No bath of water, just 3 litres sprayed round and round by arms that spin on their own recoil. A dishwasher lets in only about 3 litres at a time. No motor turns the spray arms. The holes are drilled at a slant, so each jet pushes the arm the other way, like a lawn sprinkler.
No bath of water, just 3 litres sprayed round and round by arms that spin on their own recoil. Take a dishwasher apart in 3D and see why it can use a tenth of the water your sink does.
DishwasherClearOpened 18 Jul 202612 min to playFree · no sign-up
In 60 seconds
A shower, not a bath
A dishwasher lets in only about 3 litres at a time. A pump pushes it through a heater to two spray arms, the jets hit the dishes, and the water runs through a filter back to the sump to go round again, many times a minute.
Arms that spin on recoil
No motor turns the spray arms. The holes are drilled at a slant, so each jet pushes the arm the other way, like a lawn sprinkler. With about half a bar of pressure the water leaves at about 10 m/s and the arm turns at a few tens of rpm.
A programme of stages
Cold pre-rinse, hot main wash with detergent, rinse, hot final rinse with rinse aid, then dry. Each stage lets in fresh water and pumps it away. Almost all the electricity heats water and dishes, so the long Eco programme cleans at 50 °C and uses the least energy.
Chemistry does the scrubbing
Alkali turns fat into soap, enzymes snip up egg and starch, and oxygen bleach lifts tea stains. A resin softener swaps calcium for sodium and is recharged with salt. Rinse aid lowers surface tension so water sheets off instead of drying into spots.
Hot dishes dry themselves
Evaporating water needs about 2,400 J per gram. After a hot final rinse, the dishes carry that heat, and vapour condenses on the cooler steel walls. Plastic stores too little heat to dry fully, which is why zeolite drying and auto door-open help.
About a tenth of the water
A full modern dishwasher uses about 10 litres on Eco. People in a University of Bonn study used 103 litres on average to hand-wash the same 12 place settings. Run it full: a half load uses nearly the same water and energy.
A steel box, two spinning arms, a pump, a heater and a few litres of very busy water.
A dishwasher is not a bath for plates. It is a shower that sprays the same water again and again. Only about 3 litres sit in the bottom at any moment, but a pump throws them at the dishes many times a minute.
Water comes in through the inlet valve, passes the water softener and collects in the sump, a small well under the floor. The circulation pump pushes it through a heater and up to two spray arms, one under each rack. The jets hit the dishes, run down, and go through the filter back to the sump, round and round.
At the end of each stage the drain pump empties the sump. The detergent and rinse aid wait in little dispensers on the door until the machine pops them open. A float in the base pan switches off the water if anything leaks.
No motor turns them. Every tilted jet pushes the arm the other way.
Nothing drives the spray arms. There is no motor and no gear. They spin because of Newton's third law: push water one way and the water pushes you the other way.
The little holes are drilled at a slant. Each jet leans backwards as it leaves, so it shoves the arm forwards, like the recoil of a water pistol or a spinning lawn sprinkler. The push twisting the arm is called torque. It grows with how much water comes out each second, how fast it comes out, and how far from the middle the hole is: τ = ρ·Q·v·r.
The pump pushes at only about half a bar, half the pressure of the air around you, yet the water leaves the holes at about 10 metres a second. Point the holes straight up and the arm just sits there. That is why a jammed spoon hanging through the rack leaves the dishes dirty: the arm stops, and the jets hit the same spots.
Rinse, wash hot, rinse, rinse hotter, dry: the same few litres, used again and again.
A wash programme is a short list of stages. Each one starts by letting in about 3 litres of fresh water and ends by pumping it away.
Pre-rinse: cold water knocks off loose food. Cold, because hot water would glue egg to the plate. Main wash: the detergent drops in and the heater warms the water to 45 to 70 °C. Rinse: fresh water washes away the dirty suds. Final rinse: hot water with a squirt of rinse aid, which also heats the dishes so they dry. Dry: the heat soaks out of the dishes and the water leaves them.
Here is the surprise: Eco takes the longest, often over three hours, yet uses the least energy. Almost all the electricity goes into heating water and dishes. Cleaning depends on four things: heat, chemistry, scrubbing and time. Give the detergent's enzymes more time and they can do the job at 50 °C instead of 65 °C.
Soap from fat, enzymes that snip food apart, a salt swap, and water that forgets to bead.
Spraying alone won't shift dried egg. The detergent does the real work, and it attacks each kind of dirt differently.
It is strongly alkaline. Alkali turns fat into a kind of soap that dissolves in water: that is saponification, and it goes much better once the fat melts. Enzymes are tiny protein scissors: protease snips up egg and meat, amylase snips up rice and potato starch. They work best between about 40 and 55 °C, and stop if it gets too hot. Oxygen bleach fizzes off the brown of tea and turmeric.
Hard water is full of calcium. It fights the detergent and leaves white spots. The softener is a tube of plastic resin beads that grab calcium ions (Ca²⁺) and let go of sodium ions (Na⁺) instead: ion exchange. When the beads are full of calcium, the machine washes them with salt water to swap it all back. That is why you top up dishwasher salt.
Rinse aid is a surfactant. It lowers water's surface tension, so instead of pulling itself into beads that dry into spots, the water spreads into a thin sheet and runs off.
