How does a water heater (geyser) work?

A storage geyser is a big insulated flask with a heater inside. The common Indian ones hold 10 to 25 litres. Inside the white outer body is a steel inner tank, lined with a layer of glass (or made of stainless steel) so it doesn't rust. Between the two is thick PUF foam insulation, which keeps the water hot for hours.

A heating element pokes up from the bottom: a coiled wire packed in powder inside a metal tube, usually 2 kW. A thermostat senses the water and switches it off at the temperature you set, and a thermal cut-out steps in if the thermostat fails. A magnesium anode rod quietly rusts away so the tank doesn't.

Cold water comes in at the bottom. Hot water leaves from the top, through a pipe that reaches right up inside the tank. On the cold pipe sits a brass multi-function valve: it stops hot water flowing back into your cold pipes and lets out water if the pressure climbs too high.

An instant geyser has a tiny 1 to 3 litre tank and a bigger 3 kW element. A gas geyser has no tank at all: a row of gas flames heats a copper coil as the water flows through it (see how flames and flues work in ChimneyClear).

Water is hard to heat. Every kilogram (one litre) needs 4,180 joules to warm by just 1 °C. That number is water's specific heat, and it's higher than almost any other common substance.

So the heat a geyser must deliver is Q = m × c × ΔT: mass × specific heat × temperature rise. Warming 15 L from 20 to 45 °C takes 15 × 4,180 × 25 = 1.57 million joules. A 2 kW element supplies 2,000 joules a second, and about 95% of it stays in the water, so that's about 14 minutes.

In a Delhi winter, the water in the pipes may be 10 to 15 °C instead of 25 to 30 °C in summer, so the same geyser takes about twice as long. And a hotter tank goes further: you mix it with cold water at the tap. Hot at 60 °C and cold at 20 °C make roughly two buckets of 40 °C bath water from one 15 L tank.

The element itself is just a resistor. At 230 V, a 2 kW element has a resistance of about 26 Ω (P = V² ÷ R: see Ohm's law).

Warm water is a little lighter than cold water. At 60 °C a litre weighs 983 g; at 20 °C it weighs 998 g. That 1.5% is enough: hot water floats on cold, just as oil floats on water. So a geyser's tank sorts itself into layers, hottest at the top. This is called stratification.

A geyser uses it. The element sits low down, so the water it warms rises in a gentle plume (a convection current) and collects at the top, where the hot outlet pipe draws from. Cold water comes in at the bottom, through a deflector or dip tube so it slides in gently instead of stirring everything up.

When you open the tap, the cold water pushes the hot layer up and out like a piston. The boundary between them, the thermocline, climbs steadily. You get full-temperature water almost to the end, then it turns cold within a litre or two. Without a deflector the incoming jet mixes the tank, and the water fades from hot to lukewarm much sooner.

A thermostat is an automatic switch. In most Indian geysers it's a capillary thermostat: a thin tube of liquid sits in the pocket beside the element. As the water warms, the liquid expands and pushes a switch open at the temperature you set on the knob, usually 25 to 75 °C. Others use a bimetal disc that snaps over when hot. It switches back on only after the water cools a few degrees, so it doesn't click on and off all the time.

If the thermostat sticks on, a separate thermal cut-out trips at about 85 to 90 °C and stays off until someone presses its reset button. Water also expands as it warms, about 200 mL for a 15 L tank heated to 60 °C. The non-return valve won't let it back into the cold pipe, so the pressure-relief valve lets a little out at about 6 to 8 bar. That drip is normal. Never block it: a sealed tank of very hot water can burst with the force of a small bomb.

Finally, the metal body is earthed, and an RCCB (or ELCB) in your fuse box cuts the power in a few hundredths of a second if even 30 mA of current leaks to earth, for example through a cracked element. Water and electricity are a risky mix; see Ohm's law and safety.

Steel and hot water don't get along: the iron slowly rusts. The glass lining protects most of the tank, but there are always tiny flaws. So geysers use a trick from ships and pipelines: a sacrificial anode.

Different metals hold on to their electrons differently; list them and you get the galvanic series. Magnesium gives up electrons much more readily than steel. Bolt a magnesium rod into the tank and it corrodes instead, sending electrons through the metal to the steel, which stays protected. The rod slowly dissolves, so it must be checked every year or two and replaced when it's worn down. Once it's gone, the tank starts to rust.

Hard water brings a second problem. It carries dissolved calcium, and heating makes it drop out as calcium carbonate: the chalky scale you see in kettles. It builds up on the hottest surface, the element. Scale conducts heat poorly, so the element has to run hotter and hotter to push its heat through. Nearly all of the heat still ends up in the water, but the element can overheat and burn out. Many Indian cities, like Bengaluru on borewell water, have very hard water.

A storage geyser keeps its water hot even when no one is using it, so heat seeps out through the foam all day. That's its standing loss. Heat flows through insulation like water through a sponge: Q = U × A × ΔT. Thicker, better foam (a smaller U), a smaller surface (A) and a lower temperature (ΔT) all cut it. It's the same physics as a fridge's walls (see FridgeClear).

In India, the BEE star label on a storage geyser rates exactly this: the kWh a full tank loses in 24 hours when it's kept 45 °C above the room. A 15 L five-star geyser must lose no more than 0.46 kWh a day; one star allows up to 0.68. Switching the geyser off after your bath saves most of it.

Two cleverer machines beat any element. A heat pump water heater is an air conditioner run backwards: it pulls heat out of the air and pumps it into the water, giving about 3 units of heat for 1 of electricity. A solar water heater on the roof uses black collectors or glass evacuated tubes to soak up sunlight; a typical Indian family system holds 100 litres a day and needs electric backup only on cloudy days.