A dryer that runs cooler finishes cheaper

The theory of tumble dryers is well covered elsewhere. This is about the version you meet in practice.

Drying is evaporation and evaporation is expensive

Turning liquid water into vapour requires a large amount of energy even when the temperature does not change at all. That energy has to come from somewhere, which is why drying clothes consumes far more than washing them does. Spinning faster in the washing machine removes water mechanically, and mechanical removal is dramatically cheaper than evaporation.

This is why the spin speed of the previous machine affects the running cost of the dryer more than any dryer setting. Nothing a dryer does avoids the underlying energy requirement; designs differ only in whether that energy is reused.

The vented approach heats air once

A vented machine draws in room air, heats it, passes it through the tumbling load and expels it outside. The exhausted air carries away both the moisture and all the heat that was used to evaporate it. It also draws replacement air into the room, which in cold weather means heating outdoor air at your own expense.

In practice, the approach is simple, cheap and fast, and it is the least efficient of the common arrangements by a wide margin. It requires a duct to outside, which is why it is common in some housing types and impossible in others.

Condensing keeps the water and loses the heat

A condenser machine recirculates its air, cooling it against a heat exchanger so moisture condenses and drains into a tank. Because the air is not exhausted, no duct is needed and no cold outside air is drawn into the room. The heat released when vapour condenses is dumped into the room rather than recovered, so the room warms noticeably.

The cooling side usually uses room air, which means a hot room reduces condensation and lengthens the cycle. Energy consumption is similar to a vented machine, with the convenience of needing no hole in the wall.

The heat pump reuses what condensing releases

A heat pump dryer moves heat rather than generating it, using the same refrigeration cycle found in a fridge. The cold side of the loop condenses moisture out of the circulating air, and the hot side reheats that same air. The energy released during condensation is therefore recycled into the next pass instead of being lost to the room.

In practice, air temperature stays much lower, which is gentler on fabrics and is why cycles take considerably longer.

The trade is a more complex machine with a sealed refrigerant circuit and a compressor that eventually becomes the failure point.

How the machine decides it has finished

Timed drying simply runs for a set period, which either stops early or continues heating a load that is already dry. Sensor drying measures the electrical conductivity between metal strips inside the drum as damp fabric brushes past. Wet fabric conducts far better than dry fabric, so the resistance rises steadily and levels off when the load is done.

The method misjudges small loads, which may rarely touch the strips, and heavy items whose interior stays wet. Fabric softener residue and hard water deposits on the sensors also shift readings, which is why they need occasional cleaning.

Implementations differ, and vendors are not obliged to document the differences.

Airflow is the thing that degrades

Lint is fabric fibre released by tumbling, and every dryer depends on catching it before it reaches the works. A partially blocked filter restricts airflow, which lengthens cycles and forces the heater to run for longer at higher temperature. In vented machines, lint accumulating in the duct is a genuine fire risk and is the most neglected part of the system.

In heat pump machines a secondary condenser filter clogs slowly, and a machine that suddenly takes twice as long usually needs it cleaned. Cleaning the visible filter every cycle handles most of it, while the hidden paths need attention perhaps once or twice a year.

The takeaway

Spin harder first; the cheapest water to remove is the water you never evaporate.

Once you know what it is trading away, the design stops looking arbitrary.