Central heating tank drips from its overflow

In many houses there are two cold-water tanks in the loft. As well as the main storage tank, there is a much smaller one which is the feed-and-expansion tank of the central heating system.

Dripping from its overflow does not necessarily mean a fault in the ball valve - simply that the tank was overfilled. When the system is cold there should be only a small amount of water in the feed-and-expansion tank.

As the central heating heats up the water expands, flowing back into the tank. If the tank was too full when the system v/as cold, the water will drip out through the overflow pipe. If the overflow drips, try taking about four pints of water out of the tank.

REMOVING HIE OLD VALVE Unscrew the connector nut, which joins the water- supply pipe to the inlet of the valve. Pull the connector away from the valve inlet.

Unscrew the large back nut outside the wall of the tank that holds the ball valve in place. This could be the most difficult part of the job. Apply penetrating oil if necessary.

Hold the valve with one hand as you use the spanner to turn the nut. You might find it easier to hold the valve - or the nut inside the cistern - with another spanner. Take out the old ball valve.

INSTALLING THE NEW VALVE You will notice that the new- ball valve has two large plastic nuts screwed onto its tail. Unscrew and remove one of them.

High-pressure and low-pressure valves

Ball valves are classified as either 'high-pressure' or 'low-pressure' according to the diameter of their nozzle openings.

HIGH-PRESSURE VALVES They have a smaller opening and are normally used in cold-water tanks where they are under pressure from the main water supply.

LOW-PRESSURE VALVES They have a larger opening and are used for lavatory cisterns which are fed from the cold-water tank under lower pressure.

If a high-pressure valve is fitted where a low- pressure one is needed, the cistern will fill extremely' slowly. If a low-pressure one is fitted where a high- pressure is needed, it will probably leak.

Some modern ball valves are fitted with screw-in high-pressure nozzles, but they are also supplied with a low-pressure nozzle that can be substituted, should it appear necessary.

Plumbers in London

See video clips with our works here