If that is over flowing and the ballcock is not leaking the the problem is what plumbers call pitching ie water from your heating system is pitching into the small tank and causing it to overflow.
Water tank in attic overflowing.
As the water level in the cistern rises the float pushes a washer onto the valve seating a plastic nozzle inside the valve body.
So when the water heats up and expands the tank overflows.
If its the big tank that feeds your hot water cylinder it might be that the water level is too high in tank.
If you have an overflow running from your roof tank or wc cistern sometimes just adjusting the water level can cure it.
Freezing being in a loft space that s often unheated can mean that the water in the header tank is susceptible to freezing during the winter.
They re a vital part of your home s plumbing system.
But we can dig deeper on this later on.
If the water in the tank or cistern fills up to a higher level than it should the excess water flows down the overflow pipe and runs outside rather than leaking into the room.
If the tank overfills excess water flows safely via the overflow to the outdoors.
But still you cannot come into a solid conclusion as yet since it is just an indication of a malfunction in your overflow valve.
An overflow pipe leads from a water storage tank or cistern inside your home to the outside.
If hot water is coming out of the overflow pipe turn the system off wait for the water to cool right down and set the float valve to allow less water into the tank.
Water from the mains is fed into the larger of the tanks which is the cold water storage tank.
The small water tank in you attic is for your central heating it should have a long 3 4 pipe in the shape of an inverted u above it.
If you find the hot water tank overflow pipe dripping at some point we will have to come to an assumption that there is some sort of a defect in your overflow valve.
Water is trickling out from the pipe and down onto the ground.
If you are not sure why there are two water tanks in your loft it is probably because the property has or used to have a vented central heating system.
That mysterious pipe is the overflow from the cold water header tank also called a cistern in your attic.
The most common reason for a cold water tank to overflow is a faulty ball valve.