Why Is My Toilet Running? Causes and How to Fix It

A toilet that keeps running after the flush is one of the most common plumbing problems in Ontario homes — and one of the most wasteful. A constantly running toilet can waste anywhere from 200 to 750 litres of water per day, depending on how severe the leak is. At Toronto Water's combined water and sewer rate, that's a real and measurable impact on your monthly bill, sometimes adding $50 or more each month for what amounts to a $12 rubber seal.

The good news is that the vast majority of running toilets are caused by one of just a few easily diagnosable issues, and most can be fixed by a confident homeowner in under an hour with parts from any Canadian Tire. This guide covers how a toilet's internal mechanism works, the most common causes of a constantly running toilet, how to diagnose which cause is at play in your case, step-by-step DIY repair instructions, and a clear guide to when the problem is beyond a DIY fix and it's time to call a licensed plumber.

How a Toilet's Flush Mechanism Works

Before diagnosing the problem, it helps to understand what the internal components of a toilet tank do. Lift the tank lid and set it safely aside — you're looking at a surprisingly simple system with just a few parts.

The fill valve (also called a ballcock in older toilets) is the tall assembly usually on the left side of the tank. Its job is to refill the tank with fresh water after each flush. It opens when water level drops and shuts off when the water reaches the correct level. Modern fill valves are typically plastic cylinder-style assemblies; older toilets have a ball float on an arm.

The flapper is the rubber seal at the bottom of the tank, seated over the flush valve opening. When you flush, the handle pulls the chain that lifts the flapper, allowing tank water to rush into the bowl for the flush. As the tank empties, the flapper drops back into place and seals the opening, allowing the tank to refill.

The overflow tube is the tall plastic pipe in the centre of the tank. It's an emergency drain: if the fill valve fails to shut off and the tank overfills, water flows into the overflow tube and down into the bowl rather than overflowing onto the bathroom floor.

The float — either a ball float on an arm or a cup float that slides along the fill valve shaft — signals to the fill valve when the tank is full. As the tank refills, the float rises. When it reaches the set level, it shuts off the fill valve. If the float is set too high, the water level rises above the top of the overflow tube and water drains continuously into the bowl through the overflow tube.

A running toilet means the fill valve never truly turns off — because either water is continuously leaking from the tank to the bowl through a faulty flapper, or the water level is high enough to flow continuously through the overflow tube, or the fill valve itself is failing to close.

The Most Common Causes

1. Worn or Warped Flapper (The #1 Cause)

The flapper is the single most common cause of a constantly running toilet. Rubber degrades over time — typically after 4 to 8 years of use, though Ontario's chloramine-treated municipal water (used in most Toronto-area municipalities) can accelerate rubber degradation, sometimes causing flappers to fail in as few as 3 years. A worn flapper no longer forms a perfect seal against the flush valve seat, allowing water to trickle continuously from the tank into the bowl. The fill valve detects the falling water level and runs to compensate — hence the continuous running sound.

2. Float Set Too High

If the float is adjusted too high, the water in the tank rises above the top of the overflow tube before the fill valve shuts off. Water then continuously drains into the overflow tube and down into the bowl. The fill valve keeps running to replace this water. This is often the cause when you can hear the toilet running but see the water in the tank is quite high, perhaps at or above the top of the overflow tube.

3. Faulty Fill Valve

Fill valves do eventually wear out. A failing fill valve may hiss continuously even when the tank is full, or may never fully shut off. This is often the cause in toilets more than 15 years old that have never had a fill valve replacement. You may hear a persistent hissing sound even when the water level looks correct.

4. Corroded or Scaled Flush Valve Seat

In Ontario hard water conditions, calcium scale can deposit on the flush valve seat — the smooth surface the flapper presses against. Even a new flapper cannot seal properly against a rough or pitted valve seat. If you've replaced the flapper and the toilet still runs, a scaled valve seat is the likely culprit. Run your finger around the seat and feel for rough spots, mineral crust, or visible corrosion.

5. Waterlogged or Stuck Float

In older toilets with a ball float (a hollow sphere on a metal or plastic arm), the ball can develop a small crack over time, fill with water, and become too heavy to rise to the correct shut-off level. The fill valve then never shuts off because the float never reaches the top. You can test this by manually lifting the float arm — if the water stops, the float is the issue.

How to Diagnose Each Cause

Open the tank lid and observe. These tests will identify the cause in most cases within a few minutes.

The Dye Test (Flapper Leak)

This is the single best first test. Add a few drops of food colouring or a dye tablet (available at hardware stores) to the water in the tank. Do not flush. Wait 15–20 minutes, then look in the toilet bowl. If colour appears in the bowl without flushing, water is leaking past the flapper from the tank into the bowl. Your flapper needs replacement.

Check the Water Level vs. the Overflow Tube

Remove the tank lid and look at the water level relative to the top of the overflow tube. The water level should sit approximately 2.5 cm (1 inch) below the top of the overflow tube. If water is at the same level as the tube opening or higher, and you can see or hear water trickling down the tube, the float is set too high — lower it (instructions below).

The Float Arm Test

With the tank running, manually lift the float (ball type) or push up on the cup float. If the running stops immediately when you raise the float, the fill valve is working correctly — the float just needs adjustment or replacement.

Listen for Fill Valve Hiss

If the tank is full and at the correct water level, but you can still hear a faint hiss or running sound, put your hand over the fill valve's cap while the tank is at rest. If water is still flowing, the fill valve is not shutting off fully and needs replacement.

