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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Frequently asked questions
Why does my mixer shower go cold when heating comes on?
When the central heating fires, it demands hot water from the boiler. On a combi boiler, this temporarily reduces the hot water flow to the shower — the mixer sees a drop in hot supply and compensates by drawing more cold, making the shower feel cold. A thermostatic shower valve prevents this by maintaining a set temperature regardless of supply changes.
Does fitting a thermostatic shower valve stop temperature fluctuations?
Yes — a thermostatic valve responds to supply temperature and pressure changes by automatically adjusting the hot/cold mix to maintain your set temperature. It also includes a maximum temperature stop to prevent scalding. This is the recommended upgrade for a manual mixer shower that has persistent temperature stability problems.
How much does a thermostatic shower valve installation cost?
A thermostatic bar valve costs £80–£400 depending on quality and brand. Installation by a plumber typically adds £150–£300, including removing the old valve, fitting the new one, and adjusting pipework if needed. The total cost of £250–£700 is usually considered good value given the daily comfort improvement.