Overflow pipe dripping when central heating on.

Why is my overflow dripping when the central heating is on?

If the plastic overflow pipe is dripping only when the central heating is on you need to find the small cistern and check the water level. This is a sign the water level in the central heating feed cistern is too high when it is cold.

An overflow dripping when the heating is on is a sign your expansion tank is too full.

What does the small cistern do?

The small cistern is called the feed an expansion tank. It has two jobs:

1 Feed water into the radiators, pipework and boiler.

2 When the water expands due to heating up the extra volume goes into this tank.

The water level should be set quite low in this tank to allow the expansion. Some recommend the water in the tank be about 100mm deep when cold.

When the central heating is on the water contained in the radiators, pipework and boiler heats up and expands. When hot the volume of water can increase by 4% causing the water level in the tank to rise.

If the water level is already too high this increase in volume will make the tank overflow.

Why would the water level be too high?

This can be caused by the float of the ball valve developing a leak. When its full of water it doesn’t float so well so the water level rises too high.

Luckily the ball will just unscrew and can easily be replaced. The thread on the ball is universal so the replacement should just screw back on.

Check the ball valve isn’t dripping when it is floating on the surface of the water. If it is dripping it should be replaced.

In much rarer circumstances the coil in you hot water cylinder may be split. This is really unlikely but is possible.

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 level. Its a lot easier to do while the valve isn’t in the tank.