Summary

Electric showers fail in predictable ways. The unit takes mains cold water in, heats it through a built-in element to a user-set temperature, and delivers it out the shower head. Three things can go wrong: water doesn't enter, water doesn't get heated, or the unit detects a fault and shuts down for safety. Diagnosis is logical when you understand the internal flow.

For an electrician or qualified plumber working on showers, the most important thing is electrical isolation. Electric showers are on dedicated 8.5–10.5kW circuits typically protected by a 40A or 45A MCB/RCBO. The supply is live until the consumer unit breaker is OFF and confirmed dead. Working live on a shower will kill you.

This article covers the diagnostic tree, the major component failures, the replacement procedures, the safety isolation steps, and the regulatory context (Part P, BS 7671 special location zones, water and electrical compliance).

Key Facts

Quick Reference Table

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Symptom Most likely cause Fix
No power, MCB tripped Element insulation failure, internal short Test element insulation; replace if failed
MCB tripped repeatedly Faulty RCD, water on PCB, faulty element Investigate before reset
Power on (lights showing) but no water Inlet solenoid failed, water supply isolated Check supply; if OK, replace solenoid
Power on, water comes through but cold Flow switch failed, heating element burnt out, TCO tripped Test in order; replace as found
Power on, water but only lukewarm Inlet pressure too low (combi cycling), partial element failure Check pressure; check element wattage
Water alternates hot/cold Flow surge from another tap, scaling on element Check no concurrent demand; descale
Water trickles from showerhead even when off Inlet solenoid stuck open Replace solenoid
Beeping / error code displayed Refer to manufacturer code list Brand-specific

Detailed Guidance

Step 1: Electrical isolation BEFORE opening the unit

  1. Locate the shower's dedicated circuit at the consumer unit (typically labelled "Shower" or "Bathroom shower").
  2. Switch the MCB/RCBO to OFF.
  3. Turn on the shower from the bathroom control to confirm no power (no warning lights, no indication).
  4. Pull the pull-cord isolator OFF as a secondary safety step.
  5. Use a non-contact voltage tester at the shower terminals BEFORE touching — confirm dead.

NEVER skip step 5. A pull-cord left ON or a circuit mislabelled at the consumer unit will leave live voltage on terminals you're about to grip. The 230V on a shower circuit is unforgiving.

Step 2: Diagnostic tree

Electric shower fault
├── No water at all
│     ├── Power lights ON → Inlet solenoid OR no water supply
│     │       → Check supply (turn on bath tap to confirm mains)
│     │       → If supply OK, swap inlet solenoid
│     └── Power lights OFF → No power
│             → Check MCB/RCBO state; check pull-cord; check supply cable
│
├── Water flows but cold only
│     ├── Power lights ON
│     │       → Flow switch / element / TCO
│     │       → Test continuity at element; if open = burnt out
│     │       → Test TCO; if open = tripped (replace; investigate cause)
│     └── Power lights OFF → No power (treat as above)
│
├── Hot then cold then hot
│     ├── Other taps in use → Flow drop tripping flow switch
│     └── Single appliance → Scaled element; descale
│
└── Water trickles from head when off
      └── Inlet solenoid stuck open → Replace solenoid

Replacing the inlet solenoid (most common failure)

  1. Isolate electrically (confirmed dead) and isolate water at the shower's service valve (typically a quarter-turn brass valve below the unit).
  2. Remove the front cover (usually 1–2 visible screws plus 2 hidden under the dial caps).
  3. Locate the solenoid — a square plastic-bodied unit on the inlet side, with two wire connections and a single coil.
  4. Note wire connections (photo them) before disconnecting.
  5. Undo the two solenoid retaining screws or clip.
  6. Lift solenoid out, fit replacement.
  7. Reconnect wires, refit cover, restore water, restore power.
  8. Test full cycle: cold-only and hot at each temperature setting.

Solenoid replacements typically cost £30–£60 in parts, 30–60 min labour.

Replacing the heating element

  1. Isolate electrically and water-side.
  2. Drain the unit — open the shower with the inlet isolated to drain residual water.
  3. Open the unit; element is the largest component, a copper coil enclosure with thick electrical connections.
  4. Disconnect element wiring (photo and label).
  5. Undo element retaining ring or screws.
  6. Withdraw element — may need light force if scaled in place.
  7. Fit replacement element (matched wattage — never substitute a higher wattage as the supply cable and MCB are sized for the original).
  8. Reverse assembly. Restore water (check for leaks). Restore power. Test.

Element replacement typically £80–£150 parts, 60–90 min labour. On units >10 years old, often economically equivalent to fitting a new shower — discuss with customer.

Resetting a tripped TCO (thermal cutout)

The TCO is a one-shot thermal safety device. When the element overheats (typically due to scaling or restricted flow), the TCO opens and cuts power. Two failure scenarios:

Never bypass a TCO. It exists because the element can overheat catastrophically.

When the customer should replace the whole unit

Otherwise, repair beats replace 2-to-1 on electric showers.

Cable upgrade considerations

Upgrading from 8.5kW to 10.5kW typically means upgrading from 6mm² T&E + 40A protection to 10mm² T&E + 50A protection. This is significant additional work — opening up cable runs, possibly upgrading the MCB and verifying RCD compatibility. Quote carefully; this is rarely a 30-minute job.

Frequently Asked Questions

My customer's shower trips the consumer unit RCD every time — what's the cause?

Most likely a failing element with insulation breakdown — the heating element's insulation has degraded and is leaking current to earth, which the RCD detects. Test the element insulation with an insulation tester (IR test at 500V); if below 1MΩ, replace the element. Less common: water ingress to the PCB or solenoid wiring.

Is an electric shower notifiable under Part P?

Yes, electric shower installation in a dwelling is notifiable. Either self-certify under a Part P scheme (NICEIC, NAPIT, etc.) or notify Building Control before starting. Replacement of an existing shower like-for-like on existing supply is sometimes treated as maintenance not new install — interpretation varies; safest to notify.

Can I fit a 10.5kW shower to an existing 8.5kW circuit?

No. The cable is sized for the original load. 6mm² T&E + 40A MCB cannot safely carry the additional current of a 10.5kW shower. Upgrade the cable and protection or fit a matched-rating replacement.

My customer's shower delivers lukewarm water at max temperature — why?

Likely flow rate is too high for the kW rating. A 7.5kW shower at 6 l/min struggles to heat much beyond 25°C rise (i.e. cold inlet at 10°C = lukewarm output at 35°C). Either fit a flow-restrictor in the head, or upgrade the unit kW (with cable upgrade).

What's the minimum water pressure for an electric shower?

Most modern electric showers need at least 1 bar dynamic pressure (i.e. with the shower running). Below this, the flow switch may not activate. Check the static and dynamic pressure with a gauge at the shower inlet. If pressure is borderline, a pump-fed (positive head) shower or a thermostatic mixer fed from a system boiler may be a better fit.

Regulations & Standards