Toilet Cistern Keeps Filling: Quick DIY Fixes
If your toilet cistern keeps filling up long after you’ve flushed, you’re not alone. That constant hissing is the sound of water—and money—literally going down the drain. It's one of the most common plumbing headaches, and it nearly always points to a faulty part inside the tank.
The good news? It’s usually a simple fix. The main culprits are typically a worn-out flush valve seal (often called a flapper), a dodgy fill valve, or a float that’s set incorrectly. When any of these components fail, they can no longer control the water level properly, causing a constant trickle into the bowl and forcing the cistern to keep refilling.
Understanding Why Your Toilet Keeps Filling
While the biggest immediate cost of a running toilet is a surprisingly high water bill, leaving it unchecked can lead to bigger problems. A slow, persistent leak can cause water damage over time, which can be a real nightmare to sort out. It's worth understanding an insurance claim for water damage just in case a small drip turns into a major flood.
Thankfully, fixing the problem is often far more affordable than you might think, and it’s certainly cheaper than replacing the whole toilet. Based on recent callouts, here’s a rough idea of what you can expect to pay for a professional to sort it:
- Fill Valve Replacement: Around £98.89
- Flush Valve Replacement: Approximately £95.53
- Syphon Repair: Roughly £92.50
Tackling the issue as soon as you spot it saves you from escalating bills and gives you peace of mind.
Quick Diagnosis Guide For A Running Toilet
Not sure where to start? This table breaks down the common symptoms to help you pinpoint the likely cause.
| Symptom | Likely Cause | Recommended Action |
|---|---|---|
| Water constantly trickles into the bowl. | Worn Flapper or Flush Valve Seal: The seal isn't closing properly, letting water leak from the cistern into the bowl. | Perform the dye test. If it fails, the flapper/seal needs replacing. |
| The toilet runs intermittently. | "Phantom Flush": This is also caused by a slow leak from a faulty flapper or flush valve seal. | The dye test is your best friend here. A replacement seal is the likely fix. |
| Water spills into the overflow tube. | Incorrect Float Height or Faulty Fill Valve: The fill valve isn't shutting off at the right water level. | Adjust the float height first. If that doesn’t work, the fill valve probably needs replacing. |
| You hear a hissing sound. | Leaky Fill Valve: Water is leaking from the fill valve itself, often straight down the overflow tube. | Isolate the water supply. If the hissing stops, the fill valve is the culprit. |
Gathering Your Essential Toolkit
Before you start investigating why your toilet cistern keeps filling, a little prep work will make life easier. You won’t need professional gear, but having a few basic tools to hand will simplify the job.
Grab an adjustable wrench, a flathead screwdriver, a sponge, and a bucket or old towels to mop up any spills. For most common toilet repairs, that’s all you’ll need.
Now, here’s the most important bit: turn the water off. Find the isolation valve on the pipe behind or next to the toilet and turn it clockwise until it stops. This simple action prevents a quick fix from becoming an accidental flood. For more practical advice, have a look at our top DIY plumbing tips.
Pinpointing The Exact Cause
When your toilet cistern won’t stop filling, it's almost always down to the float, the fill valve, or the flush valve. Let's find out which part is misbehaving.
First, take the lid off the cistern. Your first check should be the float mechanism. Is it tangled, catching on the cistern wall, or set too high? A high water level lets water constantly trickle down the overflow tube. Often, a quick jiggle or a simple adjustment is all it takes.
The Food Colouring Test
If the float seems fine, it's time for the dye test. This trick is a brilliant way to see if the flush valve seal (or flapper) is letting water seep from the cistern into the bowl. Add a few drops of food colouring to the water inside the cistern. Now, walk away and don't flush it for at least 15-20 minutes.
When you come back, look in the toilet bowl. If you see coloured water, you've found your culprit: a leaky flush valve seal. A constantly filling cistern is a common plumbing headache. If you're dealing with a similar but distinct issue, explore the causes of a toilet constantly flushing in our other helpful guide.
Simple DIY Fixes For A Running Toilet
Now that you have a good idea why your toilet cistern keeps filling, it’s time to get your hands dirty. These fixes are surprisingly straightforward and can likely be sorted in under an hour.
Adjust The Float Mechanism
If the water level is too high, the float is your first port of call. It's the most common culprit and the easiest to fix.
For older toilets with a ball-and-arm float, gently bend the metal arm downwards. This tells the fill valve to shut off sooner, lowering the water level.
For modern toilets with a float cup that slides on the fill valve shaft, look for an adjustment screw or a clip. Turning the screw or sliding the clip will change the float's height and the water level.
A good rule is to set the water level half an inch to one inch below the top of the overflow tube. This provides enough water for a powerful flush without it constantly trickling over the top.
When To Call A Go Assist Plumber
While most of these fixes are within reach of a keen DIYer, sometimes it’s best to call in an expert. Perhaps you’ve followed the steps, but the filling sound won't quit, or you've spotted a hairline crack in the cistern itself. These signs suggest the problem is more complex than a worn-out seal.
The UK toilet repair market shows how common these issues are, with professional cistern repairs typically costing £100-£200. For perspective, undertaking other home plumbing tasks like moving a boiler highlights why some jobs are best left to experienced professionals.
Don’t let a small leak turn into a major headache. Booking a certified Go Assist plumber for your toilet repair is the smart move. You'll get a fast, reliable fix with no hidden fees, giving you complete peace of mind.
A Few Final Questions
How much water does a constantly filling toilet waste?
A constantly filling toilet can waste between 200 and 400 litres of water daily. This adds a hefty amount to your water bill, so fixing it is good for your wallet and the planet.
Is it hard to find the right replacement parts?
Not usually. Most modern UK toilets use standard-sized fill valves and flush seals, which you can find at any large DIY or plumbing supply shop.
Top Tip: Snap a photo of the inside of your cistern on your phone before you head out. It makes matching the right parts an absolute breeze.
Can I just turn the water off at night?
Turning off the isolation valve next to the toilet is a great temporary fix to stop the noise and waste. However, it’s not a permanent solution as the underlying fault could worsen. It's always best to get to the root of the problem and sort it out properly.
