# Dryer Takes Forever? It's the Vent (Not the Dryer)

Canonical: https://buildingtalks.com/dryer-takes-forever/

> Two-hour dryer cycles mean blocked venting. Facilities-pro diagnosis with the 5 causes ranked, the IRC dryer-vent code, and the fire-risk numbers.

✓ Field Verified
Reviewed by **Al, the Building Doctor** (IUOE Local 39 · 30-Yr Pro)
·
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Airflow Restriction & Backpressure Thermal Cutout Triage · Field Diagnostics
Field Verified

IRC 2021 M1502, dryer-vent code (ignored in 70%+ of homes I inspect)

**Rigid metal duct only.** 4" diameter smooth interior. Maximum 35-ft equivalent length (subtract 5 ft per 90° turn, 2.5 ft per 45° turn). Flexible "transition" duct allowed for the last 8 ft **must be metal too**, not plastic. Plastic flex traps lint in corrugations and is the most common code violation. Replace plastic with semi-rigid aluminum or rigid metal.

## The 5 causes ranked by field frequency

| # | Cause | Field frequency | Fix |
| --- | --- | --- | --- |
| **1** | Lint buildup in vent duct | ~50% | Annual cleaning, vacuum from both ends, brush kit ($25) |
| **2** | Crushed or kinked flex duct behind dryer | ~20% | Pull dryer out, straighten or replace with semi-rigid |
| **3** | Exterior vent cap blocked (bird nest, screen) | ~15% | Clean cap, remove screen (often added illegally, birds nest in screens) |
| **4** | Plastic flex duct (illegal, traps lint) | ~10% | Replace with rigid metal or semi-rigid aluminum |
| **5** | Failed heating element (electric) or igniter (gas) | ~5% | Appliance technician, $150-$300 service |

## DIY annual vent clean, 45 minutes

- **Unplug dryer + shut off gas** (gas dryers).

- **Pull dryer 2 ft from wall.** Detach the duct from the back of the dryer.

- **Vacuum lint from dryer's lint screen housing**, pull out the lint trap, vacuum the cavity it sits in. Most homeowners never do this. There's typically a ~12-18 inch trap of accumulated lint past the screen.

- **Run a vent-cleaning brush through the duct** from both ends. $25 brush kit at any hardware store, 12 ft of flexible rod with rotating brush head.

- **Check the exterior vent cap.** Remove cap, clean lint, verify damper flapper moves freely. Remove any screen mesh, birds use it as a nesting platform.

- **Verify duct routing.** No more than 35 ft equivalent length, no kinks, no plastic transition duct.

- **Reassemble + test.** Run a dryer cycle. If it still takes >60 min for a normal load, you have a deeper restriction or an appliance issue.

## FAQ

Why does my dryer take 2 hours?

Blocked venting. Water vapor can't exit, drying takes 3× longer.

How often should I clean the vent?

Annually minimum per IRC M1502. Every 6 months if long run or plastic flex duct present.

Is a slow dryer a fire hazard?

Yes. 2,900 dryer fires/yr, $35M damage (NFPA). Lint ignites at 471°F.

Can I use plastic flexible duct?

No. IRC M1502 requires rigid metal. Plastic flex traps lint in corrugations, #1 code violation.

## Related guides

- [Humid basement](https://buildingtalks.com/humid-basement/), dryers venting into basements add 7+ gal/week of moisture

- [Best smart smoke + CO combo alarms 2026](https://buildingtalks.com/best-smart-smoke-co-combo-alarms-2026/), required near laundry per NFPA 72

**Editorial standards:** Cited authorities include IRC 2021 M1502 (dryer exhaust requirements), NFPA dryer fire statistics. Reviewed by Al, Building Doctor, SFFD Fire Safety Director, IUOE Local 39.