Dryer Vent Inspection and Cleaning

Purpose

A dryer vent is a system with a length budget and a measurable pressure, and cleaning it without measuring either leaves the customer a clean duct that is still wrong. NFPA's home fire research puts failure to clean as the leading contributing factor in home clothes dryer fires, but the second-order causes are structural: a duct too long for the appliance, foil flex trapping lint in every corrugation, screws in the airstream, or a termination the damper no longer opens.

This procedure guarantees a measured before and after, a code check on the duct itself, and a written finding when the fault is the installation rather than the lint. A shop that only sweeps sells a service that repeats at shortening intervals while the real cause stays in the wall.

Scope

Residential clothes dryer exhaust from the appliance connection to the exterior termination: inspection, measurement, cleaning, and verification. Electric and gas dryers.

It does not cover the dryer's internal lint path beyond the blower housing, nor building a new duct run, cutting a termination or altering framing, which is contractor work that step 2's stop rule hands off to. Commercial and multi-family common-exhaust systems are out of scope and follow the mechanical code path at IMC 504.

Roles and the handoff between them

Before the duct is opened

On a gas dryer, close the appliance shutoff before disconnecting anything, and treat the flexible connector as single-use: it is replaced, not reused, on reconnection. On any gas odor everybody leaves the building immediately, no switch touched, no light turned on, no phone used inside, and the utility is called from outside.

Unplug an electric dryer, or open the breaker on a hardwired unit, and prove it dead at the terminal block with a meter proved live on a known source before and after, per NFPA 70E-2021, 120.5, with work practices at 29 CFR 1910.333(b)(2).

Rotary brushing aerosolizes lint and fine dust into the room. Eye protection and respiratory protection are both required, and where your shop requires a respirator it runs under a written program per 29 CFR 1910.134 rather than a dust mask handed out of a truck box. Seal the appliance end into a collection bag or vacuum before the brush turns.

A roof or high sidewall termination is work at height. Appliance service is general industry, so the fall-protection trigger is 29 CFR 1910.28 rather than the construction trigger at 1926.501, and ladder use follows 29 CFR 1910 Subpart D. A tech alone with an extension ladder and a rotary brush is not doing this today.

The procedure

1. Establish the run before touching it. Trace from appliance to termination, noting every elbow and its angle, the material of each section, and where it is concealed. Acceptance: a sketch with each straight length and each fitting on it. Wrong looks like a tech who cleans what he can reach and assumes the rest. Stop rule: a run that cannot be traced is reported as unknown rather than certified clean.

2. Compute the allowance, taking the deduction off the allowance. The residential maximum developed length is 35 ft under IRC M1502.4.5, and each fitting reduces that maximum per Table M1502.4.5.1, so the deduction comes off the allowance rather than onto your measurement. Acceptance: measured developed length at or below the reduced allowance. IRC M1502.4.5.2 permits a longer run where the manufacturer's instructions allow it and a permanent tag stating the length is fixed near the dryer, so look for that tag before condemning a long run. Stop rule: over the allowance with no tag is an installation defect cleaning does not fix.

3. Judge the duct material and the joints. Confirm rigid metal of at least the code minimum thickness with a smooth interior, male ends pointing downstream, no fasteners projecting inside. Acceptance: rigid metal throughout the concealed run, joints taped or clamped rather than screwed. Wrong looks like foil flex in a wall cavity, which holds lint in every corrugation, or sheet-metal screws through the joints, which IRC M1502.4.2 prohibits because each becomes a lint anchor. Stop rule: non-compliant concealed duct is reported as a replacement, not brushed and left.

4. Check the transition duct separately. The flexible section between dryer and wall is a different component with different rules: listed and labeled to UL 2158A, a single length not exceeding 8 ft, not concealed, not passing through a wall. Acceptance: a listed transition, undamaged. Wrong looks like a crushed transition folded into the gap behind a dryer pushed too far back. Stop rule: unlisted or crushed means replaced on this visit.

5. Baseline the exhaust before you clean. With the duct still dirty, run the dryer on air-fluff and measure static pressure at the dryer connection, or exit velocity at the termination. Acceptance: a number written down. Many manufacturers publish a maximum external static around 0.6 inches of water column measured at the dryer with it running, so read the install manual for the model rather than assuming. Wrong looks like no baseline, which makes the after-reading meaningless. Stop rule: no instrument, no claim of improvement on the invoice.

6. Clean from both ends, capturing what comes out. Brush from the appliance end and from the termination, turning the rotary head in the direction the brush manufacturer specifies, with a bag or vacuum sealed at the open end. Acceptance: the brush passes the full length both directions with no obstruction, lint bagged. Wrong looks like brushing only from inside and pushing a plug of lint into the soffit. Stop rule: an obstruction the brush will not pass means inspect with a camera rather than force it; a rotary head can separate inside a duct.

