Dryer venting dimensions and clearances

Developed length, and the two ends people get wrong#

The number in the code is developed length, and it is not the distance from the machine to the wall.

It is measured from the point where the duct connects to the dryer, at the back of the machine, to the outlet of the terminal on the outside of the building. Both ends surprise people. The near end is the machine's own outlet, not the wall behind it, so the 6 in (15 cm) of elbow immediately behind the dryer is inside the measurement. The far end is the outside face of the hood, including the short spigot behind the damper, not the point where the duct leaves the cavity.

Developed length is also not the same as the physical length. Every fitting is converted into an equivalent length of straight duct and added in. A duct with three elbows and 4 ft of pipe has a physical length of about 5 ft and a developed length near 20 ft, and it is the second number the code is written against.

Take the measurements once, from a drawn route rather than a guessed one, in the way reading architectural drawings sets out for services generally. A route sketched on the framing before the board goes up costs nothing to change.

Why the duct is 4 in, and why bigger is worse#

A domestic dryer moves 150 to 200 cfm (255 to 340 m3/h) and its blower is rated for a static pressure of roughly 0.6 in w.c. (150 Pa). Those two numbers are fixed by the machine. The duct does not change how much air moves, only how fast it moves and how hard the blower works.

Velocity is flow divided by area, and area goes with the square of the diameter.

Going from 4 in to 6 in cuts the velocity to 44 per cent of what it was. Lint is a fibrous solid being carried in an airstream, and it stays airborne only while the air is moving fast enough to carry it. Slow the air and the lint drops out along the horizontal runs, which is why a generously sized duct silts up faster than a tight one. The general principle is standard ventilation practice, and it is the same reason a range hood duct is sized to a velocity rather than to a diameter.

The last row deserves attention because it is sold as the equivalent of 4 in round. The 3 1/4 x 10 in rectangular duct that fits inside a 2x4 stud bay has two and a half times the free area of a 4 in pipe, which is deliberate: its awkward aspect ratio costs friction, and the extra area buys that back. The consequence, which nobody prints on the box, is that the air in it is moving at 665 fpm. It is the slowest duct in the house and it is the one buried inside a wall where it cannot be inspected.

The 35 ft budget and what each fitting costs#

The IRC's own table varies the elbow figure with the radius of the fitting, and a long-radius elbow costs less than a tight one, so where a route is marginal the fitting specification is worth reading rather than assuming. Manufacturers' installation instructions override the code table in either direction.

Now spend the budget on a real installation. A dryer in a closet on an interior wall, venting sideways through a stud bay to an outside wall 18 ft away:

Elbow off the dryer to turn towards the wall 5 ft Elbow to turn up into the stud bay 5 ft Elbow at the top plate to run horizontally 5 ft Actual duct, 4 ft vertical plus 18 ft horizontal 22 ft ------------------------------------------------------ Developed length 37 ft

That fails, and it fails by less than the cost of one fitting.

Saving depth behind the machine#

The periscope is the retrofit answer where the wall is already closed. It is a flattened rectangular section that lets the machine sit within 3 in (76 mm) of the wall and lets the duct rise or fall to meet an existing hole. It costs a little static pressure and it does not remove an elbow, so it buys depth without buying length.

Whatever the connection, the machine must be able to move forward for servicing without crushing the duct. A rigid elbow with a short listed flexible transition onto it is the arrangement that survives the machine being pulled out and pushed back, and the transition has to remain visible.

Where the duct may terminate#

The exhaust terminates outdoors and nowhere else. Not in an attic, not in a crawl space, not into a soffit vent, not into a wall cavity, and not into a loft where the moisture will find the coldest timber in the building.

The clearances at the terminal:

• 3 ft (914 mm) in any direction from any opening into the building, which includes windows, doors and mechanical air intakes. The dimension runs to the openable part of the window, not to its frame, and it applies above the vent as well as beside it. Where a vent sits under a window, the sill and head heights in window sill and head heights are the numbers that decide whether it clears.
• 12 in (305 mm) above finished grade as a typical manufacturer requirement, so the terminal is above snow and splash.
• A backdraft damper is required, and it must be free to swing. A damper that a lint mat holds half open is the reason a duct runs at half its rated flow.
• No screen or mesh of any kind. A screen at a dryer terminal blocks with lint within weeks and it is the single most common cause of a dryer that has stopped drying.
• Keep the terminal clear of the outdoor unit clearances around a heat pump or condenser, which draw air and do not want a warm, humid, lint-laden supply.

Slope the horizontal run very slightly back towards the terminal. A long run condenses on cold mornings, and water that cannot drain out sits in the low point and glues lint to the duct wall.

Materials, joints and support#

Rigid metal duct with a smooth interior, 0.016 in (0.4 mm) minimum wall thickness, is the material the code is written for. Not plastic, not the thin foil concertina, not the vinyl hose sold with portable machines.

Two details do most of the work:

1. Joints run in the direction of airflow. The male, crimped end of each length points downstream, so the airstream passes over a smooth lap rather than into a step. A duct assembled the other way collects lint at every joint.
2. No fasteners that project inside. Sheet metal screws are not used on dryer duct. Every screw tip is a hook. Joints are sealed with foil tape or held with an external clamp.

Support the duct at intervals of about 12 ft (3.7 m) and at every change of direction, and keep it accessible where it can be. A cleaning access at a long horizontal run is worth designing in, because the alternative in ten years is opening a ceiling. Where the duct passes between floors, it must remain continuous through the assembly.

The other services around the machine, the socket, the isolator and their heights, follow the pattern in socket and outlet heights, and the terms used above are defined in the glossary.

When the run is simply too long#

Three routes out, in the order worth trying them.

Reroute. Because a bend is worth five straight feet, redrawing the route usually recovers more than relocating the machine. Two 45s in place of a 90 cost the same on paper but move the air more gently, and a long-radius elbow costs less than a tight one.

Boost. A listed dryer exhaust booster fan, interlocked so it runs whenever the machine does, extends the permissible run. It is a mechanical device in a lint stream and it needs cleaning, so it is a solution with a maintenance schedule attached rather than a free extension.

Delete the duct. A condenser or heat pump dryer vents nothing. It is the correct answer for a flat, an interior closet or any run that cannot be made to work, at the cost of a longer cycle and all of the machine's heat being released into the room rather than outdoors. That heat, and the moisture from any washing dried on a rack alongside it, is why an unvented laundry still needs mechanical extraction: the Approved Document F rate for a utility room is 30 l/s (108 m3/h) intermittent. The way that arithmetic works out is set out in drying rack, rail and line dimensions.