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SHOWER & BATH PLUMBING

Shower Temperature Keeps Changing

Temperature swings can result from a scaled thermostatic cartridge, uneven hot and cold pressure, changing boiler output, restricted flow or other outlets being used.

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Stop using the shower or bath if water is reaching ceilings, walls or electrical fittings

Isolate local water supplies where possible. Electric showers must be isolated electrically before any internal work, and temperature faults that create scalding risk should be treated as urgent.

MOST LIKELY CAUSES

Start with these

Thermostatic cartridge fault The mixer may no longer balance temperature correctly.
Pressure imbalance One supply changes more than the other.
Boiler flow instability Combi boilers may cycle when flow is too low.
Other water use Taps and appliances can alter available pressure.
◫ Symptom variations Match the exact pressure, temperature or leak pattern. ⌄
  • Shower alternates hot and cold
  • Temperature changes when a tap opens
  • Water goes cold at low flow
  • Only one shower is affected
  • Problem worsens gradually
✓ Quick checks Simple checks before dismantling shower or bath components. ⌄
  1. Identify whether the problem affects pressure, temperature, drainage or leakage.
  2. Check nearby hot and cold outlets for similar symptoms.
  3. Clean accessible shower heads, filters and waste covers.
  4. Inspect hoses, seals, trays and accessible joints.
  5. Stop testing immediately if water appears below or near electrics.
If the cause is isolated: keep the affected fitting out of use until repaired.
If the leak is concealed or temperature is unsafe: arrange professional inspection.
1 Step 1 — Check other water use Use controlled tests and stop if water reaches ceilings or electrics. ⌄

Process

Test the shower with taps and appliances off.

Success looks like: Temperature stabilises.
Escalate when: If not, continue with shower and boiler checks.
2 Step 2 — Increase flow slightly Use controlled tests and stop if water reaches ceilings or electrics. ⌄

Process

See whether a combi boiler maintains heat better at a higher flow.

Success looks like: Boiler cycling stops.
Escalate when: If swings remain, the mixer cartridge may be faulty.
3 Step 3 — Compare hot and cold pressure Use controlled tests and stop if water reaches ceilings or electrics. ⌄

Process

Check nearby outlets for obvious imbalance.

Success looks like: A supply-side problem is identified.
Escalate when: Balanced supplies with unstable temperature point to the mixer.
⌁ Common causes explained Why shower and bath faults develop. ⌄

Temperature swings can result from a scaled thermostatic cartridge, uneven hot and cold pressure, changing boiler output, restricted flow or other outlets being used.

Shower and bath faults often depend on when water is flowing, draining or hitting a sealed surface. Controlled separate tests help distinguish supply, mixer, waste and enclosure faults.

⌘ Advanced diagnostics Plumber or appliance-engineer checks. ⌄
  • Descale or replace thermostatic cartridge.
  • Check inlet filters and check valves.
  • Measure dynamic hot and cold pressure.
  • Assess combi boiler minimum flow and modulation.
  • Balance or regulate supplies where appropriate.
▣ Parts that may need replacing Common components linked to this fault. ⌄
  • Thermostatic cartridge
  • Inlet filter
  • Check valve
  • Mixer body component
  • Pressure-balancing component
! When to call a plumber or engineer Conditions that should end basic troubleshooting. ⌄
  • Water is leaking through a ceiling or near electrical equipment.
  • The shower temperature is dangerously hot or unstable.
  • An electric shower has an internal heating or power fault.
  • The tray, bath or concealed pipework must be removed for access.
  • A thermostatic cartridge cannot be calibrated safely.
  • The fault returns after basic cleaning or seal replacement.
? Frequently asked questions Common questions about shower and bath faults. ⌄

Why does a shower drip after it is turned off?

Large heads and hoses can hold water temporarily. Continuous fresh dripping points to a leaking control valve or cartridge.

Can failed sealant cause a ceiling leak?

Yes, but waste joints, pipe connections and tray movement must also be ruled out.

Why does shower temperature change when another tap is used?

The hot and cold pressures may be unbalanced or the thermostatic cartridge may not compensate correctly.