Header tank overflowing

[samual]

Water keeps coming out of our overflow for the header tank and making a nice water feature in the garden .
If we turn the hot water tap on for a while then it stops.
The water in the tank is sometimes warm, I gather it's meant to be cold. Hot water seems to be entering the tank and causing it to overflow.
What would cause this? It does it all year round even when we switch the hot water off.
Is it an easy, cheap problem to solve.
Any ideas?
Thanks
Sam

 

[Bahco]

It does it all year round

Hope you are not on a water meter.

Your Float valve needs replacing or resetting so that it cuts off water to the tank 1 inch below the overflow pipe. Bend the arm down gently.
See if this fixes it. If not report back as the hot water entering the tank could mean a cylinder replacement