How Electric Storage Water Heaters Work?

Electric storage water heaters are often installed above the ceiling. As a result, many people don’t get to see them and thus, they wonder how electric storage water heaters work? So, I’ll explain it.

An electric storage water heater works by taking in cold water and heating it up using an electric resistance heater to about 140°F (60°C) and storing it for later use. When you turn on the hot water tap, the pressure from the cold water line will push the hot water out of the tap.

The working principle of electric storage water heaters may look simple but they actually packed with multiple safety devices to protect us as the user.

Working Principle of Electric Storage Water Heater

Before I introduce the safety devices on an electric storage water heater, I like to simplify it by explaining to you the main components that make it work first.

Components and Operation

An electric storage water heater is basically made of 3 components; a) a tank, b) a heating element and c) a thermostat. Now, let’s take a look at how they work together.

Below is a diagram showing the basic assembly of an electric storage water heater:

Electric Storage Water Heater Components

Each electric storage water heater has two pipe connections; one for the incoming cold water and the other one for the outgoing hot water.

Initially, when you installed an electric storage water heater, you must fill it up with cold water first. Once it is filled with water, you can now turn on the power supply.

The power goes to the heating element where the heating element becomes hot due to its nature of being high electrical resistance. Hence, the water temperature is gradually raised.

The thermostat is usually pre-set at 140°F (60°C) by the manufacturer but you may adjust it depending on the water heater feature.

When the water reaches the temperature setpoint, the thermostat will cut off the power supply to the heating element automatically thereby stopping the heating process.

Usually, there is a gap of about 10°F (5°C) before the thermostat activates the heating element again to reheat the water. If the temperature setpoint is 140°F (60°C), then the water heater will run again when the water temperature drops to around 130°F (55°C).

Water Flow and Stratification

Once the electric storage water heater contains hot water at 140°F (60°C), it is ready to be used.

Now, when you turn on the hot water tap, the permanently-available pressure (either by gravity or a pump) in the cold water line will push the hot water out of the water heater.

Below is a diagram showing the water flow of an electric storage water heater:

Electric Storage Water Heater Water Flow

The pressure of the hot water is about the same as the cold water from your regular taps (without hot water) of the same bathroom because they share the same source of pressure.

However, the pressure of the hot water may be slightly lower due to the pressure drop caused by the storage water heater.

The hot water outlet is always at the top of the water heater because hot water naturally floats and cold water naturally sinks. Because of that, engineers use nozzles to control stratification so that the efficiency of the water heater is improved.

Below is a diagram showing the nozzles that help stratification:

Electric Storage Water Heater Nozzles

A good electric storage water heater has a nozzle at the cold water inlet to direct water downward as much as possible. Furthermore, the nozzle has small holes to reduce water pressure so that the storage tank won’t get stirred up.

The efficiency of storage water heaters is improved when hot water stays at the top and cold water stays at the bottom because it reduces the duration of the heating element is turned on.

Imagine if there are no nozzles. Whenever you use the hot water for a little bit, the cold water will rush into the water heater and stir up the whole thing. As a result, the overall water temperature drops and this forces the heating element to run, which obviously consumes power.

Conversely, if the cold water flows into the water heater gently and does not mix up with the hot water, the thermostat will still sense that the water is sufficiently hot and thus, it won’t energize the heating element.

Meanwhile, the nozzle at the hot water outlet ensures that only the hottest water is delivered to the tap, not the mixed and warm water.

Safety Devices on Electric Storage Water Heater

I’m not exaggerating but electric storage water heaters are prone to explode, not to mention electrocution. Hence, they have multiple safety devices to prevent tragedy from happening.

1. Thermal Cut Off

Before the power can be delivered to the heating element of an electric storage water heater, it must pass through a thermal cut off.

A thermal cut off is a mechanical safety device that trips and stops the power supply to the heating element of an electric storage water heater when the water temperature reaches around 199°F (93°C). It can be found at the back of a heating element, usually black in color.

Heating Element Thermal Cut Off

Sometimes, the thermostat can fail and when it did fail, power will continuously be supplied to the heating element regardless of the water temperature. As a result, it can boil the water and turns it into steam.

Water is not compressible but steam does. If a water heater starts to accumulate steam, its internal pressure will go up and eventually may lead to an explosion.

Hence, a thermal cut off serves as a warning that indicates the thermostat has failed should it trips.

Nonetheless, some electric storage water heaters don’t have a thermal cut off which makes them not so safe compared to those that have one.

2. Pressure Relief Valve

Further to a thermal cut off, should it also fail to stop the water heater from boiling, a pressure relief valve will activate to relieve pressure thereby preventing pressure built-up that may eventually lead to an explosion.

Pressure Relief Valve

A pressure relief valve (PRV) is a safety valve installed at the cold water inlet. Usually, it has a pressure rating of 0.8 MPa (8 Bar). Should the water heater pressure exceeds that, it’ll start to drip water in an effort to reduce the pressure.

Sometimes, electric storage water heaters are equipped with a temperature and pressure relief valve (TPV) instead of just a PRV.

Temperature and Pressure Relief Valve

A TPV does the same job as a PRV but it has an extra function to check the water temperature. It is normally installed at the hot water outlet in order to sense the water temperature.

However, a TPV or a PRV won’t work if the drain is blocked. So, always make sure that the drain tube is properly installed.

In addition, we should check the TPV or PRV regularly to make sure it is functioning. If we found that it is dripping water, it may indicate that both the thermostat and the thermal cut off are malfunctioning.

3. Isolation Barrier

Water heaters are prone to cause electrocution. However, most people don’t realize that the water heater itself is not actually causing the electrocution but other electrical appliances in the house.

It is another topic to discuss but basically, the electricity that leaks from other faulty appliances can cause electrocution through a water heater.

As a countermeasure, a pair of isolation barriers are used to extend the travel distance of water in order to increase the total electrical resistance so that the leaked electricity dissipates before it can reach the person.

Isolation Barriers

An isolation barrier can be found at both the cold water inlet and the hot water outlet. Often, people forget to account for the additional pressure drop created by isolation barriers and thus, they suffer low flow issues.


Electric storage water heaters are relatively simple engineering products. They use an electric resistance heater to heat up water and produce hot water. However, they are equipped with multiple safety devices and thus, making them look complicated.

This article was originally published on Actions will be taken for unauthorised republication of this article.

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Yu worked as a professional in the HVAC industry for more than 7 years. He is an engineer who has a passion for blogging. Ever since he created this website, Yu has helped hundreds of homeowners and engineers on HVAC related matters.

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