Cold water storage tank overflowing
A cold water storage tank overflow is one of the most common issues we see in British homes, particularly in older properties with conventional plumbing systems. Water trickling from that overflow pipe in your loft is your tank's way of telling you something's not right — usually it's a worn ball valve that's lost its ability to shut off properly once the tank reaches capacity. This problem is frustrating because it wastes water and can lead to damp in your loft if left unchecked, but the good news is that in most cases it's a straightforward fix that doesn't require specialist tools or extensive plumbing knowledge. This guide walks through the most common causes and how to resolve them yourself. If you're uncomfortable working at height or the issue persists after these steps, it's worth calling in a qualified plumber rather than risk further complications.
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Most likely cause & what to check
Access the loft tank and inspect the ball valve — the float should close the valve when the tank is full.
If the float is waterlogged or cracked, unscrew and replace it.
If the valve won't shut off, the washer inside has perished — turn off the mains, drain slightly, and replace the washer (£2).
After repair, check the water level sits 25mm below the overflow pipe.
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Frequently asked questions
Why is water constantly dripping from my loft overflow pipe?
The ball valve that controls your cold water tank isn't closing properly when the tank fills up, so excess water diverts through the overflow. This is usually caused by a waterlogged float, a cracked float, or a worn washer inside the valve itself. It's not an emergency, but it should be fixed soon to prevent water waste and potential loft damage.
Is a leaking cold water tank overflow dangerous?
No, it's not a safety hazard — the overflow pipe is designed to do exactly this as a safety measure. However, persistent overflow can cause damp and mould in your loft, damage to insulation, and waste significant amounts of water over time. If you notice water pooling in the loft rather than flowing through the pipe, stop using water and call a plumber immediately.
Can I fix a faulty ball valve myself, or do I need a plumber?
Most homeowners can replace a ball valve float or washer with basic tools and a bit of confidence — it typically takes 20-30 minutes. If you're handy with a spanner and comfortable in a loft, it's a genuine DIY job. However, if you're unsure about turning off the mains water or draining the tank, it's better to call a professional than risk creating a bigger problem.
How do I know if my ball valve float needs replacing?
Lift the float arm gently — if it feels heavy or you hear water sloshing inside the float itself, it's waterlogged and needs replacing. You can also check visually for cracks or corrosion on the float surface. A new float typically costs £8-15 from any plumber's merchant and takes just a few minutes to fit.
What's the correct water level in a cold water storage tank?
The water should sit approximately 25mm (about an inch) below the overflow pipe outlet. This small gap allows for expansion and prevents overflow under normal conditions whilst still giving you adequate water storage. If water is sitting higher than this, your ball valve definitely needs attention.
Will a faulty cold water tank overflow affect my hot water supply?
Not directly, as your hot water cylinder draws from the cold tank, but a failing ball valve can eventually cause the tank to empty or stay partially full, which reduces the pressure available for your hot water system. Fixing the overflow quickly prevents this secondary issue from developing.