How to Connect a Dryer to the Wall Vent with a Transition Duct
Why this matters
The few feet of duct between the back of the dryer and the wall collar decide how well the whole vent works. A good wall run with a crushed transition behind the dryer still dries in two cycles, overheats the dryer until its limits trip, packs lint where nobody looks, and on a gas dryer pushes exhaust carrying carbon monoxide back into the laundry room. The failure is almost always made in the last ten seconds of the job, when the dryer is pushed back against the wall and the duct folds behind it. This card covers choosing the duct, cutting and clamping it, and pushing the dryer home without crushing it. Who keeps crushing these and why is the subject of 'The Dryer Transition Duct and the Trades That Crush It'; how long the total run may be belongs to 'Dryer Exhaust Length and Fittings Card'.
Power, gas and lint
Power. Unplug the dryer before you pull it out, gas or electric; a gas dryer still has a 120 V cord for its motor and igniter. The cord and the vent share the space behind the dryer, and a cord pinched under a foot or between the cabinet and wall gets damaged.
Gas dryer. Pulling the dryer out flexes the gas connector. If you smell gas at any point, everyone leaves the house, no switches, lights or phones are operated inside, and the gas utility is called from outside. After the dryer is back in place, leak-test the connector at both ends with a noncorrosive leak-detection solution, never a flame; any change to the gas piping itself goes to a licensed gas fitter or plumber per local rules, which 'The Gas-Side Boundary for an Appliance Tech' covers.
Lint. The old transition and the wall collar are often packed with lint, which is the fuel in a dryer fire. Vacuum it out rather than shaking it loose. Wear an N95 filtering facepiece when the buildup is heavy, since vacuuming and pulling apart a packed duct throws fine lint and dust into the air you breathe; voluntary use falls under 29 CFR 1910.134 Appendix D.
Which duct
What is allowed. IRC M1502, as adopted by your jurisdiction, requires in substance that a transition duct be a single length listed and labeled to UL 2158A, no longer than 8 ft, and not concealed within walls, floors or ceilings (M1502.4.3 in the 2021 edition; check the edition your jurisdiction uses). The dryer maker's installation instructions are usually stricter and are the document the dryer's listing rests on, so read them too.
- Semi-rigid aluminum duct is the default. It holds its shape, bends without kinking at gentle radii, and has a smoother bore than foil. Most listed transition ducts are this type.
- A rigid periscope duct, a flat rectangular listed section that slides to length, solves the tight install where the dryer must sit close to the wall and the outlet lines up roughly with the collar. Check that its free area is not reduced by the flattening on the model you choose.
- Foil accordion duct that crushes under a fingertip is what you are usually replacing. Some foil products carry the listing, but they collapse and sag easily, and many dryer makers' instructions exclude them.
- White vinyl or plastic duct is not used. It is not listed for this use, dryer makers prohibit it, and it burns.
Diameter. 4 in, matching the dryer outlet and the wall collar. Reducing to a smaller size anywhere chokes the flow.
Measure at the final position
The duct has to fit the dryer where it will sit, not where it sits while you work on it. Before cutting anything:
- Mark the final position. Decide how far the dryer back will sit from the wall: the dryer instructions give the minimum clearance, and enough room for the duct to curve without folding is what really sets it.
- Measure three things: the gap from the dryer back to the wall at that position, the side-to-side offset between the dryer outlet and the wall collar, and the height difference between them.
- Picture the path. The duct leaves the dryer outlet, makes one or two gentle bends, and enters the collar. Tight bends at either end are where it kinks.
top view, dryer pushed back to its final position
WALL
==========[ COLLAR ]===============================
| 4 in wall collar, duct clamped
\
\ semi-rigid transition:
\ one gentle curve, no sag,
\ no fold, no kink at either end
\
| dryer outlet, duct clamped
+-----------------[ OUTLET ]------------------+
| |
| DRYER (back panel at the top) |
| |
+---------------------------------------------+
gap from dryer back to wall: measured and
kept, so the curve has room to stay round
Fitting the duct
- Clean the collar and the outlet. Vacuum lint out of the wall collar and as far into the wall run as the vacuum reaches, and out of the dryer outlet. If the wall run itself is packed, it gets cleaned before the transition goes on.
- Cut to length. Semi-rigid duct stretches and compresses; cut it so that, at the final position, it spans the path with gentle bends and no straight pull and no slack loop. Too long and it sags or folds behind the dryer; too short and it pulls off the collar when the dryer moves. Cut with tin snips, and fold any sharp edge back on itself, wearing gloves; a raw cut edge of aluminum duct slices skin.
