The lint trap isn't the part that starts the fire

Quick answer: Clean the vent duct once a year, from the dryer all the way to the outside flap. If your clothes need two cycles to dry, do it this weekend. If the duct is crushed foil or vinyl, replace it with rigid metal while you're back there.

Pull your dryer out from the wall tonight and look at the duct behind it. Most people never have.

You may find a silver accordion tube bent into a Z so the dryer can sit flush against the drywall. You may find it flattened to the width of a soup can. You may find lint packed in it like felt. That tube is the thing that matters. The lint trap is the part you clean, but the duct is the part that burns.

Why it clogs every year

The trap catches most of the lint. Most isn't all. The rest rides the airflow into the duct, and every ripple, bend and seam gives it something to snag on. Lint is dry, fine and flammable. Packed into a duct that's already hot, it needs only a little more heat to ignite.

Then the loop tightens. The duct narrows, air moves slower, clothes stay damp longer, the dryer runs hotter and longer, and more lint settles. Nothing dramatic happens until one day it does.

Dryer sheets make it worse in a quiet way. They leave a waxy film on the trap screen that you can't see. Once a few months, take the screen out and scrub it with a toothbrush and warm soapy water. If water pools on it instead of draining through, the film is blocking the mesh. Let it dry fully before it goes back in.

The signs, in order of how late they show up

• Towels come out damp after a full cycle.
• The clothes, or the top of the dryer, are hotter than they used to be.
• The laundry room feels humid or smells faintly of something toasted.
• You hold your hand at the outside vent and feel a weak puff, or nothing.
• The outside flap doesn't open while the dryer runs.

Any one of them is enough. Don't wait for the burning smell. That's the last one on the list.

If you have a gas dryer, a blocked vent is more than a fire problem. Exhaust from burning gas has to leave the house through that duct, and carbon monoxide is part of it. A working CO alarm near the laundry room is cheap insurance.

How to clean it yourself

Plan on an hour, maybe ninety minutes the first time. You need a dryer vent brush kit that spins on a cordless drill (the flexible rods screw together), a vacuum with a hose attachment, a screwdriver, foil tape and a helper if your dryer is heavy.

1. Kill the power. Unplug the dryer. If it's gas, shut the supply valve, and be gentle with the flexible gas line. If you aren't comfortable handling that line, stop here and call a pro.
2. Pull the dryer out just far enough to reach the back. Slow is fine. Don't drag it over the cords.
3. Disconnect the duct at the dryer and at the wall. Expect a lint shower. Wear a dust mask and eye protection.
4. Vacuum the dryer's outlet and as far into the wall duct as the hose will go.
5. Run the brush through the duct from the inside. Spin it forward only, and never reverse the drill. Add rods as you go. Pull the brush out, clean it, and repeat until little comes out.
6. Go outside. Open the exterior flap, clear the lint there, and pull out any nest a bird or mouse has built. Check that the flap closes freely.
7. Reconnect with foil tape, not cloth duct tape. The adhesive on cloth tape dries out and lets go. Clamps are fine on the ends.
8. Run the dryer empty for ten minutes with your hand at the outside flap. You should feel strong, warm airflow.

It's one of those jobs that proves its worth the first time. You'll see a pile of lint you can't unsee, and you'll never skip it again. It's one of the five house repairs you should stop paying someone else to do, and the brush kit fits alongside the ten tools that cover ninety percent of household repairs.

Fix the install, not just the clog

Here's the part that rarely gets said. Cleaning a bad duct every year is a patch. If the setup is wrong, the lint comes back faster.

Use rigid metal duct. Smooth-walled aluminum or galvanized steel, four inches across. The smooth inside lets lint slide out instead of catching. Foil accordion duct and white vinyl duct are the worst offenders, because the ridges grab lint and the material can sag, crush or burn. Many codes and manufacturers specifically warn against vinyl. Use a short length of semi-rigid metal only for the last connection to the dryer, if you need it.

Keep it short and straight. The model residential code sets a maximum run of 35 feet, and each 90-degree elbow cuts that by five feet. Your dryer's manual may set a stricter number, and the manual wins. The shorter and straighter the run, the less lint stays behind.

No screws into the duct. Screws that poke through a joint become lint hooks. Use foil tape on the seams.

No screen on the outside end. A mesh cover over the exterior opening clogs in weeks. You want a flap that opens and closes on its own.

Don't squash it. If the dryer has to sit tight to the wall, buy a recessed vent box, or a periscope-style elbow duct made for tight spaces, so the duct isn't pinched behind the machine.

When to call someone

Some vents shouldn't be a weekend project. Call a professional vent cleaner or an HVAC contractor if:

• The duct runs through the roof, or up through a second floor.
• It's well past 25 or 30 feet, or has several turns.
• You can't reach the outside termination safely without a tall ladder.
• It's a gas dryer and you're not sure about the gas line.
• You've cleaned it and the dryer still runs hot.

Ask what they use. A good one will use a rotary brush and a high-powered vacuum, will disconnect the duct at both ends, and will show you the airflow when they're done. A guy who blows air down the pipe and leaves isn't doing the job. Knowing when to stop and how to get a quote you can trust covers how to pay for the real thing and skip the pitch.

Habits that cost nothing

• Empty the lint trap every single load. It takes four seconds.
• Don't run the dryer when you leave the house or go to bed.
• Keep the area around the dryer clear. No laundry baskets stacked against it.
• Don't dry anything that's picked up gasoline, cooking oil, stain or similar. Rags used for oil finishes can heat up on their own, and a dryer is the worst place to find that out. Spread them flat outside to dry, then dispose of them properly.
• Clean the duct once a year. Twice if you have a big family, pets that shed, or a long run.

If you're buying or renting

Buying a house? The dryer vent almost never makes it into a home inspection, and it should be on your own list. Pull the dryer out during the walkthrough if the seller allows it. Look at the material, the length, and the flap outside. It's one of the things I'd add to what your home inspector won't check, and what you should.

Renting? Put your concern in writing to the landlord, with a photo. Don't take apart a vent you don't own.

Put it on the calendar, next to the smoke alarm batteries and the furnace filter. The home maintenance calendar that prevents the big repairs is a good place to hang it. Then go pull the dryer out. It takes five minutes to find out which kind of house you're living in.