How an Electric Shower Works and Common Electric Shower Faults

Electric Shower Problems | Electric Shower Fault Finding | Electric Shower Diagnostics | Electric Shower Repair

This article explains how an electric shower works and lists some of the most common electric shower faults along with their symptoms. It is a diagnostic guide and cannot cover every make of electric shower.
It can be a daunting task to engage in fault-finding an appliance if you know nothing about it. Knowledge is everything. Familiarize yourself with the information here and obtain a diagram of your shower’s internals. You should be able to identify where your fault lies quite quickly with all this information.
I found this site, showerdoc.com extremely useful for video help and to obtain spares very quickly.
And it goes without saying: Electricity can kill. Always ensure that your are working safely by isolating your shower electrically before commencing any type of work on it. If in doubt, contact a competent electrician.

Typical Electric Shower Components

Shown below, is a schematic of a typical electric shower and a photograph that illustrates what the parts of a shower look like.

Schematic: Typical Electric Shower Components

Labelled: Electric Shower Internal Parts

How an Electric Shower Works

Schematic: How an Electric Shower Works

• When the user starts the shower by pressing the stop/start button, the flow control valve lets in the water. If there is sufficient water pressure, the pressure switch will turn on the heating element(s).
• The element(s) in the heat exchanger will start to heat the water as the water passes through it.
• By controlling the flow rate of water over the heating element, the output temperature of the water can be controlled. Reducing water flow rate increases water temperature since the water is in contact with the heating element for longer. Increasing water flow rate will reduce water temperature accordingly. The graph below illustrates the relationship between water flow rate and the rise in cold water temperature for a typical electric shower with a heat output of ~9.5KW.

  

  The Relationship of Water Flow Rate to The Rise in Water Temperature in a Typical Electric Shower

• It should be noted that the temperature of the incoming water supply varies throughout the year. It can be as low as 5°C in winter and as high as 20ºC in the summer. This affects the output temperature of the shower on a seasonal basis. You may find that during the winter months, you have to turn your heat selector dial further into the red; slowing the water flow to achieve a water temperature you have become accustomed to.
• Some electric showers have high and low settings to give further control over water temperature. This is often achieved by operating just one heating element instead of two.

Electric Shower Wiring Diagram (Typical)

Schematic: Typical Electrical Shower Wiring Diagram

This is the typical wiring of an electric shower. And it’s invaluable for fault-finding.
Essentially, there are two separate circuits:

1. A low power circuit (small diameter wires) that operates a solenoid valve to start the flow of water into the unit .
2. A high power circuit (large diameter wires) that controls the heating elements in the heat exchanger and is switched on when water pressure is detected.

It is recommended that you seek the wiring diagram for your particular electric shower model to assist in fault-finding.

1. Electric Shower Low Power Circuit: Stop/Start Water Flow

Stop/Start Switch

Pressing the stop/start switch operates the solenoid valve (see below) that lets mains water enter the shower unit. It also supplies power to the neon lights. When power is switched off, the solenoid valve closes and prevents the flow of water through the unit.

Common Faults with Electric Shower Stop/Start Switches

This wiring diagram illustrates that the Start/Stop neon will illuminate when the Stop/start switch is switched on. If your electric shower will not start and this neon does not illuminate, potential causes could be:

1. No electricity supply to your electric shower. Check the obvious; fuses, a MCB trip, isolation pull-cord switched off.
2. The front cover of the shower may not have been fitted properly or it may have come away from the shower’s back plate so that there is no decent contact between the push button and the micro-switch inside. Take off the front cover and replace, ensuring switch operation is good.
3. The micro-switch is faulty. Replace the micro-switch.

How an Electric Shower Solenoid Valve Works

There are many different designs of solenoid valve assemblies, yet they all look very similar and usually have a detachable solenoid coil.

Solenoid Valve Assembly

Solenoid Coil

The solenoid coil is the part that generates magnetism when an electrical current is passed through it. The magnetism attracts the metal pin inside the valve assembly to allow mains water into the shower unit. It is very easy to detach it from the valve assembly. Insert a flat-bladed screw driver between the valve housing and the top of the solenoid coil and lever the two apart.

Common Faults with Electric Shower Solenoid Valves:

1. Sufficient mains water pressure is required to keep this valve closed. If mains water pressure is inadequate, water may pass through the valve and cause the shower head to drip. Ensure that your stop-cock is fully open and that you have full mains water pressure.
2. A damaged diaphragm will allow the constant passage of water through the valve. Even though the shower is switched off, cold water will continue to flow through the shower head. Replace the solenoid valve assembly.
3. A failure of the solenoid coil will not activate the valve’s piston and water will not flow into the shower unit. Replace the solenoid coil or the solenoid valve assembly as a complete unit.

