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Duct cleaning - laundry
If your tumble dryer is taking longer to dry than it used to - loads needing extra time, or a second cycle, to come out dry - the most common cause is a restricted vent. As lint builds up in the ducting, the moist air cannot escape as fast, so drying slows. It is a clear early warning worth acting on, because the same restriction also runs the dryer hot. Here is why your dryer takes longer than it used to, and what it means.
The short answer
A tumble dryer that takes longer to dry than it used to almost always has a restricted vent, and the usual cause is lint building up in the ducting. A dryer dries by heating air, blowing it through the tumbling laundry to pick up the moisture, and exhausting that hot, moist air out through its vent ducting. The speed of drying depends on how fast that moist air can be exhausted and replaced with fresh hot air - so the vent airflow is central to how quickly the laundry dries. When the vent is clear, the moist air escapes freely and the laundry dries in a normal cycle. As lint accumulates in the ducting over time, it restricts the airflow, so the moist air cannot escape as fast - and with the moisture leaving more slowly, the drying slows. That is why a dryer that used to dry a load in one cycle starts needing longer, or a second cycle: the restricted vent is holding the moisture in. This is worth acting on for two reasons. First, the slower drying itself is costly - more energy and time per load, and less throughput from the dryer. Second, and more important, slower drying is usually the first and clearest warning sign that the vent is filling with lint, which is also a fire risk: the same lint restriction that slows the drying makes the dryer run hotter, building towards the conditions for a dryer duct fire. So a dryer taking longer is the vent telling you it is filling up, before the more serious signs appear. Clearing the ducting - removing the lint from the whole run, not just the filter - restores the airflow, so the drying returns to normal speed and the fire risk is reduced. So the answer to why the dryer takes longer is a restricted vent, and clearing it is the fix.
Key points
How drying speed depends on the vent
To see why a restricted vent slows drying, consider how a dryer actually dries. It heats air, blows that hot air through the tumbling laundry where it picks up moisture evaporating from the wet fabric, and then exhausts the now hot, moist air out through the vent ducting - continuously, drawing in and heating fresh air to replace what it exhausts. So drying is a flow: hot air in, moisture picked up, moist air out. The speed of drying depends on keeping that flow going - exhausting the moist air fast enough to make room for fresh hot air to pick up more moisture. If the moist air cannot get out fast enough, the flow slows, and so does the drying. So the vent airflow - how fast the moist air can be exhausted - is central to how quickly the laundry dries.
When the vent is clear, the moist air is exhausted freely, the flow is maintained, and the laundry dries in a normal cycle. When the vent is restricted, the moist air is exhausted more slowly, the flow is throttled, and the drying slows - because the moisture is not being removed as fast, so it takes longer to dry the load. In effect, a restricted vent holds the moisture in: the water evaporating from the laundry has nowhere to go as quickly, so the laundry stays damp for longer and the cycle has to run longer to finish. This is the direct mechanism by which a restricted vent slows drying: it impedes the exhaust of the moist air that drying depends on removing. So a dryer taking longer than it used to is a dryer whose vent has become restricted, slowing the exhaust of moisture - which points straight to what has changed in the vent, usually lint.
Why the vent restricts
The usual reason a vent becomes restricted over time is lint. Drying laundry sheds lint continuously, and while the dryer's filter catches much of it, some passes the filter and collects in the vent ducting beyond it, accumulating gradually load by load. As that lint builds up, it narrows and restricts the ducting, reducing the airflow - so the moist air is exhausted more slowly, and the drying slows. This build-up is gradual, which is exactly why the dryer takes longer than it used to rather than suddenly failing: the vent restricts by degrees as the lint accumulates, so the drying slows by degrees, and one day you notice that loads are taking longer or needing a second cycle. The slowing tracks the filling of the vent with lint.
