A shower that keeps going hot and cold is often reacting to changes in water pressure or hot water supply. It can also be caused by a worn or scaled cartridge in the mixer valve. The valve is not always the problem, so it helps to look at the whole system.
Thermostatic Cartridge Wear or Limescale
A thermostatic mixer shower has a cartridge that is meant to keep the temperature steady. It reacts to changes in the hot and cold water and adjusts the mix. When it works well, you should not feel much change if someone runs a tap elsewhere.
Over time, limescale can build up on the cartridge, or its parts can wear. It may then react slowly or not at all. This can lead to sudden bursts of hot or cold water, or a shower that slowly drifts in temperature.
Manual Mixers and Pressure Changes
Not every mixer shower is thermostatic. A manual mixer simply blends hot and cold water at a set ratio. It does not adjust itself when the pressure changes, so it relies on both supplies staying steady.
So if someone flushes a toilet or turns on a tap, the cold supply may drop for a moment. The shower can suddenly feel hotter. When the hot supply drops, it can turn cold. With a manual mixer, this can be normal behaviour rather than a fault.
Hot Water Supply and Pressure Balance
With a combi boiler, hot water is heated as it flows. If the flow through the shower is too low, or other hot taps are used at the same time, the boiler may struggle to keep the temperature steady. Any fault with the boiler itself needs a Gas Safe registered engineer, so do not try to adjust or open it.
On gravity-fed systems, the hot water often comes from a tank in the loft, while the cold may come straight from the mains. This means the two supplies can have very different pressures. Some mixer valves do not cope well with this, and the temperature may swing. Partly closed valves, blocked filters or a scaled shower head can also limit flow and make things worse. Our page on water pressure problems explains more about pressure issues.
When a Plumber Should Investigate
If the temperature changes suddenly, there is a risk of scalding, especially for children and older people. If this happens often, it is worth getting it checked rather than putting up with it.
A plumber can check the mixer valve, filters, cartridge and supply pressures to see what is causing the swings. We can help with hot and cold shower problems on mixer showers. Electric showers are a different case, and any electrical fault needs a qualified electrician.
If your shower keeps changing temperature and you live in Swindon, Emergency Plumber Swindon can help you find the cause. Call 01793 569521 to talk it through.