2. Electric Shower High Power Circuit: Heating Element Electrical Circuit

The wiring diagram (above) shows that this heat exchanger has two heating elements. Each heating element electrical circuit contains at least two switches in series, which means that they all need to be on for the circuit to be made and for the elements to generate heat.
In circuit order, the following switches must be on for the relevant heating elements to operate.

• Heating Element #1: Thermal Cut Out (TCO) + Pressure Switch LH
• Heating Element #2: Thermal Cut Out (TCO) + Pressure Switch RH + Power Setting Switch

This means that:

• A failure of the TCO will prevent both heating elements from working.
• A failure of BOTH pressure switches will prevent both heating elements from working.
• A failure of one pressure switch or the power setting switch will cause one of the heating elements to stop working.

Electric Shower: Thermal Cut Out (TCO) Switch

Shown on top of the heat exchanger is an electrical switch known as a Thermal Cut Out switch, or TCO for short. This device senses the temperature of the water inside the heat exchanger and if it detects that the water is too hot then it shuts off the electricity supply to the heating elements. It is there to prevent the bather from being scalded.

A Thermal Cut Out Switch (TCO)

Common Faults with Electric Shower Thermal Cut Out (TCO) Switches:

1. Under normal circumstances, a TCO may switch off and on repeatedly if the incoming water temperature and selected heat setting is too high. There’s nothing wrong here. It’s actually doing its job and protecting you from being burnt. Turn down the heat setting.
2. A TCO can malfunction in a number of ways. It can fail completely and consequently fail to complete the circuit for the heating elements, resulting in no hot water. It could also fail by switching off then on at random, resulting in repeated bouts of hot and cold water. If you suspect the TCO in your electric shower, you should test it with a multi-meter. If you don’t have a muti-meter, consider by-passing the TCO by disconnecting the live input terminal (the wire from the terminal block) and connecting it securely to the output terminal of the TCO. Don’t use the shower like this as the scalding protection is lost. You can run it briefly however to see if the shower produces constant hot water. If it does, you know that the TCO is faulty.

Electric Shower: Pressure Switches / Micro Switches

Electric shower pressure switches are essentially micro switches that are able to handle high current. They are switched on by mechanical means when water pressure is detected in the shower system. Shown below is an example of a micro switch with the on/off button in white:

Electric Shower Micro Switch (Pressure Switch)

Generally, electric showers contain two of these micro switches to govern power to two heating elements in the shower’s heat exchanger. The mechanism that actually operates these switches is mounted behind them. In the photograph below, the micro switches can be seen with the thick brown wires going to and from them. Above them is the dial that selects one or both heating elements in the heat exchanger. It is labelled as HOT, MEDIUM & COLD on my shower’s face plate. It has 3 selectable positions and should not be confused with the primary dial that controls water temperature by altering water flow rate.

Micro Switches (Front)

It can be seen that there is another plastic disc beneath this dial, and it is larger in diameter. This disc is pushed towards the micro switches when water pressure is detected in the shower. That movement activates the buttons on the micro switches to allow electrical current to pass to the heating elements.

Operation of Micro Switches When Water Pressure is Detected

Note that there is a gap in the larger diameter disc and that the three-position temperature selector dial is connected to it. In the photographs, the dial is currently selected HOT and it can be seen that the larger diameter disc will operate both micro switches when water pressure is detected and the disc is shunted forwards towards them, When the selector dial is turned anti clockwise to MEDIUM, the gap in the larger disc prevents the second micro switch from operating.

Common Faults with Electric Shower Pressure Switches:

1. Look for signs of burning or melting around these switches. If any of these signs are observed, replace.
2. Check the operation of the mechanical pressure device. Make sure it is free to move when electrical power is off. Observe it’s operation with the shower’s front cover off and taking all necessary precautions. If it fails to operate, change it.
3. Ensure that the coarse temperature selector dial is set to HOT and that the corresponding mechanical device will operate both micro switches. Check movement of the selector in conjunction with the pressure switch and it’s orientation with respect to the micro switches.
4. Remove the pressure switches from their mountings and depress the micro switches in the back a few times, then re-mount. This procedure fixed the fault on my shower.

Electric Shower: Power Setting Switch

Power setting switches are electrical switches that can be selected manually to operate a second heating element  in the heat exchanger.

Common Faults with Electric Shower Power Setting Switches:

1. Check the terminal connections to the switch. Use a multi-meter to check switch integrity.

Electric Shower: Heating Elements

If you’ve concluded that the TCO and pressure switches are working properly; the neon lights are functioning and you still have no hot water, then the likelihood is that the heating elements have failed. The only way to know for sure is to test the elements with a multi-meter.

Electric Shower Heating Elements & TCO