Find the best exit path for the 4-in. duct
Whether you’re installing a new vent or upgrading an existing one, the first step is to determine where the duct will run. This can be a bit of a challenge. Keep in mind these rules of thumb. The shorter the distance and fewer the turns the better.
Use no more than 25 ft. of 4-in. duct, and subtract 5 ft. for every 90-degree turn and 2.5 feet for every 45-degree turn. For example, a dryer with a 90-degree elbow at the exhaust port and another at the top of the basement wall can run a maximum of 15 ft. Dryer manufacturers’ recommendations supersede this, so if you have to make a longer run, read the owner’s manual. If possible, position your dryer along an exterior wall to keep the vent as short as possible.
Figures A, B and C show the three most common paths from the dryer to the exterior. Vent a dryer through the roof only as a last resort. Cleaning lint buildup from roof vents is difficult, and nesting birds and squirrels often find roof vent caps irresistible. Check with your appliance dealer or local building inspector for recommendations about the best roof vent cap to use.
Avoid windows (Photo 1) and outdoor obstructions like the electrical service entry. The bottom of the vent must be at least 12 in. above the ground and 12 in. away from any obstructions such as decks or air conditioners.
Run your vent the shortest route through a rim joist or a side wall. Vent a dryer through the roof only as a last resort. Cleaning lint buildup from roof vents is difficult, and nesting birds and squirrels often find roof vent caps irresistible. Check with your appliance dealer or local building inspector for recommendations about the best roof vent cap to use.
Measure the vent route
If everything looks good outside, go back inside, mark the vent location, and plan the duct route (Photo 1). Buy enough straight duct and fittings to complete the job, including an outdoor cap. Use either galvanized steel (our choice) or aluminum duct, but don’t mix parts.
Installation techniques - outdoors
Drill a pilot hole so you can see exactly where the vent will go outside (Photo 2). If the hole is a little low or the cap doesn’t sit in the best position on the siding, it’s not too late to adjust it (Photo 3). If it’s really wrong, you can easily fill the hole and start over at a different location. If you’re cutting through a finished wall, open up a small section and check for wiring, ductwork and other hazards before drilling.
Boring a 4-1/4 in. diameter hole can be tricky. Take your time. A bimetal hole saw is easiest to use, because it’ll chew through an occasional nail without dulling and cut cleanly through vinyl or aluminum siding. Hold your drill firmly with two hands (Photo 4) and brace it against your leg if possible. Don’t lock the trigger on. If the drill tilts to one side, it may bind and jerk. Pull out the saw frequently to keep it cooler and reduce binding, and then go back into the hole. Standard hole saws are 1-1/2 to 1-3/4 in. deep, so you’ll have to stop and push out the layers of siding and sheathing, or chisel out part of the framing, before you cut completely through (Photo 5).
After the hole is finished, slide the vent cap through, square it against the siding, and attach it and caulk it (Photo 6). Screw the cap down evenly—if one corner is tighter than the others, the vent flapper won’t seal. Vent caps with a single flap and a large hood (Photo 6) work the best, but if you want a lower profile, you can use a cap that has several narrower flaps. But clean it more often; the flaps trap more lint.
Vinyl mounting block technique
How to Mount a Vent Cap on Vinyl Siding
There are two good ways to install a vent through vinyl siding. One, install a vinyl surface mounting block (shown here) that fits over the existing siding.
You may need to scribe and trim the edges to precisely fit your siding profile. Fasten the surface mounting block to the house with screws, but remember to drill slightly oversize holes through the vinyl to allow for siding movement. Two, remove a section of siding and install a special vinyl mounting block (not shown) against the sheathing. Cut the siding to fit around it. Both are available from vinyl siding dealers.
Drill and chisel technique for masonry siding
How to Make the Vent Hole in Stucco and Brick Veneer
Getting through stucco and brick requires different techniques and tools than for wood and vinyl. You’ll need a masonry bit with a hammer drill for your pilot hole. Then trace a 4-1/2 in. circle and drill a series of closely spaced holes around the circumference with the 1/4-in. masonry bit and hammer drill. Break out the masonry with a cold chisel. Snip the wire mesh back if you’re chipping out stucco. Then drill through the wood sheathing and framing with a 4-1/4 in. hole saw.
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Vent installation - indoors
Go back inside and caulk around the duct on the inside of the rim joist. You can use the plastic trim ring (included with the vent cap) for a neater appearance.
Now you’re ready to run the duct from the dryer to the vent cap. Push the lower elbow onto the dryer and the upper elbow onto the vent cap, and then measure the distance between the two, including the overlap at each end. Cut, assemble and tape the straight pieces (Photos 7 – 9).
Straight duct is available in various lengths up to 5 ft. Be sure to cut the pieces to length with tin snips before snapping the seam together (Photo 8). Elbows can be twisted to form any angle, and with two or three of them you can generally snake your way around any obstacle.
Begin assembly from either end. You can always shift the dryer slightly if you cut the final section a little short or long. However, orientation is critical when joining sections end to end. The crimped end always slides into the fitting above so that the exhaust flows smoothly from one section to the next.
Push the final elbow into the straight piece that’s connected to the cap. (We had to extend our cap; see Photo 11.) Because the pieces will be at a slight angle to each other, you may need to fuss with them to make the crimped end slide in properly.
Don’t rely on tape to hold the duct sections together—over time the adhesive will dry out and lose its stickiness. This is especially true of standard cloth or plastic duct tape. Metal foil tape is more expensive, but it will last longer. Still, any vent run longer than a few feet will require more permanent support. Strap it to the wall or ceiling framing with plastic or metal pipe strap to keep it rigid and hold it together for the long run (Photo 12).
Finally, plug the dryer in, turn it on and check outside to make sure the vent flap is opening.
A note on cleaning: Lint buildup reduces dryer efficiency, which means clothes take longer to dry. Check the vent cap frequently and brush out lint. At least once a year, snake a vacuum hose into the duct to clean it. Tapping on the duct while you do this will help knock off lint that’s stuck to the sides and joints. To avoid fires, vacuum lint from around dryer drums, burners and motors at the bottom of the cabinet.
Wear leather gloves when cutting ducts. The metal is sharp, and even brushing your hand lightly against an edge can produce a surprisingly large cut.