Flush Valve Seat Inspection

If the dye test shows a flapper leak, and a new flapper doesn't solve it, turn off the water supply and flush to drain the tank. Run your index finger slowly around the entire circumference of the valve seat. Any rough texture, visible orange or white mineral deposits, or visible pitting indicates a scaled or corroded seat. A fine-grit emery cloth can sometimes smooth light deposits; significant corrosion typically requires replacing the flush valve seat or the entire flush valve assembly.

DIY Repair: Flapper Replacement Step by Step

Flapper replacement is the most commonly needed toilet repair and is well within the ability of most homeowners. It requires no special tools and typically takes 15–20 minutes.

1. Turn off the water supply valve. The shutoff valve is typically located behind and below the toilet on the wall or floor. Turn it clockwise until it stops. If it's stiff and hasn't been turned in years, turn it slowly to avoid damaging it.
2. Flush the toilet to drain the tank. Hold the handle down to drain as much water as possible. Use a sponge or towels to remove any remaining water at the bottom of the tank.
3. Disconnect the old flapper. Unhook the two "ears" of the flapper from the pins on either side of the overflow tube. Unhook the chain from the handle arm. Remove the flapper. Note how the chain was attached.
4. Match the new flapper. Take the old flapper to Canadian Tire or a plumbing supply store and match it. Most toilets use a standard 2-inch flapper, but some models (particularly those from American Standard or Kohler) require model-specific flappers. Universal flappers ($8–$15) work on most toilets.
5. Install the new flapper. Hook the ears onto the overflow tube pins. Attach the chain to the handle arm with approximately 1 cm of slack — just enough that the chain doesn't restrict the flapper from sealing, but not so loose that it gets trapped under the flapper.
6. Turn the water supply back on and allow the tank to fill completely.
7. Perform the dye test again to confirm the new flapper is sealing. If colour appears in the bowl, check the chain slack and inspect the valve seat.

Adjusting the Float

If the water level is too high and running into the overflow tube, adjusting the float will solve it. The method depends on your float type:

Cup float (modern fill valves): Look for a small clip on the side of the fill valve shaft. Pinch the clip and slide the entire cup float down the shaft by 1–2 cm. This lowers the water level at which the fill valve shuts off. Flush and recheck the water level — it should now sit about 2.5 cm below the overflow tube.

Ball float (older toilets): The old method of bending the float arm downward still works. Alternatively, many older fill valves have an adjustment screw where the arm meets the fill valve body — turning it counterclockwise lowers the water level. The water level should visibly drop below the overflow tube top.

Fill Valve Replacement

Fill valve replacement is slightly more involved than a flapper swap but still accessible for a handy homeowner. A Fluidmaster 400A fill valve, available at most Canadian Tire locations for approximately $15–$25, is compatible with the majority of two-piece toilets and comes with clear installation instructions.

Key steps: shut off the water supply, flush and sponge out remaining water, disconnect the supply line from the fill valve shank at the bottom of the tank, remove the lock nut (turn counterclockwise — may need channel-lock pliers), lift out the old fill valve, insert the new one at the correct height (adjust per the included instructions), tighten the lock nut, reconnect the supply line, turn water on, adjust the float to the correct water level, and test. The entire process typically takes 30–45 minutes for a first-timer.

Ghost Flushing and Phantom Flushes

Have you noticed your toilet flushing on its own — a sudden burst of running water without anyone touching the handle? This phenomenon, called ghost flushing or a phantom flush, has a straightforward explanation. A slow flapper leak allows water to gradually seep from the tank into the bowl. Over several minutes to an hour, the water level in the tank drops enough that the fill valve kicks in briefly to refill it — this is the "flush" sound you hear. The toilet is not actually flushing; the fill valve is topping up the tank because a flapper leak has been slowly draining it.

Ghost flushing is an unmistakable sign of a flapper leak, even if the toilet otherwise seems to work normally. The fix is the same: flapper replacement, and if that doesn't resolve it, inspection of the valve seat.

Water Waste and Utility Impact

The water waste from a running toilet is often underestimated. Consider the numbers: a moderate flapper leak allowing a thin trickle from tank to bowl wastes approximately 400 litres per day. A significant leak — water visibly running into the bowl — can waste up to 750 litres per day or more.

At Toronto Water's 2024 combined water and sewer rates (approximately $4.24 per cubic metre for the first tier), a 400 L/day leak adds roughly 12 m³ to your monthly consumption — an additional $50 per month on your water bill, or $600 per year, for a $12 part. Addressing a running toilet promptly is one of the highest-return home maintenance actions available.

When to Call a Licensed Plumber

Most running toilet issues resolve with a flapper replacement or float adjustment. However, some situations call for a professional plumber:

• You've replaced the flapper twice and the dye test still shows a leak
• The valve seat is visibly corroded or has significant mineral buildup that emery cloth hasn't resolved
• The toilet is rocking at the base or water is on the floor around the toilet
• Your toilet is 20+ years old with original parts and has had multiple recent issues
• The fill valve replacement didn't resolve a hissing fill valve
• Water is running from the base of the toilet (a sign of wax ring failure, not a tank issue)

Mechora Group's residential plumbing team serves Toronto, Mississauga, Brampton, Vaughan, Markham, and Oakville. We diagnose and repair all toilet issues — from simple flapper replacements to complete toilet replacements for toilets that have reached the end of their useful life. All work is performed by licensed, insured plumbers under the requirements of the Ontario College of Trades.

For a newer toilet in otherwise good condition, a flapper and fill valve together cost under $40 in parts and can resolve 90% of running toilet issues. For a toilet that's 20+ years old and causing repeated problems, replacement is often the more economical long-term choice — particularly when provincial toilet rebate programs are factored in. Our plumbers are happy to give you an honest assessment of which path makes more sense for your situation.