fixmyplumbing

Its quite rare but the coil that holds the water from the boiler inside your hot water tank can split.  If this is the case then the lowest of the tanks will be overflowing.

To prove:

1. Turn water supply off either at service valves near the tanks or turn stopcock off to whole house.
2. Open hot taps until water level in the highest tank is the same or lower than overflowing tank.

When the water level in the higher tank is the same as or lower than the overflowing tank the overflowing should stop.

As the split in the coil may be quite small it may take a while for the water level in the overflowing tank to go down.

The only solution to this problem is to replace the cylinder.