This is why cleaning the filter, though good practice, does not fix a dryer that has slowed: the lint causing the restriction is in the ducting beyond the filter, where cleaning the filter does not reach. A laundry can clean its dryer filters diligently and still find the dryers slowing, because the ducting is filling with the lint that passes the filter. So a dryer taking longer is usually a sign that the ducting - not the filter - needs clearing, because that is where the restricting lint is. Occasionally the restriction has another cause (a crushed or kinked duct, a blocked outside vent flap, an over-long duct run), but lint build-up in the ducting is by far the most common, especially when the dryer has gradually slowed over time rather than changing suddenly. So the slowing points to a filling vent, and the filling is usually lint in the ducting - which is what needs clearing to restore the drying speed.
Why it is worth acting on
A dryer taking longer is worth acting on for two reasons, one about cost and one about safety. The cost reason is straightforward: a dryer that takes longer to dry each load uses more energy and time per load, and dries fewer loads in a day - so a restricted vent quietly raises the running cost and cuts the throughput of the dryer. In a commercial laundry running dryers hard, that adds up: more energy per load across many loads, and less laundry processed in the working day. So the slower drying is not just an inconvenience but a continuing cost, which clearing the vent recovers by restoring the drying to normal speed and efficiency.
The safety reason is more important. Slower drying is usually the first and clearest warning sign that the vent is filling with lint - and that same lint build-up is a fire risk. As the vent restricts, the dryer not only dries more slowly but runs hotter, because the restricted airflow cannot carry the heat away - and a lint-filled vent running hot is exactly the condition that causes dryer duct fires. So a dryer taking longer is the vent's early warning that it is filling up, appearing before the more alarming signs (excess heat, a burning smell) that come later as the danger builds. This makes the slower drying valuable as a warning: it flags a filling, restricting vent while there is still plenty of time to clear it, before the fire risk grows serious. So acting on a dryer that takes longer is not just about recovering the drying speed and efficiency; it is about heeding the earliest warning of a filling vent and clearing it before the lint build-up becomes a fire hazard. The slower drying is the cheap, early signal - worth reading and acting on rather than working around by simply running longer cycles.
What to do
The fix for a dryer that takes longer is to clear the vent ducting - removing the accumulated lint from the whole run, not just the filter - so the airflow is restored, the moist air is exhausted freely again, and the drying returns to its normal speed. Because the restricting lint is in the ducting beyond the filter, clearing the ducting is what actually addresses the cause; cleaning the filter alone, while worth doing, will not restore a dryer that has slowed because of lint in the ducting. So when a dryer takes longer than it used to, the action that fixes it is clearing the ducting, which reaches where the restriction is. This also removes the flammable lint and lets the dryer run cooler, so it reduces the fire risk at the same time as restoring the drying speed - dealing with both the cost and the safety reasons for acting.
The better approach, in a commercial laundry, is not to wait for the dryers to slow but to keep the ducting on a regular cleaning schedule, so the lint never builds up enough to restrict the airflow in the first place - keeping the drying at full speed and the fire risk low continuously. Waiting for the slowing means accepting a period of reduced efficiency (and rising fire risk) before acting; scheduled cleaning avoids that by keeping the vent clear proactively. But whether responding to the slowing or preventing it, the key is that it is the ducting that needs clearing, on a frequency matched to how hard the dryers run. So the answer to a dryer taking longer is: clear the ducting to restore the airflow (which also reduces the fire risk), and ideally keep it on a schedule so it does not slow again. The slower drying is the vent asking to be cleared - and clearing it is both the fix and the prevention of something worse.
The takeaway
A tumble dryer takes longer than it used to because its vent has become restricted, and the usual cause is lint building up in the ducting. A dryer dries by exhausting hot, moist air out through the vent, so the drying speed depends on how fast that moist air can escape; as lint accumulates and restricts the ducting, the moist air escapes more slowly, so the drying slows - loads needing extra time or a second cycle. The slowing is gradual because the lint builds up gradually, which is why the dryer takes longer than it used to rather than failing suddenly. And because the restricting lint is in the ducting beyond the filter, cleaning the filter does not fix it - the ducting needs clearing.