The heat stored in hot dishes does the drying. Plastic stores too little.
Turning water into vapour takes a lot of energy: about 2,400 joules for every gram. So drying needs heat. Machines find it in three ways.
Heating element: an old-style hot element warms the air. It works, but it costs extra electricity. Condensation drying: the final rinse is hot, so the dishes come out hot and full of stored heat. The steel walls cool down faster, so vapour from the dishes condenses on the walls and runs down to the drain, just like steam on a cold bathroom mirror. Zeolite: a mineral under the tub that soaks up water vapour and gets hot as it does. The heat that dried the zeolite out during the main wash also warmed the wash water, so it is nearly free.
Why does plastic stay wet? A plastic box is light and holds little heat for its size. It cools down before its water can evaporate. A heavy plate stores more than 3 times as much heat per square centimetre. Many machines now pop the door open a crack at the end to let the damp air out.
A full dishwasher uses about 10 litres. Washing the same by hand often takes ten times more.
It feels as if a big machine must use more water than your two hands. It doesn't. A modern dishwasher washes a full load with about 10 litres, because it sprays the same few litres round and round.
At the University of Bonn, researchers asked 113 people across Europe to wash 12 place settings by hand. On average they used 103 litres of water and 2.5 kWh to heat it, and took 79 minutes. The most careful used about 30 litres; the most wasteful, far more than 100. A modern full-size machine on Eco needs about a tenth of that.
Two catches. A machine uses almost the same water and energy whether it is full or half empty, so run it full. And if you wash by hand in cold water, as many people in India do, hand-washing uses no heating energy at all, but still far more water.
How much water sits in a modern dishwasher while it washes?
About 3 litres. Only a few litres fill the sump. The pump sprays them round and round instead of filling the tub.
What makes the water come out of the spray arms?
The circulation pump. The circulation pump pushes water from the sump up through the pipes to the arms.
Where does the water go after it hits the dishes?
Through the filter, back to the sump, and round again. It is recirculated. Only at the end of a stage does the drain pump empty it.
What makes a dishwasher’s spray arm turn?
The push back from its slanted jets. Each tilted jet shoves the arm the other way, like a lawn sprinkler.
What happens if all the holes point straight up?
It hardly turns at all. Straight-up jets push only downwards on the arm, with no sideways push to twist it.
Why does a spoon hanging through the rack spoil the wash?
It blocks the arm, so the jets stop sweeping round. A stuck arm sprays the same spots over and over, and misses the rest.
Where does most of a dishwasher’s electricity go?
Heating the water and the dishes. The pump needs only tens of watts. The heater needs about 2,000 W whenever it is on.
Why can the Eco programme clean at a lower temperature?
It gives the detergent longer to work. Heat, chemistry, scrubbing and time all clean. Less heat can be made up with more time.
Why is the first rinse cold?
Hot water would cook egg and other protein onto the plates. Protein like egg sets hard when heated, so loose food is knocked off before the water warms up.
What does the enzyme protease do?
Snips up proteins like egg. Protease cuts proteins into small pieces that wash away. Amylase does the same to starch.
Why does a dishwasher need special salt?
To wash calcium off the softener’s resin so it can work again. Salty water swaps sodium back onto the resin beads and flushes the trapped calcium away.
How does rinse aid stop spots on glasses?
It lowers surface tension so water runs off as a sheet. With weaker surface tension, water spreads and drains instead of drying as drops full of minerals.
In condensation drying, where does the water go?
It condenses on the cooler tub walls and runs down to the drain. The walls cool faster than the dishes, so vapour from the hot dishes condenses on the steel.
Why does plastic come out wetter than a plate?
It stores much less heat, so it cools before its water can evaporate. Light plastic holds little heat per square centimetre. Evaporating water needs that heat.
What does zeolite do in a dishwasher?
Soaks up water vapour and gives out heat. The mineral adsorbs vapour, drying the air, and the heat it releases warms the dishes.
About how much water does a modern dishwasher use for a full load?
About 10 litres. It recirculates a few litres in each stage, so the whole Eco programme takes roughly 9 to 11 litres.
In the Bonn study, how much water did people use to hand-wash 12 place settings, on average?
About 100 L. The average was 103 litres, with a very wide spread from careful to wasteful washers.
Why should you run a dishwasher only when it is full?
It uses about the same water and energy either way. Fill volumes and heating hardly change with the load, so a half load wastes half the water and energy.
Words worth knowing
Sump
The small well under the tub floor where wash water collects before the pump sends it round again.
Recirculation
Pumping the same few litres from the sump to the spray arms over and over during one stage.
Reaction force
The push back on a nozzle as its jet shoots water out. It is what turns the spray arms.
Torque
A twisting push. A force far from the centre twists more.
Enzyme
A protein that speeds up one reaction, like snipping starch or egg into small pieces that wash away.
Ion exchange
Resin beads that trap calcium ions from hard water and release sodium ions instead.
Surface tension
The pull between water molecules that makes drops round. Rinse aid weakens it so water sheets off.
Latent heat
The energy needed to turn water into vapour: about 2,400 joules per gram.
Zeolite
A porous mineral that soaks up water vapour and gives out heat as it does.
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