Mixer shower running too hot — cannot get cold water
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Scalding hot water from showers is a safety hazard, especially for children and elderly users. Do not use the shower until the temperature is controlled.
A mixer shower that runs scalding hot, even when turned to its coldest setting, is a frustrating and potentially dangerous fault that catches many homeowners off guard. The problem usually stems from either a broken thermostatic cartridge inside the valve—which acts like the shower's temperature control brain—or a simple issue like the cold water supply being blocked or isolated. This guide walks you through practical checks and repairs that most confident DIYers can handle, starting with the simplest checks before moving to cartridge replacement. Thermostatic cartridges are relatively affordable spare parts and often the permanent fix for this issue. If you're uncomfortable working with your shower valve, or if these steps don't resolve the problem, it's worth calling a Gas Safe registered engineer or qualified plumber to investigate further and ensure the fault isn't hiding a bigger supply issue.
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Most likely cause & what to check
Check both supplies — confirm cold water is reaching the shower valve by briefly opening the cold isolation valve on its own. If no cold water is present (e.g. the cold feed tank is empty or the isolator is closed), the shower will run on hot only.
On a thermostatic mixer shower, the thermostatic cartridge controls temperature. A failed cartridge can cause the valve to default to hot. The cartridge can be replaced without replumbing — costs £20–£100 depending on the brand.
Check the anti-scald stop on the temperature handle — most thermostatic showers have a factory-set maximum temperature limiter. This is usually a small tab or screw under the temperature handle that limits rotation. Ensure it has not been removed or damaged.
Some shower thermostatic cartridges have a maximum temperature adjustment screw on the cartridge body — consult the shower manufacturer's installation guide.
If the cartridge is more than 8 years old, particularly in a hard water area, replacement is the most reliable fix. Brands such as Mira, Triton, Bristan, and Grohe all have spare cartridges available.
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Frequently asked questions
Can a mixer shower really run only on hot water?
Yes—if the cold water supply is isolated, blocked, or the cold feed tank is empty, the shower will deliver only hot water regardless of handle position. It's also the most common cause of this fault, so it's always worth checking the isolation valves under the sink or in the airing cupboard first. A failed thermostatic cartridge can also default to hot-only mode, so don't assume it's a supply issue without testing both feeds.
Is it safe to use a scalding shower while I sort the problem?
No—scalding water causes serious burns in seconds, particularly for children, elderly relatives, and people with reduced sensation. Turn off the shower at the supply if you can, or isolate it completely until the temperature is properly controlled. This is genuinely one of the few plumbing faults where safety must come before convenience.
How do I know if the thermostatic cartridge has failed?
If cold water is reaching the valve (confirmed by opening the cold isolation on its own) but the shower still runs hot, the cartridge is almost certainly the culprit. Cartridges also tend to fail after 8–10 years, especially in hard water areas where limescale builds up inside—replacement is usually cheaper and faster than trying to clean or repair it.
Do I need to turn the water off to replace a shower cartridge?
Yes, you'll need to isolate both hot and cold supplies at the isolation valves serving that shower. Some showers have individual isolators under the control panel; others rely on the main hot and cold isolators in your airing cupboard or under the sink. Always drain the system slightly by opening a tap lower down to release pressure before you start work.