fixmyplumbing

This is very rare and should not happen but I have seen it once.  Thermostatic mixing valves mix hot and cold water together so there is no chance of scalding.  The same source of hot and cold water from a tank should be used when they are installed. But in some cases mains pressure cold water and tank fed hot water is used.  If the non return valve that keeps the two sets of water apart fails then the mains water can get through into the hot water pipework.  This starts to fill the tank up and makes it overflow.

Solutions

• Re-pipe with hot and cold supplies from the same tank fed source.
• Replace the valve.
• Disassemble the valve and clean the non return valves .
• Add new non return valves on the hot and cold inlet pipes.