A running toilet: the silent water waster in your home

A faint trickle from the bathroom when nobody has flushed for hours. A thin stream sliding down the back of the bowl. The toilet is running, and it may have been for weeks.

A running toilet is one of the most common leaks in any home. It is also one of the most overlooked. The water goes straight down the drain, so there is no damp patch to spot. That is why it is often left for months.

Here we look at why a toilet keeps running, how to check yours, what you can fix yourself and when it is time to call a plumber.

Why does a toilet keep running?

Water comes in through a fill valve. A float rises with the water level and closes the valve when the cistern is full. When you flush, a flush valve at the bottom of the cistern opens and lets the water rush into the bowl. The flush valve then closes and the cistern refills.

When a toilet keeps running, the cause is almost always one of these.

• A worn flush valve seal. The rubber washer at the bottom of the cistern gets hard, cracked or coated in limescale. It no longer seals properly and water seeps into the bowl all the time. This is the most common cause.
• The fill valve is set too high. The water rises above the overflow pipe, a tube in the cistern that stops it from flooding the bathroom. The surplus then runs into the bowl without ever stopping.
• The fill valve is stuck. Grit or limescale keeps it from closing fully, so the cistern tops itself up constantly.
• Float problems. The float can catch on the side of the cistern or take on water.
• Worn parts in an older cistern. Links, buttons and fixings all age. A flush button that does not spring back fully can hold the flush valve slightly open.

How to tell if your toilet is running

A small leak can be completely silent. Here are a few ways to check.

Listen. Go into the bathroom when the house is quiet. A soft hiss or trickle when nobody has flushed is a clear sign.

Look in the bowl. Small ripples on the surface of the water, or a thin stream down the back of the bowl, mean water is escaping from the cistern.

Notice the refills. If you hear the cistern quietly refilling for a few seconds now and then without anyone flushing, it is losing water that then gets replaced.

Try the dye test. This is the most reliable way to find a silent leak.

1. Lift the lid off the cistern.
2. Add a few drops of food colouring. A strong colour such as blue or green works best.
3. Do not flush. Wait half an hour or longer.
4. Look in the bowl. If the colour has appeared, the flush valve is leaking.

No colour in the bowl but the cistern still refills on its own? Check the water level. If it sits level with the top of the overflow pipe, the fill valve is the culprit. Flush afterwards so the dye does not stain the bowl.

Why it matters even though the water goes down the drain

Nothing gets wet and nothing breaks, so it is easy to shrug off. But there are good reasons not to.

The water never stops. A leak that runs day and night for months adds up to a lot of water.

You may pay for it. If your home has a water meter, you pay for every drop that passes through it. A running toilet can push your bill higher than it should be, with no obvious reason why. Read what a water leak really costs to see how small leaks turn expensive.

It is a sign of wear. A seal that has started to let water through rarely gets better on its own.

Nobody notices. A toilet in a guest bathroom or a downstairs cloakroom can run for months without anyone going in there to hear it.

How to fix a running toilet yourself

Most cistern faults can be fixed without a plumber. Always turn off the water to the toilet first. Use the isolation valve, the small valve on the pipe feeding the cistern, which you can usually turn with a flat screwdriver. Then flush once to empty the cistern.

Adjust the fill level. If water runs over the overflow pipe, lower the level. Most fill valves have an adjusting screw or a clip that sets the height of the float. Lower it a little at a time until the water settles a short way below the top of the overflow.

Clean or replace the seal. Lift out the flush valve and look at the rubber washer. If it is only dirty or scaled up, wiping it and the seat it sits on may be enough. If it is hard or cracked, replace it. Take the old one to the shop so you get the right size.

Check the float. Make sure it moves freely and does not rub against the side of the cistern. If it rattles with water inside, replace it.

Replace the fill valve. If the valve is stuck and cleaning does not help, swap the whole unit. Follow the instructions that come with the new valve.

Turn it off while you wait. If you cannot fix it straight away, close the isolation valve between flushes. That stops the leak until the replacement part arrives.

When to call a plumber

Call a plumber if:

• the isolation valve will not close or is leaking itself
• the cistern is concealed behind a wall and you cannot reach the parts
• water is leaking where the cistern meets the pan or at the floor
• the toilet keeps running even after you have replaced the seal and adjusted the level
• the cistern or the pan is cracked

Why a running toilet shows up on the mains water pipe

The water from a running toilet never reaches the floor. It flows from the cistern into the bowl and away down the drain. A floor sensor next to the toilet will never react, however long the leak goes on. We compare the two approaches in our article on whole house leak detectors vs floor sensors.

But every drop that runs through the toilet has to enter the house first. It passes through the mains water pipe. A monitor fitted there sees the leak as a small but constant flow. And that flow carries on through the night, when nobody in the house is using any water.

Water Grip clamps onto the incoming water pipe and measures the flow continuously. When it recognises abnormal flow, such as a small flow that never stops overnight, it sends an alert to the app. You can fit it yourself in a few minutes, without a plumber.

The alert tells you water is running somewhere. Then it is up to you to track it down. Do the dye test on every toilet in the house, check taps and appliances, and turn off the water where the leak is.

Summary

A running toilet is common and hard to notice. The usual cause is a worn flush valve seal. Other causes are a fill valve set too high, a valve that has stuck or a float that misbehaves.

Do the dye test on every toilet in the house once a year. It takes half an hour and a few drops of food colouring. And if you want to know when water is flowing while nobody should be using it, the mains water pipe is the right place to measure.