7. Re-measure and compare to the baseline. Repeat step 5 identically, same dryer setting. Acceptance: the reading improved and now sits within the manufacturer's limit for the model. Wrong looks like a reading that barely moved, meaning the restriction is geometry rather than lint. Stop rule: no meaningful improvement sends the finding back to steps 2 and 3, and the customer is told cleaning was not the answer.

8. Reconnect, verify, and run a full heat cycle. Refit the transition, and on a gas dryer fit a new listed connector and leak check every joint with a fluid listed for gas piping. Acceptance: no bubble at any joint, damper opening freely at the termination while the dryer runs, and a full heat cycle completed with no cycling on a thermal limit. Wrong looks like a job closed on a cold-air test. Stop rule: any bubble means close the gas, break the joint and remake it; a damper that does not open means the termination is replaced.

The record this produces

A vent finding sheet, and it is what sets the next interval.

• The run: the sketch, each straight length, each fitting and its angle, the material of every section including concealed portions.
• The allowance calculation: the 35 ft starting allowance, the deductions taken and for which fittings, the reduced allowance, and the measured developed length beside it.
• Compliance findings: duct material, joint method, screws found, transition listing and condition, termination and damper condition.
• Before and after: the same measurement twice, the dryer setting used, and the manufacturer's published limit beside them.
• Recovered lint: noted as a quantity or photographed, which is what the customer actually believes.
• Disposition and interval: clean and compliant with a next-service interval, or a named installation defect referred out.

The allowance calculation changes the shop's relationship with this customer. Once a run is on paper as over-length, every future drying complaint has a documented cause, and the shop stops being asked to clean twice a year to compensate for something a brush cannot reach.

One run of this procedure, filled in

Second-floor laundry, electric dryer, complaint that towels take three cycles.

• Step 1: run traced. Straight sections total 24 ft of developed length, three 90 degree elbows and one 45, terminating at a gable sidewall, concealed through a joist bay.
• Step 2: allowance starts at 35 ft. Using the code table's deductions of 5 ft per 90 and 2 ft 6 in per 45, the deductions total 17 ft 6 in, so the permitted developed length falls to 17 ft 6 in. Measured developed length is 24 ft, which is over by 6 ft 6 in, or 37 percent above the permitted length. No manufacturer tag was found near the dryer. Fail. Stop rule taken: the run is an installation defect and is reported as such.
• Step 3: concealed section is rigid metal, but two accessible joints are assembled with sheet-metal screws. Recorded as a defect to correct.
• Step 4: transition duct is a listed UL 2158A product but crushed flat where the dryer had been pushed back against it. Replaced on the visit.
• Step 5: baseline static measured at the dryer connection on air-fluff, above the limit published in the install manual for this model.
• Step 6: brushed both ends, screws removed and joints re-taped, a substantial bag of lint recovered.
• Step 7: re-measured identically. The reading improved but stayed above the manufacturer's limit, which is exactly what step 2 predicted, because the geometry did not change when the lint left.
• Step 8: reassembled, full heat cycle run, no limit cycling during the run.

Drying times will improve and will not become normal, and the finding sheet says so before the customer discovers it. The honest close is that the cleaning was worth doing and the duct still needs a shorter path, with a contractor referral the same day. A shop reporting only "vent cleaned, lint removed" is back in four months owning a problem it had already measured.

When the job cannot run as written

The termination cannot be reached safely. A steep roof, ice, or a two-story sidewall with no safe ladder set. Clean from the appliance end only, state on the sheet that the termination was not inspected, and book the return with the right access.

The duct is concealed with no access. Say plainly what was cleaned and what was not, and quote a camera inspection rather than certifying a run nobody saw.

The brush will not pass. A collapsed duct, a crushed elbow, or a bird nest at the termination. Stop rather than force it. A separated brush head inside a wall converts a cleaning call into demolition.

The dryer is gas and the connector is old. It is replaced, not reused. With no connector on the truck, the dryer is left disconnected, the shutoff closed and capped, and the customer told it is out of service until reconnected.

The customer only wants the cheap sweep. Do the sweep, but keep the measurement and sign off no run as compliant that you did not check. A sheet saying "not assessed" is honest; one that says nothing reads as approval.

References

• IRC M1502: M1502.3 termination, M1502.4.1 material, M1502.4.2 joints and fasteners, M1502.4.3 transition duct, M1502.4.5 with Table M1502.4.5.1 for length and fitting deductions, in the edition your AHJ has adopted.
• IMC 504 for the mechanical-code path covering commercial and common-exhaust systems. UL 2158A for listed transition ducts.
• 29 CFR 1910.134 for respiratory programs; 1910.28 and 1910 Subpart D for fall protection and ladders in general industry; 1910.333(b)(2) and NFPA 70E-2021, 120.5 for electrical work.
• NFPA research on home clothes dryer fires, for the failure-to-clean finding in the opener.