Why a duct is measured in equivalent feet
A dryer blower produces a modest pressure and it has to push a large volume of warm wet air against whatever the duct does to it. Straight smooth pipe costs very little. A turn costs a great deal, because the air has to change direction, and it does so by separating from the inside of the bend, creating turbulence that the blower pays for. Rather than ask an installer to compute pressure drops, the trade converts every fitting into the length of straight duct that would cause the same resistance. Add those to the real straight run and you have the equivalent length, which is the number that is compared against the allowance.
The commonly used deductions for 4 inch duct are 5 feet for a smooth radius 90 degree elbow, 2.5 feet for a smooth radius 45, and double those for the mitered versions — 10 feet and 5 feet respectively. The mitered figures are the ones that catch people out. A pressed, sharply angled 90 costs as much as ten feet of pipe, so two of them behind a dryer have consumed twenty feet of a thirty-five foot budget before the duct leaves the laundry room.
The transition piece between the dryer and the wall is counted here at twice its measured length, because it is almost always ribbed rather than smooth and it is almost always installed with slack in it. That is a judgement rather than a published deduction, and it is deliberately conservative: a four foot flexible transition that has been compressed into eighteen inches of accordion behind a pushed-back dryer is worse than any number on this page.
What the allowance actually is
The figure this page compares against is yours to supply, because it belongs to the dryer rather than to the duct. Thirty-five feet of equivalent length for 4 inch rigid duct is the number commonly permitted where the manufacturer states nothing, and it appears in enough installation instructions that people treat it as universal. It is not. Some dryers permit considerably more, particularly those with more powerful blowers, and some permit less. The instructions for the machine are the governing document and local requirements can differ again.
The reason the allowance exists is not merely drying performance. A duct beyond its limit runs slower, and air moving slowly through a duct drops the lint it is carrying instead of taking it outside. Lint accumulates, the effective diameter shrinks, the air slows further, and the process compounds. That is a fire risk and it is also the mechanism by which a dryer that worked acceptably at installation becomes one that needs two cycles three years later. A run that is comfortably inside its allowance is self-cleaning in a way a marginal one is not.
Reading the symptoms
| Symptom | Usually means |
|---|---|
| Loads need two cycles | Restricted duct, or a duct at its limit that has begun to accumulate lint |
| The laundry room is humid during a cycle | A leaking joint, a disconnected duct, or a duct that terminates indoors |
| Clothes come out very hot but still damp | Heat going in, moisture not getting out — a classic airflow restriction |
| The exterior damper barely moves | Little air arriving at the end of the run |
| Lint visible around the wall cap | Air is slow enough to be dropping its load before the exit |
The test that settles most of these costs nothing: run the dryer on a heated cycle and put your hand at the exterior termination. Strong warm airflow that clearly pushes the damper open means the duct is working. A weak breath means it is not, and the arithmetic on this page is the place to find out whether the route was ever going to work or whether it has simply filled up.
Fixing an over-length run
The fixes are ranked by how much length they return and none of them involve making the dryer work harder. Replacing mitered elbows with smooth radius ones halves their cost and is often the single largest recovery available. Removing turns entirely is better still, and it is worth asking whether the duct is taking a scenic route to keep the exterior cap on a less visible wall. Shortening the flexible transition to the minimum that lets the dryer be pulled out for service, or replacing it with a rigid periscope fitting, buys back several feet and lets the machine sit closer to the wall as a bonus.
Where the route genuinely cannot be shortened, upsizing the duct is sometimes permitted and the instructions will say so, and booster fans listed specifically for dryer exhaust exist for long runs. Both are answers of last resort compared with venting out the nearest exterior wall. What is never an answer is venting indoors, in any form, with any filter or water trap attached. A dryer puts several pints of water into the air over a load, and doing that inside a building produces condensation on cold surfaces, mould in wall cavities and a laundry room that smells of it.
Once the duct is right, the running-cost side of a dryer is worth its own look. The laundry cost calculator splits a load into washer power, dryer power, water and detergent, and a restricted duct shows up there as drying time, which is where the money is. For the clearance behind the machine that the duct and its elbows consume, the appliance clearance calculator handles the depth arithmetic, and the home maintenance schedule is where the annual duct cleaning belongs.
Questions people ask
How long can a dryer vent be?
The maximum belongs to the dryer, not to the duct, and it is in the installation instructions. Thirty-five feet of equivalent length for 4 inch rigid duct is the figure commonly permitted where nothing else is stated, and it is a total that includes the deductions for every fitting rather than a straight measurement. That distinction is the whole point: a run with four 90 degree elbows has already spent twenty of those thirty-five feet before any pipe is counted. Measure the route, count the fittings, and add them the way this page does.
Why does an elbow count for five feet?
Because the air pays for the turn. Flow separates from the inside of a bend and becomes turbulent, and the blower has to overcome that just as it overcomes friction along straight pipe. Converting the fitting into an equivalent length of straight duct is a way of expressing that resistance in a unit installers can add up with a tape measure. The sharper the turn the higher the cost, which is why a mitered 90 counts double a smooth radius one. It is also why two 45 degree elbows are cheaper than one 90 where the geometry allows it.
Can I use flexible foil duct for the whole run?
No. Foil and vinyl flexible duct is not acceptable for a concealed dryer run: the ribbed interior catches lint at every corrugation, it sags between supports and collects more, it crushes when the dryer is pushed back, and the materials themselves are a fire concern in a duct that carries lint and hot air. The only place flexible or semi-rigid duct belongs is the short exposed transition between the dryer and the wall connection, kept as short as the machine allows, never concealed inside a wall or floor, and never joined to another length of it.
My dryer takes two cycles. Is the duct or the dryer at fault?
Check the duct first, because it is free and it is usually the answer. Run a heated cycle and feel the airflow at the exterior termination — strong warm air that pushes the damper open means the duct is doing its job and the problem is the machine. A weak breath means the duct is restricted, and the two causes are a run that was too long from the start and a run that has filled with lint. Do the arithmetic on this page for the first and pull the duct apart for the second. Also check the lint screen and the housing under it, which is a five second job that fixes a surprising share of these complaints.
How often does a dryer duct need cleaning?
Annually is the interval most people land on, and the honest answer is that it depends entirely on how close the run is to its limit and how much you dry. A short straight run out a near wall moves air fast enough to carry most of its own lint outside and can go longer. A long run with several elbows accumulates faster and is the one that matters more, which is an unfortunate combination. The observable signal is drying time: when loads start needing noticeably longer than they did, the duct has narrowed, and that is true regardless of what the calendar says.