Cold water tank in loft overflowing

The overflow pipe from a cold water storage tank is there for safety — if water constantly drips from the overflow pipe outside (usually at soffit or eaves level), something is wrong with the water level control inside the tank.

Access the loft and inspect the cold water tank (a square or round polyethylene tank, typically 100–227 litres). Look at the ball valve — a float on an arm that rises with the water level and shuts off the inlet when the tank is full.

The most common cause is a waterlogged or sunken ball float. Lift the float arm manually — if this stops the flow of water, the float itself is the problem. Old copper or plastic floats can fill with water or crack. Replace the float (£3–£8) by unscrewing it from the threaded rod.

If lifting the arm does not stop the flow, the ball valve seat is worn or the rubber washer has failed. You can replace just the washer (unscrew the back nut, remove the piston, and replace the rubber washer — £1–£2) or replace the whole ball valve (Portsmouth or equilibrium type, £8–£20).

Also check whether the arm can be adjusted — bending the arm slightly downward (on older Portsmouth valves) reduces the level at which the float cuts off, giving more freeboard before overflow.

While in the loft, confirm the tank has a lid (to prevent contamination), is insulated on sides and top (not underneath — you want heat from below to prevent freezing), and that the overflow pipe slopes continuously downward to outside.