How does a storage water heater work?

In its basic principle a storage electric water heater is a simple device: it keeps a certain amount of water in an insulated tank and heats it to a set temperature with an electric heating element.

Behind that simplicity, however, stand several components that control temperature, pressure and safe operation at the same time. To understand water heater faults, the role of the safety valve or the possibility of a dangerous rise in temperature and pressure, you first have to understand what happens inside the heater during normal operation.

The main parts of a storage water heater

The design differs between manufacturers and models, but a classic electric storage heater usually contains:

• a water tank;
• thermal insulation;
• an electric heating element;
• a working thermostat;
• an independent thermal cut-out;
• a cold water inlet connection;
• a hot water outlet connection;
• a safety valve;
• a magnesium or other sacrificial anode on the corresponding types of tank.

Each of these parts has its own function, and the correct operation of the heater depends on all of them working together.

The tank

The tank is a closed vessel that holds the water.

In most domestic heaters it is made of steel sheet with internal anti-corrosion protection, most often enamel, while there are also heaters with stainless steel tanks and other design solutions.

Around the tank there is a layer of thermal insulation whose purpose is to reduce heat loss to the surroundings. That is why the water stays hot even after the heating element has stopped working.

During operation the tank is under mains water pressure, so it is not the same thing as an ordinary open vessel with water in it.

How does water get into the heater?

Cold water enters the lower part of the tank through the cold water connection.

A safety valve or a safety group is usually fitted on that connection.

When a hot tap is opened, pressurised water from the mains enters the heater and pushes the heated water towards the outlet connection.

In other words, the heater does not „pump“ hot water.

What drives the flow is the pressure of the water mains.

As long as water enters on the cold side, approximately the same amount of water can leave on the hot side.

Why is hot water taken from the top of the tank?

Heated water is less dense than colder water and naturally rises towards the upper part of the tank.

That is why the system is built so that hot water is taken from the upper part while fresh cold water enters at the bottom.

In this way the aim is that the cold water that has just arrived mixes as little as possible with the hottest water in the upper part of the heater.

Inside the tank there can therefore be a pronounced temperature stratification: the water at the top can be considerably hotter than the water at the bottom.

The electric heating element

Heating the water is the job of a resistive electric heating element.

When electric current flows through it, electrical energy is converted into heat which is transferred to the water.

In domestic heaters the elements often have a power of a few kilowatts, depending on the volume and the design of the appliance.

A higher power allows faster heating, but the final water temperature is not set by the power of the element, it is set by the thermostat.

A 2 kW element, for example, does not „heat the water harder“ than a 1.5 kW element in terms of the final temperature. It only transfers energy to it faster, while the thermostat decides when the power will be cut off.

The role of the thermostat

The working thermostat follows the water temperature and controls the power supply to the heating element.

When the water temperature is below the set value, the thermostat allows the element to work.

When the temperature reaches the set value, the thermostat breaks the circuit and the element stops working.

After some time the water cools down through heat losses, or because part of the hot water has been used and replaced by cold water.

When the temperature falls below a certain limit again, the thermostat switches the element back on.

That is why a water heater does not draw its rated power continuously through the whole day. The element switches on and off in cycles.

The working thermostat is not the only temperature protection

Besides the working thermostat, a heater usually also has an independent thermal cut-out.

Its purpose is not to regulate the normal water temperature but to switch the element off if the temperature exceeds the permitted limit because the working thermostat has failed.

Those are two different functions.

The working thermostat:

regulates the temperature during normal operation.

The thermal cut-out:

reacts when the normal regulation fails.

On many appliances, once such a protection has tripped, a manual reset or a service call is needed.

What happens to water as it is heated?

Water expands as it is heated.

That means the same mass of hot water takes up a slightly larger volume than when it is cold.

In an open vessel that is not a problem, the water level simply rises a little.

But a storage water heater is part of a pressurised plumbing system.

Because of that, the expansion of water during heating can cause a rise of pressure inside the tank.

This brings us to one of the most important safety elements of a water heater, the safety valve.

What is the safety valve for?

A safety valve has several functions, depending on its design.

Its basic task is to prevent the pressure in the tank from exceeding a safe value.

In some designs the valve allows a controlled return of a small amount of water from the heater back towards the water mains when the pressure rises through heating.

If the pressure keeps rising and reaches the value set for safety relief, the valve opens an outlet to the atmosphere and releases water.

That is why occasional dripping from the safety valve during heating does not necessarily mean the valve is faulty.

Its design and the way it works deserve a separate explanation, so we will devote a text of its own to the safety valve.

What is the magnesium anode for?

A magnesium anode is commonly used in enamelled steel tanks.

Enamel is the basic protection of the steel against contact with water, but no protective layer is absolutely perfect. At microscopic damage, at mounting points and on other exposed surfaces there can be a possibility of steel corrosion.

Magnesium is electrochemically more active than steel, so under the right conditions it is consumed as a sacrificial material, protecting the steel tank.

That is why the anode is not a part fitted „for the whole life of the heater“.

Its condition changes over time and certain designs provide for periodic inspection and replacement.

What happens when we open a tap?

An example shows the work of the whole system best.

Suppose we have a heated 80 litre water heater.

We open the hot water tap.

The pressure of the water mains pushes cold water through the inlet connection into the lower part of the tank.

At the same time hot water from the upper part of the tank is pushed towards the tap.

As the hot water is used, more and more cold water enters the tank.

The temperature inside the heater falls.

The thermostat registers the drop in temperature and switches the element on.

The element heats the water again until the thermostat reaches the set temperature and cuts the power.

Then the cycle repeats.

Why do we sometimes get more „usable“ hot water out of a heater than its volume?

An 80 litre heater does not necessarily mean that at the tap we can only use 80 litres of water at a temperature suitable for a shower.

If the water in the heater is heated to, say, 60 °C, and for a shower we use a mixture of about 38, 40 °C, the hot water from the heater is mixed with cold water.

That is why from 80 litres of very hot water we can get a larger amount of mixed water at a lower temperature.

Exactly how much depends on the temperature of the water in the heater, the temperature of the cold water and the temperature wanted at the outlet.

Why should a water heater not be seen as just „a bucket with a heating element“?

Although the principle is simple, a water heater is in fact:

a closed pressure vessel filled with water, with an electric heating element of several kilowatts.

Because of that, the following have to be controlled at the same time:

• the water temperature;
• the electrical supply to the element;
• the pressure in the tank;
• the electrical protection of the user;
• the mechanical soundness of the tank;
• the correct operation of the safety valve.

When all those systems work correctly, the operation of a water heater is simple and predictable.

When one of them fails, the kind of consequence depends on which part failed and whether the remaining protections are sound.

That is exactly why the next texts will deal separately with how the safety valve of a water heater works and what actually has to happen for a storage water heater to be able to explode.