Shower going hot and cold: why it happens

Combi boiler or stored hot water?

If your hot water comes straight from a combi boiler — no cylinder in an airing cupboard — the boiler is heating the water as the shower runs, and anything that interrupts that process shows up as a temperature swing. If you have a cylinder, the shower is mixing stored hot water with cold, and the cause is more often the shower valve itself.

You can usually tell in seconds: no cylinder means a combi. If you are not sure, look for a white wall-mounted boiler with no tank anywhere else in the house.

Combi boiler going hot and cold

On a combi, the classic pattern is the water going cold for a few seconds, then hot again, in a cycle. That usually means the boiler is firing, stopping, and refiring. Common reasons include a blocked plate heat exchanger — the part that transfers heat to your tap water — a failing flow sensor, or the boiler overheating and cutting out to protect itself.

Another pattern is the shower going cold when someone else runs a tap or the washing machine fills. A combi can only heat so much water at once, and if the incoming mains flow is limited, sharing it makes the shower temperature wander. Our guide to diverter valve problems covers a related fault where hot water is diverted the wrong way.

Mixer shower and cylinder systems

With stored hot water, the usual suspect is the shower's thermostatic mixing valve. These valves blend hot and cold to a set temperature and react to pressure changes. When the valve's internal filters clog with debris or the thermostat cartridge wears out, the temperature drifts or swings.

A tell-tale sign is the shower being fine for the first few minutes, then drifting as the stored water cools or as other outlets are used. Cleaning or replacing the shower cartridge often cures it. If the hot water itself runs out quickly, the problem may be the cylinder or its immersion heater rather than the shower.

Safe checks you can try

• Note the pattern: cold bursts in a cycle point to the boiler; gradual drift points to the shower valve or stored water running down
• Check whether other taps affect the shower — that points to flow and pressure, not a faulty part
• Check the boiler pressure gauge if you have a combi; low pressure can cause erratic behaviour
• Clean the shower head — a scaled head restricts flow and can trigger cut-outs on some combis
• Do not open the boiler casing; anything inside it is Gas Safe registered engineer territory

When to call an engineer

If the boiler is cycling on and off, showing a fault code, or the problem is getting worse, it needs proper diagnosis. A blocked heat exchanger or a failing sensor will not fix itself, and running the boiler through constant cut-outs can shorten its life. Our boiler repairs service covers exactly this kind of fault-finding.

Need it sorted? Book a repair or a new shower

If the boiler is at fault, book a boiler repair. If the shower valve itself is worn out, our shower installation service can replace it with a modern thermostatic unit. Ring 07429 632804 or use the quote form, and describe the pattern — hot-then-cold in a cycle, or gradual drift — as it tells Chris a lot before he arrives.