It is worth acting on for two reasons. The slower drying costs energy, time and throughput - a continuing cost that clearing the vent recovers. And more importantly, slower drying is usually the first, clearest warning that the vent is filling with lint, which is also a fire risk: the same restriction that slows the drying makes the dryer run hotter, building towards a dryer duct fire. So a dryer taking longer is the vent's early warning, appearing before the more serious signs. Clearing the ducting restores the airflow and the drying speed, and reduces the fire risk at the same time - and keeping it on a regular schedule stops it slowing again. Read the slower drying as the early signal it is, and clear the ducting.
Questions
Because the usual cause - lint building up in the vent ducting - accumulates gradually, so the restriction develops by degrees and the drying slows by degrees. A little more lint collects each load, narrowing the ducting slightly more, reducing the airflow slightly more, and slowing the moisture exhaust slightly more - so the drying gets slower over time rather than stopping suddenly. This gradual slowing is characteristic of a lint-restricted vent, and it is why you notice loads gradually needing longer or a second cycle rather than a sudden failure. A sudden change would more likely point to something else (a fault, a crushed duct, a blocked flap), but a gradual slowing over time is the signature of lint building up in the ducting.
Not if the slowing is caused by lint in the ducting, which is the usual case. Cleaning the filter is good practice and catches much of the lint, but the lint that restricts the vent and slows the drying collects in the ducting beyond the filter, where cleaning the filter does not reach. So a dryer that has slowed because the ducting is filling with lint will not be restored by cleaning the filter alone - the ducting itself needs clearing. This is a common source of confusion: laundries clean the filters diligently and are puzzled that the dryers still slow, because the real restriction is further along the vent. To restore the drying speed, clear the ducting, which is where the restricting lint actually is.
It can be a warning of one. Slower drying is usually the first and clearest sign that the vent is filling with lint, and that same lint build-up is a fire risk: as the vent restricts, the dryer runs hotter (the restricted airflow cannot carry the heat away), and a lint-filled vent running hot is the condition that causes dryer duct fires. So a dryer that takes longer is often flagging a filling vent before the more serious signs - excess heat, a burning smell - appear. That is why slower drying is worth acting on for safety, not just efficiency: it is an early warning that the vent needs clearing, given while there is still time to clear it before the fire risk grows. Reading it as a warning and clearing the ducting addresses both the speed and the risk.
Occasionally, yes, though lint in the ducting is by far the most common cause of gradual slowing. Other possibilities include a crushed or kinked duct restricting the airflow, a blocked or stuck outside vent flap stopping the exhaust leaving, an over-long or poorly routed duct run, or a fault in the dryer itself (such as a failing heating element, which would slow drying differently). But when a dryer has gradually got slower over time, lint building up in the ducting is the usual explanation, because it develops exactly that way. A cleaning contractor clearing and inspecting the ducting will find whether lint is the cause and address it; if a cleared duct does not restore the drying, the other causes can be investigated.
More than it might seem, especially in a commercial laundry. A dryer taking longer uses more energy per load (running longer to achieve the same drying) and more time per load, and dries fewer loads in a working day - so a restricted vent quietly raises the energy cost and cuts the throughput. Across many loads and a busy laundry, the extra energy and lost capacity add up to a real continuing cost. Clearing the vent recovers this by restoring the drying to normal speed and efficiency, so each load uses less energy and time and the dryer processes more. So the slower drying is not just an inconvenience but a measurable cost, which is part of why acting on it - clearing the ducting - is worthwhile even before considering the fire-risk reason.
Keep the vent ducting on a regular cleaning schedule, so lint does not build up enough to restrict the airflow again. Because the slowing is caused by lint gradually accumulating in the ducting, clearing it once restores the drying, but the lint will build up again over time as the dryer runs - so to keep the drying at full speed (and the fire risk low), the ducting needs clearing periodically, at a frequency matched to how hard the dryers run. A busy commercial laundry needs it more often. Rather than waiting for the dryers to slow again, keeping the ducting on a proactive schedule keeps it clear continuously, so the drying stays fast and efficient and the vent never fills to the point of restriction or fire risk. A cleaning contractor can advise the right frequency.
A dryer that takes longer usually has lint restricting the ducting beyond the filter - we clear the whole run so the drying returns to normal and the fire risk drops. Ask us about your laundry ducting.
