Thermostatic mixer showers
A thermostatic mixer uses a cartridge that constantly adjusts the blend of hot and cold to keep the temperature steady. When the cartridge wears or becomes clogged with scale and debris, it stops reacting quickly enough, and the temperature swings. Filters at the inlets can also block, starving the valve of one supply.
Cleaning or replacing the cartridge and filters usually fixes it. In hard water areas, this is a routine bit of maintenance every few years.
Pressure imbalance
Mixers work best when hot and cold arrive at similar pressures. If the hot water comes from a low-pressure loft tank and the cold from the mains, the mix is easily upset – especially when someone runs a tap or flushes a toilet. A plumber can rebalance the supplies, fit a valve designed for unequal pressures, or change how the shower is fed.
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Combi boilers and electric showers
With a combi boiler, the shower may run cold if the flow is too low for the boiler to keep firing, or if another tap draws hot water at the same time. Cleaning the shower head and checking the boiler's minimum flow helps.
Electric showers heat water as it passes through. If the flow drops – a scaled head, a kinked hose or low mains pressure – the unit can overheat and cut out, then come back on, giving bursts of hot and cold. Scale inside the unit causes similar problems.
When to call a plumber
Call a plumber if cleaning the head doesn't help, if the shower scalds without warning, or if the problem started after other plumbing work. For electric showers that trip, smell of burning or show scorch marks, switch off at the isolator and get a qualified person to check before using it again. Boiler faults need a Gas Safe registered engineer.
