Summary

"My shower is fine for the first minute then goes cold" is one of the most common domestic complaints a plumber will hear. It's rarely a straightforward "no hot water" fault because there IS hot water — just not enough of it, or not reliably. The customer's description is unreliable; you need to test.

The fault is different depending on the hot water source (combi vs cylinder vs electric shower), and different again depending on the shower type (mixer vs thermostatic mixer vs digital). Get the system type wrong in your head and you'll be chasing the wrong parts. Start every visit by identifying the boiler/cylinder type on the wall and then the shower type in the bathroom before touching anything.

The commercial risk is quoting a part before diagnosing. A £180 diverter valve won't fix a scaled plate heat exchanger. A £250 mixer cartridge won't fix an undersized combi. Both jobs need a flow rate + temperature test at the appliance, and both take 30 minutes with a bucket and thermometer.

Key Facts

Quick Reference Table

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System Symptom Pattern Most Likely Cause Test
Combi Cold within 60–90 sec, boiler still firing Plate HX scaling / DHW blockage Flow rate at kitchen tap vs shower
Combi Cold after 30–60 sec, boiler modulating Undersized combi vs shower demand Compare kW output to shower flow × 35°C
Combi Cold intermittently, radiators warm during shower Diverter valve sticking Feel rad temperature during DHW draw
Unvented cylinder Cold after 8–15 min Cylinder emptying Time from full-hot to cold
Vented cylinder Cold after 5–10 min Cylinder recovery vs draw rate As above
Any Cold when toilet flushed Cold mains pressure drop Pressure gauge on incoming
Any Cold when kitchen tap on Pressure imbalance Two-outlet test
TMV shower Cold when hot demand elsewhere TMV losing hot pressure Isolate outlet, retest
Mixer Warm-hot drift over 2 min Cartridge cross-flow Replace cartridge

Detailed Guidance

Diagnostic decision tree

SHOWER RUNS COLD DIAGNOSIS
==========================
Q1: Is the boiler a combi or does the property have a cylinder?
├── COMBI
│   Q2: Does hot go cold within 60–90 seconds?
│   ├── YES → PHE scaling OR undersized boiler
│   │   → Measure kitchen tap flow at hot (l/min × temp rise)
│   │   → If output <30 kW effective, boiler undersized for shower
│   │   → If output OK at tap but poor at shower, check PHE / shower cartridge
│   └── NO (goes cold slowly) → Diverter valve OR flow modulation
│       → Feel radiators during shower (should be cold)
│       → If radiators warm, diverter valve sticking
│
├── UNVENTED CYLINDER
│   Q2: How long until cold?
│   ├── 5–15 min → Cylinder normal draw-down
│   │   → Cylinder size vs demand mismatch, OR
│   │   → Coil undersized, poor recovery
│   ├── <5 min → Immersion element failed OR coil blocked
│   │   → Test immersion 500V megger + resistance
│   │   → Isolate boiler, run immersion only
│   └── Depends on other users → System draw-down, normal
│
└── VENTED CYLINDER
    Q2: Cold-water tank in loft (gravity feed)?
    ├── YES → Head pressure low, TMV struggling
    └── NO / pump-boosted → Pump cavitation or worn impeller

Combi-specific diagnosis

Every combi has a rated DHW output (usually stamped on the front panel or in the manual). Common UK sizes: 24 kW, 30 kW, 35 kW, 42 kW. The rule of thumb:

If the shower is a rainfall-style 15 l/min head being fed by a 24 kW combi, the combi is the problem. There is no "fix" to the boiler; it will always modulate down under that demand. The customer's options are: change the shower head to a lower flow-restricted model, or replace with a larger combi (a 30 kW is often like-for-like on flue and connections; check the model).

Testing DHW output — put a bucket under the kitchen mixer tap (usually shortest run from combi). Run hot fully, time 10 seconds, measure volume. Multiply by 6 for l/min. If flow at kitchen tap is less than half the boiler's spec, the DHW side is scaled or restricted before it reaches the shower.

Plate heat exchanger scaling — the most common combi fault in hard-water areas after 5–7 years. Symptoms:

Fix: PHE flush with descaler (e.g. Fernox DS-40 or Sentinel X800) via a purpose-made pump kit, or PHE replacement. Flush £120–£220 labour + chemicals; PHE swap £200–£450 depending on boiler make (Vaillant ecoTEC ~1 hour, Ideal Vogue ~1.5 hours, Worcester Bosch Greenstar ~2 hours).

