A shower often changes temperature because the balance between the hot and cold supplies shifts, for example when someone flushes a toilet or runs a tap elsewhere in the home. In a mixer shower, a worn or scaled-up thermostatic cartridge can also struggle to keep the temperature steady, though the cause depends on the type of shower and how it is supplied.
Not every temperature problem is a plumbing fault. Working out which kind of shower you have is a useful first step.
Mixer Showers and Supply Balance
A mixer shower blends hot and cold water from your home’s supplies. If another tap or toilet draws water from one side, the mix can suddenly change, giving a burst of hot or cold.
Thermostatic mixers are designed to correct for this, but they rely on a cartridge that senses the temperature. Over time, limescale and debris can make it slow to react or stick at one setting. Small filters where the pipes join the valve can also clog and starve one side.
Non-thermostatic mixers do not correct themselves, so they are more likely to swing in temperature when water is used elsewhere.
Problems With the Hot Water Supply
Sometimes the shower is working as it should, but the hot water reaching it is changing. A hot water cylinder that has run low, or a hot water system that is not keeping a steady output, can cause the temperature to drop or surge.
If the hot taps elsewhere in the home also run hot and cold, the problem may not be the shower. Faults with the heating or hot water appliance need an appropriately qualified heating engineer, which is outside plumbing work on the shower itself.
Electric Showers
Electric showers heat cold mains water inside the unit. If an electric shower keeps changing temperature, the cause may be a low or changing water flow, or a fault inside the unit.
Do not open an electric shower or attempt any repairs on its electrical parts. Switch it off at its isolating switch if you are worried, and contact an appropriately qualified electrician or the shower maker for advice on the unit itself.
Safe Steps and When to Get Help
Test the water with your hand before stepping in, and take extra care with children and older people, as sudden hot surges can scald.
Note when the change happens. Does it happen when another tap is used, after the shower has run for a while, or at random? Is the problem in one shower or every hot tap? These clues help narrow down the cause.
If you have a mixer shower that will not hold its temperature, our shower repair service in Oxford can check the valve, cartridge and filters, and explain whether the issue sits with the shower or the wider supply.