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Dryer Elbows Hood {{ visualStage.marker }}
Pick a starting route, then edit the measured length, fittings, and selected limit.
Choose the units used for the measured route and maximum length field.
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Use the actual duct path length; fitting penalties are entered separately below.
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Count sharp right-angle elbows in the exhaust duct route.
elbows
Count angled offsets or two-piece bends that are not full 90-degree elbows.
elbows
Keep product-specific long-turn fittings separate from standard elbows.
elbows
Common 35 ft is a quick screen; manufacturer modes let you match a specific dryer table.
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This is the selected pass/fail budget for the equivalent length result.
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Use the manual's equivalent value for one standard 90-degree elbow.
ft
Use the dryer table when the fitting value is model-specific.
ft
Leave at 0 when the measured straight duct already covers the full manufacturer-counted route.
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Use this only for optional restriction screening, not as a replacement for the manufacturer manual.
Add a conservative allowance when the termination hood is more restrictive than a clean open damper.
Use feet of equivalent length for any fitting or route condition not listed separately.
ft eq
Used for the maintenance warning only.
loads/wk
Used for inspection cadence guidance; it does not change equivalent length.
months
Metric Value Use Copy
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Tags: Home Maintenance, Household
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Dryer vent length is not just the tape-measured distance from the dryer to the outside hood. Airflow has to pass through straight duct, elbows, transition duct, and the termination. Each bend or restriction makes the blower work as if the run were longer, so the practical question is equivalent length: the measured route plus allowances for fittings and restrictions.

Equivalent length matters because restricted exhaust can slow drying, raise appliance temperature, collect lint, and make a future dryer incompatible with the existing route. A short straight wall exit is forgiving. A second-floor laundry with several elbows, a roof cap, flexible transition duct, and a long attic run needs closer review because the same physical distance can behave like a much longer duct.

Dryer exhaust route with straight run, elbows, exterior hood, and equivalent length marker

Residential codes and dryer manuals both matter. A common code-style reference uses a 35 ft maximum for a smooth metal exhaust duct and subtracts or adds equivalent allowances for elbows. Many dryer manufacturers publish their own vent tables, and long-vent models may allow more than the common reference when the installation follows the manual. The stricter number usually controls for a real inspection or appliance replacement decision.

Material and maintenance change the meaning of the length number. Smooth rigid or semi-rigid metal carries air better than crushed, sagging, or accordion-style transition duct. A clean wall hood is different from a roof cap or sticky louver. Lint buildup, bird screens, and damp lint at low points can turn an acceptable route into a poor performer.

Equivalent length is a planning and documentation value, not a complete airflow test. If clothes take longer to dry, the laundry room gets unusually hot, or the outside hood barely opens while the dryer is running, treat the route as a maintenance issue even when the calculated length appears acceptable.

How to Use This Tool:

  1. Choose a Route preset if one matches the installation. Presets seed common duct lengths, fitting counts, and limit assumptions.
  2. Select Input units, then enter Measured straight duct. Measure the duct path from the dryer connection to the exterior termination, not including imaginary fitting allowances.
  3. Count Standard 90-degree elbows, 45-degree elbows, and Long-sweep 90-degree elbows separately so sharp bends and smoother bends are not treated the same.
  4. Set Length basis. Use the common 35 ft reference for a broad screen, manufacturer/custom when you have a model-specific table, and conservative manufacturer-style values when the route or manual calls for stricter allowances.
  5. Enter Maximum allowed equivalent length from the selected basis or dryer manual.
  6. Open Advanced for custom elbow penalties, flexible transition length, termination restriction, other fitting allowance, laundry load count, and months since cleaning.
  7. Read Vent Length Table and Limit Checks before using Vent Budget Map or Placard Note for documentation.

If the result is over the selected limit, shorten the duct, reduce elbows, improve the termination, or choose a dryer whose manual allows the route. Do not solve a blocked route by increasing the selected maximum unless that value comes from the actual dryer instructions.

Interpreting Results:

Equivalent length is the main result. It adds straight duct, elbow penalties, flexible transition allowance, termination allowance, and any other fitting allowance. Remaining allowance is the selected maximum minus the equivalent length.

  • Over limit means the route exceeds the selected maximum and should not be treated as acceptable without redesign or manufacturer confirmation.
  • Near limit means the route clears the maximum but leaves little room for future appliance changes, lint buildup, or measurement error.
  • Label length appears when the route is above the common 35 ft reference. Keep the calculated equivalent length with the installation record so the next dryer can be checked.
  • Fitting share helps show whether elbows are doing most of the damage. Replacing sharp turns with approved long-sweep fittings may improve the budget without moving the dryer.
  • Cleaning cadence combines route utilization, fitting share, usage, flex allowance, and time since cleaning. It is a maintenance clue, not a fire-risk certificate.

Use the result as a route screen. Confirm final compliance with the dryer manual, local code requirements, visible duct material, exterior airflow, and any installer or inspector notes.