Cylinder-specific diagnosis

For unvented cylinders, calculate hot-water bath equivalents:

Cylinder capacity ÷ (shower flow × mix ratio) = shower duration.

Example: 150L cylinder, 10 l/min shower, 65°C stored blended to 40°C at outlet. Mix ratio ~60% hot : 40% cold. Hot draw per minute = 6 l/min. Cylinder duration at that draw = 150 ÷ 6 = 25 minutes with no recovery.

But recovery: 24 kW indirect coil recovers roughly 400 l/hour from 15°C to 65°C. So during a shower, cylinder is being emptied at 6 l/min and refilled at 6.7 l/min. Should be sustainable indefinitely IF the boiler is firing to demand.

Common failures:

Shower valve / cartridge diagnosis

Once the hot supply is confirmed adequate (kitchen tap runs hot indefinitely at 8+ l/min while the shower is running cold), the fault is at the shower valve.

Bar mixer valve (exposed thermostatic) — remove the cover, isolate outlets, feed with mains, check TMV cartridge for scale. Descale in citric acid, refit. If still failing, replace cartridge (typically £45–£85 OEM parts for Aqualisa / Mira / Bristan / Grohe).

Concealed thermostatic mixer — often more expensive to fix. Cartridge only accessible after removing the tile-side face plate. If the customer is set on a repair, price the cartridge + a tile-replacement contingency. If tiles are old/rare, consider whether replacement is more economical.

Electric shower — different beast. See shower not heating. Cold-after-hot on an electric shower typically means the element is failing or scale is fouling the thermal cut-out.

Digital shower (Aqualisa Quartz, Mira Platinum, etc.) — most have self-diagnostic error codes. Check the unit's remote or app. If the pump/mixer is in the loft, listen for it cycling on-off.

The two-outlet test (essential)

Turn on both the shower and the kitchen hot tap simultaneously.

This test isolates the fault to before or after the shower valve in about 60 seconds and tells you where to look next.

Pressure balance and TMVs

A thermostatic mixer valve is engineered to hold outlet temperature stable even if inlet pressures shift, but only within its rated range (typically ±20% inlet pressure). Two common real-world triggers:

Fix isn't the TMV — it's ensuring adequate cold main flow. On mains-fed systems, an accumulator (mains water boosting) buffers demand. On low-pressure systems, a whole-house pump.

Combi diverter valve

The diverter is a plastic-and-brass assembly inside a combi that mechanically switches primary flow between the central heating circuit and the DHW plate exchanger. When you turn on a hot tap or shower:

If the diverter sticks partway — very common on Worcester Bosch Greenstar and Ideal Logic models after 6–8 years — some primary flow leaks into the CH circuit during DHW mode. You'll feel radiators warm up during a shower. DHW temperature drops because heat is being lost to the CH loop.

Test: run the shower, feel the flow-and-return pipes at the boiler AND a radiator. If the radiator warms up during shower use, diverter is sticking. Repair is either full diverter valve assembly (£90–£180 parts, 1–1.5 hours labour) or the diverter cartridge/paddle only if the make allows (Worcester does; Ideal often full assembly).

When it's not the plumbing

Frequently Asked Questions

Can I just fit a bigger shower head restrictor?

Reducing shower flow with a restrictor works when the fault is undersized combi vs high-flow head. It doesn't work if the fault is scale or diverter. Diagnose first; a restrictor is a fix, not a diagnosis. Aim for a shower head at or below the boiler's DHW spec at your desired temperature.

The boiler is 8 years old — should I just replace it?

Not necessarily. A well-descaled combi with a fresh PHE and diverter can run another 10 years. Cost of PHE + diverter fitted ~£400–£700. New A-rated 30 kW combi installed £2,000–£3,000. Do the maths honestly for the customer.

Do I need to isolate the whole supply to test the shower cartridge?

No — the shower should have local isolation valves under the bath, behind the cistern, or in an airing cupboard. Find them first. If none exist, offer to fit them at the same visit (£30–£70 add).

The customer says "it worked fine last week" — does that rule out scaling?

No. PHE scaling can develop from mildly-symptomatic to fully-fault in a matter of days once the scale layer reaches a critical thickness. "Fine last week" is not diagnostic.

Why do I need to check the cold pressure?

Because TMV shower valves modulate against hot AND cold pressures. If the cold main is dropping to 0.5 bar when the shower is running, the TMV cannot deliver a balanced blend. Fit a pressure gauge on the cold main tee and watch it during shower operation. If pressure crashes, the fault is upstream cold supply, not the shower.

Regulations & Standards