Getting the water out of laundry costs more than heating it did

Evaporation is expensive in a way heating is not

Water is already awkward to heat, because it stores more energy per degree than almost any common substance. But turning liquid water into vapour is in a different class again: the energy needed to evaporate a given mass of water at boiling point is several times the energy needed to raise that same mass from cold to boiling.

None of that energy raises the temperature of anything. It goes into breaking the attractions between molecules so they can leave the liquid, which is why a pan holds steady at boiling however hard you heat it and why sweat cools skin. Latent heat is the term, and laundry is where a household meets it most expensively.

A washed load holds a substantial mass of water in the fabric, and whatever removes it has to supply the latent heat unless the water is flung out mechanically instead. That distinction — evaporate it or fling it out — is the whole economics of drying.

Which makes spin speed the cheap lever

Spinning uses very little energy. The motor is already there, the cycle is short, and what it achieves is the removal of water by force rather than by heat. Every gram thrown out in the drum is a gram the dryer does not have to evaporate, and evaporation is the expensive step.

The gains are not linear, since the water clinging most stubbornly to a fibre is the hardest to remove and the returns fall away at the top of the speed range. There is also a real cost in fabric wear and creasing, and some items should not be spun hard at all. But for ordinary cottons the highest sensible spin is close to free.

It is one of the few household energy adjustments needing no purchase, no installation and no change of habit beyond a different button. Rare enough to be worth saying.

Wash temperature is a smaller number than folklore suggests

Most of the energy in a wash cycle goes on heating water rather than on turning the drum, so the temperature setting is the dominant variable within the machine itself. Today’s detergents are formulated to work at low temperatures, and for ordinary soiling the result at a low setting is generally indistinguishable.

There are genuine exceptions worth respecting. Heavily soiled work clothing, items handled during illness and fabrics that harbour residues all have reasons for a hotter wash, and a machine that’s run permanently cold can develop biofilm in the drum and drain that a periodic hot cycle clears.

So the sensible position is low by default with occasional hot cycles, rather than either extreme adopted as a rule. A smaller claim than the ones usually made about laundry, with the advantage of being true.

Dryers differ mainly in where the heat ends up

A vented dryer heats air, passes it through the clothes, and expels it outside carrying the moisture with it. Simple and effective, but the air it expels came from inside the house and is replaced by cold air drawn in, so in winter the heating system pays a second time for the same load.

A condenser dryer cools the moist air, collects the water in a reservoir and releases the heat into the room. Nothing is expelled, which suits a house that hasn’t an external wall available, and the room becomes warm and slightly humid. In summer that is unwelcome; in a cold utility room it is arguably not wasted.

A heat pump dryer takes the heat released when moisture condenses and reuses it to warm the incoming air, recycling the latent energy instead of throwing it away. That recovery is why consumption falls so markedly. The trade is a longer cycle at a lower temperature, which is gentler on fabrics and slower to finish.

Drying indoors relocates the cost rather than removing it

A rack of wet washing in a living room is often treated as free drying, and it is not. The water still has to evaporate, the latent heat still has to come from somewhere, and it comes from the room’s air — which means the heating system supplies it, quietly, without appearing as an appliance on any monitor.

The vapour then has to leave the building or it will condense on the coldest surface available, which is usually a window reveal or an outside wall. A house drying laundry indoors through the winter without adequate ventilation is generating a moisture load it has no route to remove, and that shows up as mould rather than as a bill.

Outdoors, the same evaporation happens using energy from the air and the sun, at no cost at all. Where climate and space allow, a line remains unbeatable — a deflating conclusion for an article about appliances.

What is actually worth doing

Spin harder, dry outside when the weather permits, and if drying indoors is unavoidable, do it in a room that can be ventilated to the outside rather than in the middle of the house. These cost nothing and address the largest part of the problem.

Replacing a working dryer purely on efficiency grounds is a harder case to make, and for a household drying a load or two a week it is difficult to justify at all. The measure that matters there is how often the machine runs, not what it consumes when it does.

Where the case is strong is a household drying frequently and year-round with no realistic outdoor option — a large family, a flat without a garden, a wet climate. There the difference between the dryer types is large enough to be worth deciding deliberately rather than replacing like for like when the old machine dies.