Technical Details:

Dryer exhaust length is modeled as a developed route plus equivalent allowances. Straight duct contributes its measured length. Fittings contribute length penalties because they disturb airflow, collect lint, and add resistance. The final comparison is against a selected maximum length from a code-style reference, a manufacturer chart, or a custom value.

The calculation keeps measured length and fitting penalties separate because those values point to different fixes. A long straight run may require a shorter route or a dryer rated for longer exhaust. A fitting-heavy route may improve by reducing elbows, using long-sweep fittings where allowed, or correcting a restrictive termination.

Formula Core:

Equivalent length is the sum of measured duct and the selected allowances.

Lequiv = Lstraight + (N90×P90) + (N45×P45) + (Nlong90×Plong90) + (Lflex×FlexFactor) + Termination + Other
Basis Default maximum Default elbow allowances When to use it
Common 35 ft reference 35 ft 5 ft per 90-degree elbow and 2.5 ft per 45-degree elbow. Initial residential screen when the dryer manual is not yet available.
Manufacturer chart / custom limit User-entered User-entered or default fitting allowances. Best choice when the exact dryer model publishes a vent table.
Conservative manufacturer-style User-entered Higher fitting penalties for stricter manuals or uncertain routes. Useful when route restrictions, roof terminations, or manual details justify caution.

For example, 24 ft of straight duct with three standard 90-degree elbows and one 45-degree elbow under the common basis is 24 + 3 x 5 + 1 x 2.5 = 41.5 ft. Against a 35 ft maximum, the route is 6.5 ft over. If one 90-degree elbow is replaced with a long-sweep fitting that carries a 2.5 ft allowance, the equivalent length drops to 39 ft, still over the common reference but closer to a manufacturer-specific route budget.

Signal Boundary Action
Limit failure Equivalent length is greater than the selected maximum. Redesign the route or confirm a higher manufacturer-rated maximum.
Tight route Equivalent length is within about 10% of the selected maximum. Check measurements, fittings, duct material, and exterior hood condition.
Fitting-heavy route Fittings account for roughly one-third or more of equivalent length. Look for sharp bends that can be removed, rerouted, or replaced with approved long-sweep parts.
Cleaning review Long utilization, many fittings, frequent loads, flex restriction, or long time since cleaning. Inspect airflow and clean the exhaust path before treating the route as healthy.

The cleaning score is deliberately conservative. It points to maintenance conditions that can reduce airflow, but it does not measure static pressure, lint load, dryer temperature, or actual exhaust velocity.

Limitations and Safety Notes:

Dryer exhaust design is safety-sensitive. The result should be checked against the appliance instructions, adopted local code, and the visible condition of the installed duct.

  • Plastic or foil accordion-style duct can trap lint and kink; manufacturer and safety guidance commonly calls for rigid or semi-rigid metal duct.
  • Exterior hoods, screens, roof caps, and crushed transition duct can block airflow even when measured length is acceptable.
  • Longer drying time, damp clothes after a normal cycle, unusual heat, or weak outside airflow should trigger cleaning or service.

Worked Examples:

Simple wall exit. A laundry closet with 8 ft of straight rigid duct, one 90-degree elbow, and a clean wall hood is well below the common 35 ft reference. Vent Length Table should show the elbow penalty separately from the straight run, and Limit Checks should leave usable remaining allowance.

Second-floor route. A 30 ft route with four standard 90-degree elbows has 50 ft equivalent length under the common basis before flex or hood restrictions. The result is over a 35 ft reference, so the practical fix is a shorter route, fewer bends, or a dryer manual that explicitly supports the route.

Manual-backed long route. A dryer manual may allow a longer maximum with a smooth metal duct layout. In that case, choose Manufacturer chart / custom limit, enter the published maximum, and keep the Placard Note with the installation record.

FAQ:

Is straight duct length enough?

No. The result uses equivalent length, so elbows, flex, hood restriction, and other allowances are added to measured straight duct before the limit check.

Why does the calculator have a manufacturer basis?

Dryer manuals can publish model-specific maximum vent lengths. Use the manufacturer/custom basis when you have that table and want the selected maximum to match the appliance.

What if my calculated route is below the limit but clothes still dry slowly?

Treat slow drying as an airflow symptom. Clean the lint screen, exhaust duct, exterior hood, and behind-dryer area, then check for crushed flex, sticky dampers, or hidden lint.

Should I count the flexible transition duct?

Yes when it is part of the exhaust path. Enter its length and type in Advanced so the equivalent-length budget reflects added restriction.

Glossary:

Equivalent length
The effective duct length after adding fitting and restriction allowances to measured straight duct.
Transition duct
The short connector between the dryer outlet and the fixed exhaust duct.
Termination
The exterior hood, louver, roof cap, or wall cap where dryer exhaust leaves the building.
Long-sweep elbow
A smoother bend that may carry a lower equivalent-length penalty when the selected basis allows it.
Placard note
A record of the calculated equivalent length and selected maximum for future dryer selection or inspection.

References: