A running toilet is the single most common residential plumbing problem and the most-ignored. The leak is silent — water runs from tank to bowl continuously, refilling silently, never making a sound — and a running toilet wastes 200+ gallons of water per day. On Ottawa's metered water, that adds up quickly on your utility bill. The fix is almost always an inexpensive part you can install in 20 minutes.
This guide walks through diagnosis (which part has failed) and repair for the three common failure modes: bad flapper, malfunctioning fill valve, and water level too high. One of the three is the cause for 95% of running toilets.
Before you start
Three things to settle:
1. Diagnose by the symptom. Constant slow refill (every 10-30 min): flapper is the most likely culprit. Random refills (every 1-3 hours): could be flapper or fill valve. Water spilling into the overflow tube: water level too high or fill valve stuck open. Loud running: fill valve failure.
2. Three primary parts. Flapper: rubber valve at the bottom of the tank that seals against the flush outlet. Fill valve: the tall vertical mechanism that refills the tank after a flush. Float: the part that signals the fill valve when to stop. Each can fail independently.
3. Universal-fit replacements. Modern flappers and fill valves are mostly universal-fit (Fluidmaster 400A, Fluidmaster 5300, Korky 528). Verify the model number on your toilet's fill valve before buying — some older or specialty toilets need brand-specific parts.
What you'll need
Tools
- Adjustable wrench
- Sponge and bucket
- Old towels
- Phillips screwdriver
- Slip-joint pliers
Materials
- New flapper (Fluidmaster 502 or matching)
- New fill valve (Fluidmaster 400A or 5300)
- Plumber's tape (Teflon)
Diagnose before replacing
A 30-second diagnosis tells you which part to buy. Drop a few drops of food colouring into the tank water (do not flush). Wait 15 minutes. If colour appears in the bowl, the flapper is leaking. If colour stays in the tank but you hear constant water running, the fill valve is the problem.
This free test saves the cost of buying parts you don't need.
Step-by-step instructions
Diagnose with the dye test
Add 5-10 drops of food colouring (any colour) to the tank water. Don't flush.
Wait 15 minutes. Check the bowl water — if coloured, the flapper is leaking. If not coloured but you hear water running constantly, the fill valve is stuck or the float is set too high.
Shut off the water and drain the tank
Find the shut-off valve behind the toilet (typically near the floor). Turn clockwise to close.
Flush the toilet to drain the tank water into the bowl. Hold the flush handle to drain as much as possible. Sponge or vacuum out residual water — the tank should be nearly empty before working inside.
If flapper is the culprit: replace it
The flapper is a rubber valve at the bottom of the tank, connected by a chain to the flush handle.
Disconnect the chain from the handle lever. Slide the old flapper off its mounting pegs at the base of the tank.
Slide on the new flapper, ensuring it sits flat against the flush outlet seat. Reconnect the chain to the handle, leaving 1/2 inch of slack — too tight and the flapper won't seat; too loose and the handle won't lift the flapper enough to flush.
If fill valve is the culprit: replace it
Disconnect the water supply line at the bottom of the tank.
Loosen the lock nut underneath the tank that secures the fill valve to the tank. Lift the old fill valve out from the top.
Insert the new fill valve through the tank hole, with the rubber gasket against the bottom inside surface. Tighten the lock nut from below — hand-tight plus a quarter turn.
Reconnect the supply line. Adjust the fill valve's height per the manufacturer's instructions (typically the top of the valve sits 1 inch below the tank rim).
Adjust the water level
Most fill valves have a float adjustment — either a screw on top or a clip on the side. The water level should sit about 1 inch below the overflow tube top.
If water spills into the overflow tube, the level is too high — adjust the float to lower the shutoff point. If the bowl doesn't refill enough, raise the level.
Turn on water and test
Open the shut-off valve slowly. The fill valve should fill the tank and shut off automatically when the float reaches the set level.
Flush a few times. After each flush, the tank should refill to the set level and stop. No continuous water running.
Listen for 5 minutes after the tank is full. If you hear running water, the flapper is still leaking or the fill valve isn't shutting off cleanly.
Re-do the dye test
After 30 minutes of normal operation, repeat the dye test to confirm the leak is fixed. No colour in the bowl after 15 minutes = success.
If colour still appears, the new flapper isn't seating cleanly — verify it's flat against the flush seat and the chain has correct slack.
Common mistakes that cost you
- Replacing parts without diagnosing. The dye test takes 15 minutes and tells you exactly which part has failed. Don't replace the fill valve when the flapper is the issue.
- Wrong-size flapper. Older toilets use 2-inch flappers; newer ones use 3-inch flush valves. Match flapper to flush valve size.
- Chain too tight or too loose. Tight: flapper doesn't seat fully and tank runs. Loose: handle doesn't lift flapper enough to flush. 1/2 inch slack is the rule.
- Not waiting after install to verify. A new flapper sometimes seats poorly initially. Wait 30 min and re-test before declaring success.
- Over-tightening the fill valve lock nut. Cracks the tank. Hand-tight plus a quarter turn is enough.
Frequently asked questions
How much water does a running toilet waste?
200+ gallons per day for a continuously-running toilet. On Ottawa's metered water rates, plus sewer fees that roughly double the cost, that waste adds up fast on your bill. An inexpensive flapper pays for itself many times over each year.
Even an intermittent runner (filling once an hour) wastes 50+ gallons/day.
Why does my toilet flush twice?
The flapper is closing too soon. Either the chain is too tight, the flapper is the wrong size for the flush valve, or the flapper has a 'low-flow' design that closes early.
Replace the flapper with a model rated for your toilet, or adjust chain to allow the flapper to stay open through the full flush cycle.
How often should I replace toilet parts?
Flappers: every 3-5 years preventively. Rubber degrades from chemical exposure to chlorinated water.
Fill valves: every 5-10 years. Internal components wear and seal poorly over time.
What if the dye test shows no leak but the toilet still runs?
The fill valve is stuck on, or the water level is set above the overflow tube (causing constant overflow into the bowl).
Check water level first. If it's spilling into the overflow tube, lower the float. If water level is fine but fill valve runs anyway, the valve has failed and needs replacement.
When to call us instead
If your toilet has had multiple flapper/fill-valve replacements that haven't fixed the issue, the toilet itself may have internal cracks or a damaged flush valve seat. Replacing the toilet may be more economical than continued repairs.
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