bob prefers snips made by
Malco, which cost less than
$35 online. Unless you
enjoy trips to the ER, wear
gloves when cutting pipe—the stuff is razor sharp.
Each pipe needs support. You can use just about
any support you want, but adjustable steel support
brackets are quick and easy. And don’t
forget to screw the pipe to the joist hanger so the
pipes won’t rattle when someone stomps across
the floor above. Every connection needs three
screws. They don’t have to be evenly spaced.
Use 1-in. galvanized zip screws designed for sheet metal.
Figuring out the right combination
of turns to get an elbow to point in
the right direction can be perplexing.
Bob recommends moving one “gore”
(elbow ring) at a time, starting with the connected
side. And don’t make 90-degree turns if you don’t have to. A 90-degree elbow creates the same resistance as adding 5 ft. of pipe.
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
In addition you will need a hole cutter, a right angle drill or right angle attachment and leather gloves.
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.
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