A shower’s temperature often changes because the balance of hot and cold water reaching it changes, for example when someone uses a tap or flushes a toilet elsewhere. It can also be caused by a worn or clogged valve or cartridge inside the shower, or by a problem with how hot water is supplied to the property.
Finding out when the changes happen can help narrow down the likely cause. A sudden burst of very hot water can also be a scald risk, so it is worth taking seriously, especially in homes with children or older people.
Other Taps and Appliances
If the shower runs hot or cold when another tap is turned on, a toilet is flushed or an appliance starts filling, the supply to the shower may be changing. This is more noticeable with some types of shower and some water systems than others.
If the changes only happen at busy times of day, when several people are using water, this may be the reason.
A thermostatic shower is designed to reduce these swings. If it no longer does, the thermostatic part of the valve may not be working as it should.
Worn or Clogged Valves
Inside a mixer shower, a cartridge or valve blends the hot and cold water. Over time, limescale, grit or wear can stop it moving freely, so it may not keep the temperature steady. You may notice the control becoming stiff or the temperature changing without the control being moved.
Some showers also have a maximum temperature stop that limits how hot the water can get. If this has been knocked or adjusted, the shower may not reach the temperature it used to.
This can often be dealt with by cleaning or replacing parts, depending on the shower. It is not always the cause, though, and the same symptoms can come from the supply.
Electric Showers
In an electric shower, the temperature depends on how fast water flows through the unit. If the flow changes, the temperature can change too. A unit that keeps cutting out, switching between hot and cold, or showing signs of overheating should not be used until it has been checked. Do not open the unit, as it contains electrical parts.
Hot Water Supply
If hot taps elsewhere in the home are also changing temperature, the problem may be with how hot water is produced or stored, rather than the shower itself. That may need a different kind of assessment.
Note whether only the shower is affected or other hot taps too, and when the changes happen. Our page on repairing a faulty shower explains how shower faults, including temperature problems, are assessed and repaired.
If your shower keeps changing temperature, a plumber can check the shower and the water supply to it and explain the options.