How does a water heater supply hot water?

1. Turn the hot tap and warm water arrives in seconds. Yet water enters every home cold, through a pipe under the street. Something in the house heats it.
2. Most homes have a tall storage tank. Cold water enters at the top but runs down a long pipe, called a dip tube, so it arrives at the very bottom.
3. A heating element, or a gas burner, warms the water at the bottom. Warm water is lighter than cold water, so it rises and the hottest water gathers at the top.
4. Open a tap and hot water leaves through a pipe at the top. At the same moment cold water flows in at the bottom. The tank always stays full.
5. Use hot water faster than the heater can make it and the hot layer shrinks. When it is gone, the tap runs cold and the tank needs time to recover.
6. A tankless heater stores nothing. When a tap opens, a strong burner switches on and heats the water as it races through a coiled pipe. It never runs out, but it can only heat so much at once.

Tap each part of the tank. Watch what it does when you find it.

• Cold inlet and dip tube. Cold water comes in at the top, then runs down this long tube so it lands at the bottom of the tank, right beside the heater.
• Heating element. Electricity makes this metal loop hot, like the loop in a kettle. It sits low in the tank, where the coldest water is. Gas heaters use a flame under the tank instead.
• Thermostat. The thermostat senses how hot the water is. It switches the heater on when the water cools and off when it is hot enough, so the tank never overheats.
• Insulation jacket. Between the inner tank and the outer shell is a thick layer of foam. It works like a winter coat, slowing the heat that tries to leak out.
• Hot outlet. This short pipe takes water from the very top of the tank, where the hottest water gathers, and sends it to the taps.
• Safety relief valve. Heated water takes up more room and pushes on the tank. If the push or the heat ever gets too high, this valve opens by itself and lets a little water out safely.
• Drain valve. Bits of mineral from the water settle at the bottom over the years. A plumber opens this valve to flush them out.

Tap the faucets to open one, two or three at once. Watch the hot layer in the tank and the temperature from the tankless heater.

Experiment: Three taps run at once for a long time. What happens at each heater?

1. Tap one faucet. Watch both heaters for a few seconds.
2. Open all three faucets. Watch the red layer in the tank.
3. Look at the water coming out of the tankless heater. Is it still hot?

What we found: The tank gave lots of hot water at once, but its hot layer was used up and the taps ran cold. The tankless heater never ran out, yet with three taps open the water raced through too fast to get properly hot.

Turn on JavaScript to try it yourself.

Two homes need hot water in different ways. Pick a tank or a tankless heater for each one, then run the morning. Everyone must get warm water.

Water comes into your home cold. To make it warm, a heater has to give it energy, and water needs a lot of energy to warm up. That is why a kettle takes minutes, not seconds, to boil.

A tank heater solves this by starting early. It heats a big store of water slowly and keeps it hot inside a thick coat of insulation. When you open a tap, hot water leaves the top of the tank and cold water flows into the bottom through the dip tube. Hot water is lighter than cold, so the two stay in layers, hot above cold, like oil floating on water.

If lots of people shower in a row, the hot layer gets used up faster than the heater can replace it. Then the water runs cold and everyone has to wait.

A tankless heater has no store at all. A sensor notices water starting to flow, and a very strong burner or element heats the water in the few seconds it spends inside a coiled pipe. It can keep going all day. But if too many taps open together, the water rushes through too quickly to get hot. Here the thermos and kettle comparison breaks down a little: a real tank refills itself with cold water the moment you pour.

Key words

Dip tube

A long tube that carries incoming cold water to the bottom of the tank.

Thermostat

A switch that senses temperature and turns the heater on or off.

Stratification

Hot water floating in a layer on top of cold water.

Heat exchanger

A part that lets heat pass from a flame or element into water without the two touching.

Specific heat capacity

How much energy it takes to warm something up. Water needs a lot.

Standby loss

Heat that leaks out of stored hot water while nobody is using it.

Check your thinking

1. Where is the hottest water in a tank heater?

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   At the top, because warm water rises. Warm water is lighter than cold water, so it floats up and gathers at the top.

2. Why can a tank heater run out of hot water?

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   People can use the hot water faster than the heater makes it. The tank stays full of water, but the hot part is used up and the heater warms the new cold water only slowly.

3. What happens if you open too many taps on a tankless heater?

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   The water comes out lukewarm. The water rushes through too fast for the burner to heat it fully.

4. What does the dip tube do?

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   It sends cold water to the bottom so it does not mix with the hot water. Keeping the cold water low leaves the hot layer at the top ready to use.

5. Which home is the best match for a tankless heater?

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   Six people who shower one after another. A tankless heater never runs out, so a long line of showers is easy. Its limit is how much it can heat at one moment.

6. Why does a storage tank waste some energy even when nobody uses hot water?

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   Heat slowly leaks through the insulation, so the heater must top it up. This standby loss is the cost of keeping a store of hot water ready all day.