dryer-vent-fire-guard

Use when clothes take multiple cycles to dry, the laundry room is hot/humid, or you want to audit a dryer vent for lint buildup and fire risk. Computes vent equivalent length vs code max, dry-time creep diagnosis, and a clean/replace/repair ladder.

Overview

Clothes dryers cause ~13,000–15,500 reported home structure fires per year in the US alone, and a third of them trace to failure to clean the dryer vent. The failure mode is silent: lint accumulates inside the vent run for years, drying time creeps from 40 to 90 minutes, everyone blames "the dryer getting old" — while the exhaust restriction superheats the lint itself (an excellent fuel) behind the machine.

This skill turns the two signals every household already has — how long a load takes to dry and what the vent looks like behind the dryer — into a code-based verdict and an action ladder.

When to Use

• A normal load needs 2+ dry cycles, or drying takes >60 minutes.
• Laundry room feels hot/humid, or you smell burning while drying.
• Dryer is hot to the touch on top; clothes come out very hot.
• You just moved into a home with an unknown vent history (always audit).
• Annual maintenance planning, or after noticing lots of lint at the outdoor hood.
• Post-incident: dryer shut off mid-cycle (thermal fuse tripped — a classic restriction symptom).

Don't use for: dryer won't start at all, drum won't turn (electrical/belt issues), gas burner diagnostics inside the machine — those are repair topics, not venting.

How It Works

1. Run the dry-time test. Time a standard load (bath towels, high spin, from wet). Baseline for a healthy vent + mid-size dryer: 35–50 minutes. 60+ minutes or a second cycle = restricted airflow until proven otherwise.
2. Inventory the vent run. Behind the dryer, note: duct material, length along the whole path, count every 90° elbow, duct diameter, and where it terminates.
3. Compute equivalent length (see Data & Physics) and compare against the code allowance — this is what scripts/dryer_vent_audit.py does.
4. Grade fire risk from material (vinyl/foil = violation), vertical rises, crush points, screencaps, and dry-time evidence.
5. Apply the action ladder (clean → replace duct → reroute/shorten → pro cleaning).
6. Set the recurrence: 1×/year minimum; 2×/year for large households or pets.

Data & Physics

Code allowance (IRC M1502 / manufacturers): max 35 ft of equivalent length for a 4″ rigid duct. Every 90° elbow costs 5 ft of allowance. Flexible duct adds friction: figure its effective length at ~2× its actual length.

Equivalent length = actual length × material factor − the allowance already spent by elbows. If the result exceeds 35 ft, the vent is out of spec and will lint up no matter how often you clean it — reroute or upsize.

Why restriction = fire: restricted exhaust → humid air stays in the drum → longer cycles → element/burner runs longer → lint (trapped where airflow is slowest: elbows, transitions) bakes at ever-higher temperatures. Lint ignites around 250–300 °C and burns fast.

Dry-time verdict bands (standard towel load): ≤50 min HEALTHY · 51–60 WATCH · 61–90 RESTRICTED · >90 or 2+ cycles SEVERE (clean now, today).

Termination rules: must vent outdoors — never into an attic, crawlspace, garage interior, or wall. Exterior hood must have a flap damper, not a screen (screens catch lint = plug). Box "periscope" vents behind the dryer must not be crushed.

The Action Ladder

1. Clean now (any verdict above HEALTHY): unplug dryer (gas: shut valve), pull the machine, vacuum the wall-side duct collar and the dryer's internal duct with a shop vac + flexible brush from BOTH ends. Expect a shocking lint harvest on runs older than 2 years.
2. Replace vinyl/foil flex with semi-rigid or rigid aluminum immediately — vinyl duct is a code violation on dryers, full stop.
3. Shorten/reroute if equivalent length exceeds 35 ft: straightest path wins; every elbow removed buys back 5 ft.
4. Pro vent cleaning ($100–200) when the run is long, vertical, in-wall, or inaccessible from both ends.
5. Fix termination: add/replace an exterior hood with damper, remove any screen, confirm flap isn't jammed with lint.

Worked Examples

python3 scripts/dryer_vent_audit.py --demo runs a 12-ft foil flex run with 4 elbows and a 75-minute dry time → SEVERE: out of code, foil violation, clean-today directive.

python3 scripts/dryer_vent_audit.py --material rigid --length-ft 18 --elbows 2 --diameter-in 4 --dry-minutes 48 --loads-per-week 8 → healthy but near the edge: 28 ft equivalent, schedule annual clean.

Common Pitfalls

1. Cleaning only the lint screen trap. The screen catches ~80%; the rest coats the run. The trap is not the vent.
2. Judging by "it still dries eventually." Multiple cycles IS the symptom — each one is money ($0.50–1.00/load) and heat soak into trapped lint.
3. Reusing crushed/kinked flex after moving the dryer. One hard kink can halve airflow; semi-rigid costs $10.
4. Venting "temporarily" into a bucket or indoors. Adds gallons of moisture to the house (mold risk) and is a code violation; lint spread is uncontrolled.
5. Assuming a new dryer fixes slow drying. The vent came with the house, not the dryer.
6. Screen on the exterior hood "to keep birds out" — birds are real, but the screen plugs first; use a damper hood and check it seasonally.
7. DIY brush without vacuuming: pushing lint deeper into elbows can complete the blockage. Always vacuum out after brushing.

Verification Checklist

• Dry-time test done with a standard load and timed honestly
• Duct material, length, elbow count, diameter recorded from the actual run
• Equivalent length computed and compared to 35 ft
• Exterior termination inspected: outdoors, damper, no screen
• Cleaning performed from both ends (or pro booked) before next heavy use
• Next clean date written into the home-maintenance calendar