Toilet Won't Stop Running: Float Valve, Flapper, Overflow & Siphon Diagnosis

Summary

A running toilet is the most wasteful domestic plumbing fault in terms of water volume — a leaking cistern can waste up to 400 litres per day, equivalent to 8 full baths. Under the Water Supply (Water Fittings) Regulations 1999, a cistern must be fitted with an effective float valve and overflow, and must not waste water. Water companies in the UK are increasingly auditing properties for running toilets as part of leakage reduction programmes, and some offer free repair kits to householders.

The UK has two distinct toilet flushing systems: the siphon flush (traditional, common in properties built before 2000, uses a siphoning action to empty the cistern) and the flap valve flush (modern, uses a rubber flap that lifts when flushed, more common in dual-flush cisterns from the 2000s onwards). These two systems fail differently. The diagnosis of a running toilet must therefore first identify which flushing mechanism is installed.

A second critical distinction is whether the water is overflowing into the bowl silently (via the flush path) or visibly overflowing via the overflow pipe to the exterior of the building. Silent overflow into the bowl is diagnosed by adding food colouring to the cistern — if it appears in the bowl without flushing, the flush seal is leaking. Visible overflow to the exterior indicates the float valve is not shutting off, causing the cistern to overfill.

Key Facts

• Water waste — a continuously running toilet wastes 200–400 litres per day; at UK average water rates, this costs £150–£300 per year (metered)
• Siphon flush — traditional UK flush mechanism; water siphons over the top of the siphon tube into the pan; cannot backflow; the red rubber diaphragm (flap washer) is the most common failure point
• Flap valve flush (flapper) — modern mechanism; a rubber flap seals the outlet; flap fails when rubber degrades, allowing water to trickle into the pan continuously
• Float valve types — high-pressure Portsmouth valve (brass, traditional), low-pressure diaphragm valve (modern, Torbeck-type), side-entry vs bottom-entry; BS 1212 Parts 1–3 govern float valves
• Ballcock adjustment — on older float valves, the float arm can be bent downward slightly to reduce the water level; on modern valves, an adjustment screw or collar changes the float height
• Water level — should be at least 25mm below the overflow outlet; if water reaches the overflow, the cistern is overfilling
• Silent leak test — add a few drops of food colouring to the cistern; if colour appears in the bowl within 15 minutes without flushing, the flush valve seal is leaking
• Dual-flush cisterns — use a flap valve with two modes (full 6 litre and reduced 4 litre flush); the rubber seal around the entire outlet is a more complex failure than a simple flapper
• Overflow pipe — must be fitted so that any overflow is visible externally (drip from exterior wall); if the overflow discharges to the pan, a leaking float valve is masked
• Continuous trickle into pan — if water trickles into the pan between flushes, the flush valve seal has failed; the cistern remains full but slowly drains
• Cistern not filling — a float valve that sticks closed causes the cistern to not refill; this is the opposite problem (no flush) but related to the float valve mechanism
• Syphon diaphragm kits — universal diaphragm kits (red rubber disc) fit most syphon mechanisms; cost under £5; critical to get the correct diameter (usually 89mm)

Quick Reference Table

Detailed Guidance

Decision Tree

TOILET WON'T STOP RUNNING — START HERE
              |
              v
Is water RUNNING INTO THE PAN between flushes?
              |
          YES | NO
              |  \
              |   v
              |  Is water OVERFLOWING EXTERNALLY
              |  (dripping from exterior wall)?
              |       |
              |   YES | NO
              |       |   \
              |       |    v
              |       |  Hissing sound from cistern?
              |       |  Check water level in cistern
              |       |  -- is it below overflow?
              |       |   YES: Normal (ignore)
              |       |   NO: Float valve issue
              |       v
              |  FLOAT VALVE not shutting off
              |  --> Adjust float height first
              |  --> If still overflowing: replace float valve
              v
IS IT RUNNING INTO THE PAN?
              |
              v
Add food colouring to cistern.
Wait 15 minutes WITHOUT flushing.
Does colour appear in bowl?
              |
          YES | NO
              |   \
              v    v
         Flush    Problem may
         valve    be intermittent
         seal     -- monitor
         failure
              |
              v
What type of flush mechanism?
              |
        ------+------
        |            |
        v            v
   SIPHON         FLAP VALVE
   (press-flush,  (button flush,
   older toilet)  dual flush)
        |            |
        v            v
   Replace        Replace
   siphon         flapper or
   diaphragm      flush valve seal

Identifying Your Flush Mechanism

Siphon flush: Typically found in toilets with a single flush lever (handle) on the side or top. When the lever is pressed, it lifts a plastic or metal siphon mechanism. To confirm: remove the cistern lid and flush — you should see a siphon assembly with a large vertical tube.

Flap valve flush: Found in most dual-flush toilets and many post-2000 single-flush toilets. Has two buttons on top of the cistern. When pressed, the flush valve lifts a large rubber flap at the base of the cistern.

Identifying which has failed: The siphon mechanism cannot fail in a way that allows continuous water trickle during idle — the siphon only flows when activated by a flush. If you have a siphon toilet and it is running continuously, the siphon diaphragm (the red rubber disc inside the mechanism) has developed a hole. Water then bypasses the siphon and trickles into the pan.

For flap valve toilets, the rubber flap or the rubber seal around the flush valve outlet is the failure point. The rubber degrades with age, especially in hard water areas, allowing a continuous trickle.

Replacing a Siphon Diaphragm

The siphon diaphragm is a circular rubber disc inside the siphon mechanism. It is the most commonly replaced toilet part.

1. Turn off the water supply to the cistern (service valve below or to the side of the cistern).
2. Flush to empty the cistern.
3. Disconnect the siphon from the flush lever linkage (usually a clip or hook).
4. Depending on toilet type:
   • Close-coupled: The siphon may be accessible from inside the cistern, or the cistern may need to be removed from the pan (undo the bolts securing the cistern to the pan).
   • High-level cistern: Unscrew the large nut securing the siphon to the cistern base.
5. Once the siphon is removed, the diaphragm is accessed by splitting the siphon body (some have a bayonet lock, others unscrew).
6. The old red rubber diaphragm will be visible — it may have a hole or be distorted. Remove and discard.
7. Fit the new diaphragm. Universal kits contain multiple size discs — use the one that fits snugly inside the siphon cup.
8. Reassemble in reverse order. Reconnect the flush lever linkage.
9. Restore water supply. Test: fill cistern, wait 15 minutes, check for colour trickle (use food colouring).

Replacing a Flap Valve / Flush Valve Seal

For flap valve (dual-flush) cisterns, the repair depends on the brand. Geberit, Siamp, and Fluidmaster are the most common UK brands.

Generic flap valve replacement:

1. Isolate and drain the cistern as above.
2. The flush valve sits centrally in the cistern base. Disconnect the flush button linkage (usually unscrew the button assembly).
3. The flush valve typically twists to unlock from the cistern base. Rotate 90° anti-clockwise and lift out.
4. At the base of the flush valve, you will find the rubber seal (a flat ring or a shaped rubber gasket). Remove and replace with the matching seal from a repair kit.
5. For a Fluidmaster or Siamp universal flush valve, the entire valve assembly may need replacement rather than just the seal.
6. Reassemble. Test for trickle as above.

Adjusting or Replacing a Float Valve

If water is overflowing externally, the float valve is not shutting off correctly. The float ball or float cup rides up as the cistern fills; when it reaches the set level, it closes the water inlet valve.

Adjustment on old Portsmouth-type (brass) float valve:

• The float ball is attached to a horizontal brass arm.
• To lower the water level, bend the arm downward slightly (toward the cistern base).
• The float shuts the valve off at a lower level, reducing the water height.

