Shower goes cold: combi boiler flow and mixing checks
Quick summary
If your shower goes cold then hot, start by checking whether it happens only when other taps run (pressure/flow changes) and whether your boiler is maintaining hot water output.
Safety first
- If you suspect an electrical fault with an electric shower, isolate power and use a qualified electrician/installer.
- If you smell gas or suspect CO: stop and call 0800 111 999 / seek urgent help.
- Do not open the boiler casing.
Full checklist: Safety guidance
What to check (in order)
- Check whether the issue happens only during the shower or also at hot taps.
- Test a hot tap: does the temperature stay steady for 1–2 minutes?
- Run the shower and then turn on a cold tap elsewhere: does the shower temperature change sharply?
- If you have a thermostatic mixer shower, note whether cold pressure changes affect it (some mixers fail/scale).
- If combi: note boiler display/temperature during shower; does it cycle or show a fault?
What the result means
- Only changes when other water is used: pressure/flow imbalance; could be mains pressure or plumbing layout.
- Hot taps also go cold: boiler or hot water generation issue (capacity, sensor, plate HX scaling).
- Thermostatic mixer struggles: cartridge/mixer issue or scale/debris.
What you can safely do
- Try another hot tap to see whether the issue is shower-only or whole-house.
- Run only the shower for a minute before adding cold water or using other outlets.
- Check whether the pressure changes when another tap is opened.
- Note whether the problem is immediate, intermittent, or after a few minutes.
When to call a professional
- You can’t get stable hot water anywhere.
- Boiler shows fault codes or cycles excessively.
- You suspect scald risk or electrical issues.
Engineer notes
Differentiate supply-side vs appliance-side. Measure DHW flow rate and temperature stability; check mains cold fluctuations. For combis, consider DHW NTC, flow sensor/switch, diverter behaviour (model dependent), plate HX scaling and inlet filter debris. For mixer showers, inspect cartridge/filters and check pressure balance.
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