Understanding how an electric shower works is the secret to faster, safer DIY repairs. Unlike a mixer shower, an electric unit is essentially a high-powered water heater that combines plumbing and electronics in one box. In this guide, I’ll walk you through the journey of water from the mains pipe, through the heating tank, and out of your shower head, explaining what each internal part actually does.
The 3 Stages of an Electric Shower
To make sense of the repair, think of the shower in three stages:
1. The Entry (The Solenoid Valve) When you press the 'Start' button, electricity hits the solenoid valve. This is an electromagnetic gate that opens to let mains-pressure water flow into the unit. If this doesn't open, your shower stays dry.
2. The Pressure Check (The Microswitch) The water then pushes against a diaphragm in the pressure switch. This is a safety check—if your mains pressure is too low, the switch won't click, and the heating elements won't turn on. This prevents the tank from boiling dry and melting.
3. The Heat Exchange (Heating Elements & TCO) Once the pressure is confirmed, electricity flows to the heating elements inside the canister. As water passes over these elements, it heats up instantly. The Thermal Cut-Out (TCO) sits on top, monitoring the temperature to ensure it never reaches scalding levels.
Flow path
- Cold Entry: Water enters via the solenoid valve.
Pressure Check: The pressure switch confirms flow is safe.
Heating: The stabiliser valve slows the water as it passes the elements.
Outlet: Hot water exits via the PRD-protected outlet pipe.
Common Symptoms & What They Mean
Water is stone cold: Usually a failed TCO or a burnt-out heating element.
Shower starts then cuts off: Often a faulty solenoid coil getting too hot and failing.
Low pressure light is on: A blockage in the head, a kinked hose, or a problem with your home's mains water pressure.
Whether you have a Triton T80, a Mira Sport, or a Bristan, the basic principle of operation remains the same.
Electric Shower Troubleshooting: Top 2026 FAQs
Q: Why is my electric shower suddenly running ice cold?
A: This is usually caused by a tripped Thermal Cut-Out (TCO) or a failed heating element. If the TCO has operated, it's often due to a lack of water flow caused by a blocked shower head or a kinked hose. Use a multimeter to test for continuity across the TCO and elements.
Q: Why does my shower keep going hot and cold (cycling)?
A: This is known as "cycling." It happens when the water flow is too low. The shower gets too hot, the safety TCO trips (turning the heat off), the water cools down, the TCO resets (turning the heat back on), and the cycle repeats. The fix: Descale your shower head and check your hose for internal collapses.
Q: Why is water leaking from the bottom of the shower unit?
A: If water is gushing from the bottom, your Pressure Relief Device (PRD) has burst. This is a safety feature that "blows" to prevent the heating tank from exploding when there is a major blockage. You must replace the PRD and clear the blockage (usually a scaled-up shower head) before using it again.
Q: My shower has power but no water is coming out. What's wrong?
A: The most likely culprit is a faulty solenoid valve coil. The coil is responsible for lifting a plunger to let water into the unit. If the coil's internal wiring breaks, the valve stays shut. You can test this with a multimeter; a healthy coil usually reads between 3.5kΩ and 4.0kΩ.
Q: Can I replace an 8.5kW shower with a 10.5kW model?
A: Not necessarily. A 10.5kW shower draws significantly more current and usually requires 10mm² cabling and a 45A or 50A circuit breaker. Most older 8.5kW showers use 6mm² cable. Running a 10.5kW shower on 6mm² cable is a fire hazard. Always consult a Part P electrician before upgrading.
Q: Why does my shower smell like burning plastic?
A: Switch it off immediately at the fuse box. A burning smell usually indicates a loose wire at the terminal block or a failing pull-cord switch. Because showers pull high current, any loose connection will generate intense heat, melting the plastic and potentially starting a fire.
Electric Shower sizes and the cable sizes needed.
8.5kW35A - 37A6mm² or 10mm² Standard daily use
9.5kW39A - 41A10mm² Better winter performance
10.5kW43A - 46A10mm² High flow / Large families
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