Adjustment on modern diaphragm float valves (Torbeck/BallFix type):

• These have an adjustment screw or collar on the side of the valve body.
• Turning the adjustment lowers the shut-off level.
• The float cup moves on a vertical spindle; the height at which it shuts off is set by the collar position.

Replacing a float valve (any type):

1. Isolate the water supply (service valve under the cistern or main stopcock).
2. Flush to empty the cistern.
3. Place a towel under the cistern. Disconnect the water supply hose or compression fitting from the float valve tail.
4. Inside the cistern, the float valve is secured by a large plastic or brass nut (backnut) on the outside of the cistern wall. Unscrew and remove the valve.
5. Fit the new valve. Hand-tighten the backnut, then a quarter turn with a spanner — do not overtighten plastic backnuts.
6. Reconnect the supply. Set the water level using the adjustment (aim for 25mm below the overflow).
7. Restore supply and check for leaks at the connection.

Frequently Asked Questions

How much water does a running toilet waste?

A constant trickle (not a full flush) into the pan wastes approximately 200–400 litres per day. This equates to 73,000–146,000 litres per year. At the average UK water rate (approximately £2.00 per cubic metre on a meter), this costs £146–£292 per year. Toilets with a visible overflow running waste even more — a fully open float valve can waste several thousand litres per day.

Can I repair a running toilet myself?

Yes — it is one of the most accessible plumbing DIY tasks. There is no notifiable work involved (Water Regulations apply, but minor repair of existing fittings does not require notification). The tools needed are basic: an adjustable spanner, channel-lock pliers, and optionally a sponge and bucket. Replacement parts (diaphragm kits, float valves) are stocked by all plumbing merchants and most DIY stores.

My toilet cistern makes a hissing noise all the time — what is this?

A continuous hiss from the cistern indicates the float valve is not fully closing — water is continuously entering and continuously overflowing via the overflow or via a flush valve leak. Check the water level inside the cistern: if it is at or above the overflow pipe outlet, the float valve is not shutting off. Either the float is waterlogged (especially older hollow plastic floats) or the valve diaphragm is worn.

The cistern fills but immediately starts running again — why?

If the cistern fills to the normal level and then immediately begins trickling into the pan without being flushed, the flush valve seal is definitely the culprit. Confirm with the food colouring test. This is almost always caused by rubber seal degradation — common in hard water areas where limescale accelerates rubber deterioration.

My toilet is very old with an outside cistern — can I get parts?

High-level cisterns (Victorian/Edwardian pattern) with a pull-chain are still served by the same siphon diaphragm system as modern toilets. Universal siphon diaphragm kits (89mm) fit most high-level siphon mechanisms. Float valves for high-level cisterns (low-pressure type, BS 1212 Part 2) are also still available. If the ceramic cistern itself is cracked, replacement cisterns of matching style are available from salvage yards.

Regulations & Standards

• Water Supply (Water Fittings) Regulations 1999 — Schedule 2, Paragraph 14: cisterns must be watertight and fitted with an effective float-operated valve; Paragraph 16: overflow pipes must be installed to discharge in a conspicuous position

• BS 1212:1990 (Parts 1–3) — float operated valves; Part 1 (Portsmouth), Part 2 (diaphragm), Part 3 (diaphragm with equilibrium design)

• BS EN 14055:2011 — WC cisterns for flushing

• Water Regulations Advisory Scheme (WRAS) — cistern and float valve approval guidance

• Waterwise: Leaky loo campaign — water waste from leaking cisterns, free repair kits

• CIPHE — plumbing installation and maintenance guidance

• Thames Water: Fix a leaky loo — practical guide with video

• dripping tap — tap washer and float valve diagnosis

• water regulations — Water Regulations compliance for plumbing fittings

• bathroom refit — full bathroom refit including WC replacement

• toilet types — WC types: close-coupled, wall-hung, concealed cistern