If the overflow leaks when the heating is on it is a sign the water level in the central heating feed tank is set too high when cold.
The water in the central heating system expands when it heats up. There is normally space left in the tank for this expansion. This can be caused in several ways, the most common are:
Split ball from ball float.
The water level in the tank is set by an air filled plastic ball float. The ball float is pushed up by the rising water level. When it reaches the correct level it turns off the fill valve.
When the ball is full of water it doesn’t float on top so the water level rises too high. The ball is attached to the long arm of the float valve.
It will either be a (newer) bent arm or a straight (older) arm. The thread is the same on both so the ball will just unscrew and can easily be replaced.
Remember to turn off the water or tie up the arm so the valve doesn’t open without a ball float.
Water level set too high
The water level in the tank should be set with quite a large gap between the water level when cold and the level of the overflow.
The water level is set by adjusting where the ball is attached to the arm of the valve.
The valve will have some adjustment on it in the form of a sliding threaded nut that connects the ball to the arm of the valve.
In some cases the water level is still too high at maximum adjustment. For an extra low water level the arm of the ball valve can be bent. Its a lot easier to do while the valve isn’t in the tank.
Leaking ball valve
If the ball valve in the tank is leaking the water level in the tank will rise. At some point the overflow will run continuously. Before that point (especially in winter when the heating is in use) it will overflow when the heating is on.