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, dryers venting into basements add 7+ gal/week of moisture
- 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.