Sidcup Toilet Keeps Running? Very Hard Water Is Why

A toilet that keeps running in Sidcup is nearly always one of two parts failing to close: the fill valve, which lets water into the cistern, or the flush valve seal, which holds it there. Sidcup’s water is very hard, about 331 mg/l in Thames Water’s Sidcup zone, and scale builds up on both. Lift the cistern lid: if the water is above the overflow, the fill valve is at fault; if the level looks right but water trickles into the pan, the flush seal is. Turn off the small isolation valve on the cistern feed to stop it running until the part is cleaned or replaced.

How hard is Sidcup’s water?

Sidcup, Blackfen, Foots Cray and Chislehurst are all in Thames Water’s Sidcup supply zone, which averaged about 331 mg/l of calcium carbonate in 2024 (Watergrade, republishing Thames data). Thames Water calls anything over 300 “very hard”. It is harder than Bromley town (about 280) and Orpington (about 291).

That matters for toilets because the fill valve and the flush seal both rely on small rubber parts closing tightly against plastic. A thin layer of scale is enough to stop them sealing.

Two faults, two symptoms

The fill valve will not shut off

The fill valve is the part on one side of the cistern, connected to the water supply, usually with a float on an arm or a float that slides up a column. When the cistern fills, the float rises and closes the valve.

If scale or grit stops the valve closing, water keeps coming in. It rises past the normal level and runs into the overflow. On an older toilet, the overflow is a pipe out through the wall, so you see water dripping or running outside. On most modern toilets, the overflow runs inside the cistern into the pan, so you see a constant trickle in the bowl.

The flush valve seal is leaking

The flush valve is the large part in the middle that lifts when you press the button. At its base is a rubber seal. If scale builds up on the seal or the seat it sits on, water leaks slowly past it into the pan. The fill valve then tops the cistern up, and the cycle repeats. You may hear the cistern refilling every few minutes, with nobody using it.

How to tell which one it is

1. Take the cistern lid off and put it somewhere safe. It is heavy and breaks easily.
2. Look at the water level. There is usually a line marked inside the cistern, or a level a couple of centimetres below the top of the overflow tube.
3. If the water is at or above the top of the overflow, the fill valve is not shutting.
4. If the level is normal, look in the pan. A ripple or trickle running down the back of the bowl means the flush seal is leaking.
5. To be sure, put a few drops of food colouring in the cistern and do not flush for half an hour. Colour in the pan means the flush seal.
6. Turn off the isolation valve on the cistern feed, a small valve on the pipe, usually with a slot for a flat screwdriver. Turn the slot across the pipe to shut it.

Can I fix it myself?

Sometimes. With the isolation valve off:

• Fill valve: many modern fill valves have a top that unscrews or twists off. Rinse out grit and scale, check the small rubber diaphragm, and refit. If the float is adjustable, check it is not set too high.
• Flush seal: on a push-button flush, the valve usually twists out of its base. Wipe the seal and the seat clean, and refit. If the seal is cracked, hard or out of shape, it needs replacing.

If the valve is scaled solid, the plastic has gone brittle, or you cannot get the parts apart, the whole valve is usually better replaced. A new fill valve or flush valve is a small job for a plumber.

Why it costs you money

Thames Water customers on a meter pay for every litre that goes down the overflow. A toilet that runs on night and day can add noticeably to a bill, and nobody sees it happening.

An external overflow dripping in winter can also freeze and block, and then the cistern has nowhere to go but over the top.

The external overflow pipe

If you see water dripping from a pipe outside, near the bathroom, that is usually the toilet cistern overflow. Higher up near the roof, it is more likely the cold water tank in the loft. Our guide to an overflow pipe dripping outside a Bexley house explains how to trace which one.

Sidcup’s 1930s bathrooms

Most of Sidcup’s homes are 1930s semis and chalet bungalows, and most bathrooms have been refitted at least once. Some older toilets have a high-level cistern or an old-style siphon flush, which uses a different mechanism and usually needs a diaphragm replacing rather than a seal. If you are not sure what you have, send us a photo when you call.

Keep scale at bay

• Clean the fill valve and flush seal once a year if the toilet starts to hesitate.
• Do not use in-cistern blocks that dissolve the rubber parts.
• A whole-house water softener reduces scale on every valve, tap and appliance; Thames Water advises keeping an unsoftened drinking tap if you fit one.

Dripping taps have the same cause. See leaking taps in Sidcup.

When to call

Call us if the toilet is overflowing on to the floor, the isolation valve will not turn or is missing, or you have tried a clean and it still runs. We handle overflowing toilets in Sidcup. More on the town is on our Sidcup area page.

Need a plumber now?

Call 07868 281022 and tell us your postcode, what the toilet is doing, and whether you have turned the water off. We agree the price with you before any work starts.