fixmyplumbing

The water level in a tank is too high so it is running out the overflow. Go into the loft and take the lid off each tank. There are normally two but can be virtually any number depending on the size and type of system you have. Take the lid off each tank and check how high the water level is in each one.

If the ball valve is passing there will be a constant drip on the valve.
If the ball float on the end of the valve has split and filled with water the float won’t float so the valve will not shut off.
If the coil in the hot water cylinder has burst the water of the heating system and hot water cylinder will try to find their own level. This will make the lowest of the tanks overflow.
If there are thermostatic mixing valves on your hot water system where the cold comes from the mains the non return valve could have stuck open.