Why does the water temperature from my shower mixer sometimes fluctuate from freezing cold to scalding hot?

A1. The most common reason for this symptom is that you are using a manual mixer valve supplied from a system with other appliances demanding water while you are showering. In other words the pressure of the hot and/or cold supply is fluctuating. If for example somebody flushes a toilet elsewhere in the house the cold water pressure drops as the cistern fills and the result is usually a sudden increase in temperature at the shower head. Similarly if somebody turns on a hot water tap elsewhere your shower will go cold. Washing machines and dishwashers will have the same effect. The above description can apply to both gravity and mains pressure hot water systems (including Combi boiler systems). If you have recently had your central heating system or boiler changed, make sure your shower valve is compatible. Manual mixer valves for example do not give adequate control over water temperature when used in conjunction with a combination boiler or a main pressure hot water system.Remedy: ...  more