Mixer shower going hot and cold — temperature not stable
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Most likely cause & what to check
A non-thermostatic (manual) mixer shower blends hot and cold water at the valve. If someone flushes the toilet or runs a tap elsewhere, the cold supply drops momentarily — leaving you with a blast of hot water. This is the most common cause of temperature fluctuation.
The permanent fix is to upgrade to a thermostatic shower valve. A thermostatic mixer valve detects and compensates for pressure changes automatically, maintaining your set temperature regardless of what else is running. Budget £80–£200 for the valve; fitting is 2–4 hours of plumber time.
As a cheaper interim measure, fit a pressure-reducing valve (PRV) on the cold main to the bathroom — this creates a more stable cold supply pressure. However, this only helps if the problem is cold pressure drop; if the hot pressure is the variable one, it will not help.
Check whether your hot water supply is adequate — if the boiler or cylinder cannot keep up with demand (e.g., someone is running a bath at the same time), the hot side will drop and the shower will run cold. Use the shower at a time when no other hot water is being drawn.
If you have a combi boiler, very long pipe runs between the boiler and the shower mean you get stored cold water from the pipe before the hot arrives — this is not a pressure issue, it is a pipe-length issue. Insulating the hot pipes reduces recovery time between uses.
Ask a plumber to assess whether a thermostatic valve or a pumped shower system would be the right upgrade for your situation.
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