- Slide each end over its fitting. Fully onto the dryer outlet and fully onto the wall collar, so the duct overlaps each by its full collar depth.
- Clamp both ends. A 4 in worm-gear clamp or a spring clamp at each end, tightened over the overlap. No sheet-metal screws into the duct or through the collar: IRC M1502, in substance, prohibits screws or similar fasteners that protrude more than 1/8 in into the duct, and a screw through a transition duct or its collar is the usual offender; each one catches lint and starts a plug.
- Foil tape on joints only. If the dryer instructions call for it, foil tape (not cloth duct tape, which dries out and falls off) goes around the outside of the clamped joint to seal it. It never replaces the clamp, and it never patches a hole in the duct; a damaged duct is replaced.
- Check the outside hood. The termination flap must open freely and close by gravity. IRC M1502, in substance, requires a backdraft damper at the termination and no screen, since a screen packs with lint. If the hood has a screen or a flap that sticks, that is part of the job.
Pushing the dryer home
This is where the job fails. Push the dryer back slowly, a few inches at a time, while watching the duct. Watch from the side of the dryer with a flashlight, have a second person watch from above, or hold a phone camera behind the dryer to see the duct. Stop if the duct starts to flatten, fold or kink, pull the dryer forward, and fix the cause, which is usually duct too long, a bend starting too close to the outlet, or a dryer pushed closer than the measured gap. Plug the cord back in, and on a gas dryer check that the gas connector is not kinked as the dryer goes back. Then level the dryer.
A worked case
A gas dryer taking two cycles to dry a normal load, and the cabinet top hot to the touch. The vent run in the wall is short and clean, checked with a brush from the outside hood.
Behind the dryer: a foil accordion duct about 7 ft long, piled into a heap behind the cabinet, flattened where the dryer back rests on it, with a fist of lint caught in the lowest fold. One end is held on the wall collar by duct tape, no clamp.
The dryer is unplugged and pulled out, gas connector untouched, and the old duct is bagged. The collar and the dryer outlet are vacuumed. Final position: the customer wants the dryer about 6 in from the wall, and the outlet sits 12 in to the right of the collar and 3 in lower.
A semi-rigid listed transition is cut so it spans that path in one long curve, a little over 2 ft in this layout, rather than the 7 ft that was there. Clamps at both ends, foil tape over the clamped joints as this dryer's instructions call for. The outside hood flap is checked and swings freely.
The dryer goes back a few inches at a time with a phone camera behind it. At about 7 in from the wall, the camera shows the curve starting to flatten near the outlet. The dryer comes forward, the duct is shortened by about 1 in and its bend eased away from the outlet, and the second push goes to 6 in with the duct still round. The gas connector is leak-tested at both ends with solution: no bubbles.
What would have changed the call. If the customer had needed the dryer at 2 in from the wall, no round duct would hold its shape in that space, and a listed periscope duct becomes the choice. If the wall run had been long, with several elbows, the transition's length and bends count toward the maximum run, and the length card decides whether a booster fan or a reroute is needed. If the dryer had been electric, the leak test drops out; the cord is checked for pinching either way.
The finish test
- Run the dryer on an air-only or no-heat setting with the door closed.
- Go to the outside hood. The flap stands fully open, and air comes out hard enough to feel strongly with your hand at the hood. A flap barely lifting, or weak airflow, means a restriction is still in the path: the transition, the wall run, or the hood.
- Run on heat for ten minutes and feel the exhaust: warm and moist, strong flow, and the dryer cabinet top no hotter than a hand rests on comfortably.
- On a gas dryer, with it running, recheck that no exhaust is coming into the room at the back of the dryer.
Write on the ticket: what was removed, the duct type and length fitted, clamp type, hood condition, the push-back check, the leak test on a gas dryer, and that strong airflow was confirmed at the hood.
References
- International Residential Code, Section M1502 (clothes dryer exhaust), as adopted by your jurisdiction; M1502.4.3 in the 2021 edition for transition ducts
- UL 2158A, Outline of Investigation for Clothes Dryer Transition Duct (the listing a transition duct carries)
- See related: The Dryer Transition Duct and the Trades That Crush It; Dryer Exhaust Length and Fittings Card
- See related: The Gas-Side Boundary for an Appliance Tech
- 29 CFR 1910.134 Appendix D, voluntary use of respirators (OSHA general industry); dryer maker's installation instructions