If your toilet is constantly running or keeps refilling every few minutes, it is not just annoying. It can waste a surprising amount of water and sometimes hints at leaks you cannot yet see. The good news is that most issues are straightforward to diagnose once you understand what is happening inside the cistern.

How a toilet cistern works in simple terms

Inside your cistern are a few key parts that control how the toilet fills and flushes. Knowing what they do helps you work out where the problem might be.

The fill valve lets water into the cistern after you flush. Attached to it is a float, which rises with the water level and tells the valve when to shut off. The flush valve (or siphon) releases water into the pan when you press the handle or button, then seals again so the cistern can refill.

There is also an overflow pipe inside the cistern. If the water level gets too high, it runs into the overflow and down into the pan instead of over the rim of the cistern. This is often why you hear repeated refilling or see a thin trickle of water into the toilet bowl.

7 common causes of a constantly running toilet

Most running toilets come down to a handful of faults. You can often spot the likely cause just by lifting the cistern lid and watching what happens after a flush.

1. Worn flush valve seal or washer

The flush valve has a seal or washer that keeps water in the cistern when you are not flushing. Over time it can harden, crack or become misshapen. When that happens, water seeps through into the pan and the cistern keeps topping up.

Look inside the cistern and watch the area where water leaves the tank. If you see a constant trickle into the pan even when everything else looks still, the seal may need replacing. A clear close-up photo here can help you compare what you see at home to a healthy seal.

2. Mis-set float level

If the float is set too high, the cistern will try to fill above the overflow level. Water then runs into the overflow continuously, and the fill valve never properly shuts off.

Visually, you will notice the water level sitting right at or over the top of the overflow pipe. A simple diagram showing the correct level a couple of centimetres below the overflow rim can make this much easier to understand.

Gently bend a metal float arm down so the water stops lower in the cistern.
On modern plastic floats, use the adjustment screw or slider on the fill valve.
Flush and check that the final level sits just below the overflow opening.

3. Faulty fill valve

Even with a correctly set float, a worn fill valve can fail to shut off properly. You might hear a constant hissing or see water trickling into the overflow. Sometimes tapping the side of the valve makes the noise briefly stop, which suggests the internal mechanism is sticking.

Replacing a fill valve is a common plumbing job. Confident DIYers can change like-for-like valves, but if you are unsure or have limited access in the cistern, it is sensible to call a plumber.

4. Debris in the valve

Small bits of limescale or grit from the pipework can lodge inside the fill valve. This prevents it from closing fully and leads to a slow, continuous flow. It is especially common in hard water areas.

With the water isolated, some valves can be opened and gently cleaned out. A photo of your specific valve model, compared with a diagram from the manufacturer, will help you decide whether this is realistic as a DIY task.

5. Cracked or damaged overflow pipe

If the overflow pipe is cracked or poorly fitted, water can dribble through it even when the level is not particularly high. This can be harder to spot, as you may only see movement when the cistern is nearly full.

Shine a torch inside and inspect the overflow from top to bottom. Any obvious crack or loose joint is a strong sign that a plumber will need to replace it.

6. Stuck or warped flapper (on some toilets)

Many UK toilets use a siphon rather than a flapper, but some modern cisterns, especially imported models, do have a rubber flapper. If it warps, gets stiff, or the chain catches, it may not close fully after a flush.

If you see a flexible rubber flapper over the outlet, press it gently by hand. If the running stops, it is likely not sealing and will need adjusting or changing. A before-and-after photo can be helpful if you are replacing it yourself.

7. Water level set too high overall

Sometimes everything works, but the general fill level has been set too high for the design of the cistern. Even small changes to incoming water pressure can then tip it into constant overflow.

Mark the current water line with a pencil, then lower the float slightly and test again. Aim for the water to stop 2 to 3 centimetres below the overflow pipe or any “max fill” line printed inside the cistern.

Quick DIY checks before you call a plumber

There are a few simple tests you can do that do not require tools and give you a clearer picture of the problem.

Food colouring test for silent leaks

If you suspect water is slipping from the cistern into the pan but cannot see movement, add a few drops of food colouring to the cistern and wait 20 to 30 minutes without flushing. If coloured water appears in the pan, you have an internal leak past the flush valve seal or flapper.

A photo of the coloured water in the bowl can be very useful if you later speak to a plumber, as it proves the leak path.

Check water level against the overflow

Lift the lid, flush once, and watch where the water settles. If it sits level with or above the overflow opening, that points towards a float or fill valve issue rather than a seal problem.

If you have a smartphone, take a quick photo of the settled water level relative to the overflow. You can use this both for your own comparison and to show a professional.

Using the service valve to isolate the water

Locate the small isolation (service) valve on the pipe feeding the toilet, usually a little silver or chrome fitting with a slot for a flat screwdriver. Turn it a quarter-turn so the slot runs across the pipe to reduce or stop the flow.

If the running stops when you turn the valve, the problem is inside the cistern rather than in the supply pipework. This also buys you time and limits water waste until a plumber from Plumbing Correct can attend.

Risks of ignoring a constantly running toilet

A running toilet often starts as a minor nuisance but can lead to bigger issues if left as it is. Continuous internal leaks keep the cistern damp and can cause fittings to degrade faster.

More seriously, a fault that begins as a slight overflow can develop into water appearing around the base of the pan or behind the toilet. Persistent moisture here can damage flooring and skirting boards, and in upstairs bathrooms it can affect ceilings below.

If you notice any damp patches, soft flooring, or discolouration near the toilet, treat it as a priority rather than something to watch.

What a plumber will typically do on a visit

A qualified plumber will start by inspecting inside the cistern, checking each component in turn while flushing and refilling. They may repeat the food colouring test, check the isolation valve, and measure the water level relative to the overflow.

Once they have pinpointed the fault, common fixes include replacing a worn flush valve seal, fitting a new fill valve, resetting the float height, or changing a damaged overflow pipe or flapper. After the repair, they will flush several times to ensure the cistern fills and stops correctly and will check around the toilet for signs of existing leaks or damp.

Diagnosis of the running or refilling issue
Replacement or adjustment of faulty parts
Testing for silent leaks and proper shut-off
Check of the isolation valve and visible pipework

Need help in Richmond, Twickenham or Clapham?

If your toilet keeps running, the cistern will not stop filling, or you can see water escaping around the pan, it is time to get it sorted properly. A quick visit can prevent damage to the flooring and put an end to that constant trickling sound.

Plumbing Correct helps homeowners across Richmond, Twickenham and Clapham with running toilets, leaks and all general plumbing services. For urgent issues or flooding risk, you can also use their emergency callout service.

To book a repair or get friendly advice, contact Plumbing Correct on 02036334011 today.

Toilet constantly running? 7 causes and what to do next