How moisture sensing works
Wet fabric conducts a small current; dry fabric barely does. Each time damp clothes tumble across the two strips, they bridge the gap, and the control counts those contacts. As the load dries, the contacts fade, and the dryer moves into its cool-down and ends.
Samsung describes its sensor bars exactly this way, as strips inside the drum that stop the cycle once moisture is no longer detected. ENERGY STAR lists automatic termination among the features of certified dryers, because stopping on time saves energy and spares fabric from over-drying. LG calls the same part a humidity sensor and warns that a dirty one can lead to either too much or too little drying.
Finding the strips on your dryer
On most front-load dryers sold in the US, open the door and look at the lower front of the opening, just below or beside the lint screen: two curved, shiny strips set into the plastic. Some models mount them on the rear bulkhead at the back of the drum instead. On a stacked or compact dryer, a flashlight helps.
Sensing on ventless dryers
Heat-pump and condenser dryers use the same strips on many models, and some also read the humidity or temperature of the air leaving the drum. The film problem is the same on both designs, so the cleaning applies to ventless dryers too.
Source: Samsung — How to clean your Samsung dryer — Sensor Dry uses bars inside the drum that stop the cycle when moisture is no longer detected; clean them with a soft cloth and mild soap; LG — How to clean the dryer humidity sensor — A contaminated humidity sensor can cause over-drying or incomplete drying; clean its surface regularly; ENERGY STAR — Clothes dryers — Certified dryers use automatic termination; small loads can take longer to dry without the tumbling of a full load
What dryer sheets leave behind
A dryer sheet works by melting a thin layer of softening agent onto the load as it warms. Some of that coating lands on whatever else the clothes touch, including the drum, the lint screen and the sensor strips. Electrolux states that fabric softener sheets can leave a residue on the moisture sensor bars that keeps them from accurately sensing wet clothes.
The film is nearly invisible. The strips may still look shiny, yet a fingertip drawn across them feels slightly greasy or tacky instead of clean metal. Liquid softener and heavy detergent carried over from the washer can add to the layer.
- Loads come out damp after an automatic cycle but dry fine on a timed one.
- Automatic cycles have become noticeably shorter than they used to be.
- The display jumps to its cool-down stage a few minutes into a full load.
- Heavy items, like jeans or towels, are dry on the surface but wet at the seams.
Source: Electrolux — Not drying and extended drying time — Softener sheets leave residue on the moisture sensor bars; wipe the bars with rubbing alcohol
Cleaning the strips
Manufacturers suggest slightly different cleaners. Electrolux says to wipe the bars with rubbing alcohol; Samsung says a soft cloth and mild soap, then dry them with a towel. Either works on ordinary film. Alcohol cuts heavy, waxy buildup faster, and it evaporates without leaving a trace.
Scratches in the strips hold residue and speed the next buildup. Leave the door open for a while after using alcohol so the fumes clear before the dryer runs.
A few things do more harm than good around the strips:
- Scouring pads, steel wool or abrasive cleaning powder.
- Spray cleaners that leave a scented or waxy coating of their own.
- Soaking the area, since water can run down into the wiring below the strips.
- Pliers or a screwdriver to pry at the strips, which are held by clips and wiring.
Source: Electrolux — Not drying and extended drying time — Softener sheets leave residue on the moisture sensor bars; wipe the bars with rubbing alcohol; Samsung — How to clean your Samsung dryer — Sensor Dry uses bars inside the drum that stop the cycle when moisture is no longer detected; clean them with a soft cloth and mild soap
When cleaning does not help
Clean strips that still give short cycles, or cycles that never end, point somewhere else.
Cycles that run far too long
If an automatic cycle keeps going long after the clothes are dry, the strips are not the usual suspect. A blocked vent keeps the air in the drum humid; a wire from the strips that has broken or shorted can make the control read the load as permanently wet. Check the duct first, then book.
Small loads fool it
One shirt or a pair of socks rarely touches the strips, so the control has too little to go on. ENERGY STAR notes that small loads can take longer to dry without the tumbling effect of a full load. For a couple of items, a timed cycle is more reliable than a sensor cycle.
The dryness setting
Automatic cycles usually offer a dryness level, from damp through very dry. If someone set it to less dry, loads will feel damp with perfectly clean strips. Check the knob or the menu before assuming a part has failed.
Source: ENERGY STAR — Clothes dryers — Certified dryers use automatic termination; small loads can take longer to dry without the tumbling of a full load
Less film going forward
Using half a sheet, or a sheet only on synthetic loads where static is worst, cuts the buildup. Wool dryer balls soften without leaving a coating. Whatever you choose, pair it with a periodic wash of the lint screen, since the same residue closes up the mesh and slows airflow.
A monthly wipe of the strips takes a minute, so it fits easily with the lint-screen wash. Households that use a sheet on every load are the ones most likely to see short cycles return within a couple of months.
