How to Identify Your UK Flush Valve or Siphon
Learn 4 professional methods to identify your exact flush valve or siphon model. Complete visual guide with measurements, brand recognition, and valve type identification.
Start With Your Problem
Before identifying your specific valve or siphon, let's diagnose what's actually wrong. These 5 problems account for 90% of all toilet cistern issues in UK homes.
Constantly Running Toilet
You can hear water trickling into the bowl continuously. Your water bill has increased significantly.
Most Likely Cause:
- • Failed flush valve seal (dual flush)
- • Perished siphon diaphragm (lever flush)
- • Worn inlet valve washer
Quick DIY Fix:
Turn off water supply, flush toilet empty, remove flush valve and inspect seal. If perished or damaged, replace the seal (£5-15) or entire valve (£15-45).
Weak or Incomplete Flush
Toilet requires multiple flushes to clear. Not enough water volume during flush cycle.
Most Likely Cause:
- • Blocked siphon (limescale buildup)
- • Failed siphon diaphragm
- • Incorrect water level adjustment
- • Partially closed isolation valve
Quick DIY Fix:
Check water level in cistern (should be 1-2 inches below overflow). Adjust float valve. If still weak, remove and clean siphon or replace diaphragm (£8-12).
Cistern Won't Stop Filling
Water continuously enters cistern and overflows into the bowl or overflow pipe. Hissing sound after flushing that never stops.
Most Likely Cause:
- • Failed inlet/fill valve
- • Stuck float mechanism
- • Worn inlet valve washer
- • Debris in valve preventing closure
Quick DIY Fix:
Turn off water supply. Remove inlet valve top and check for debris. Clean and reassemble. If washer is perished, replace entire inlet valve (£15-35).
Flush Button Stuck or Broken
Dual flush button won't press, stays stuck down, or feels loose. Lever handle snapped or won't return to position.
Most Likely Cause:
- • Broken flush button mechanism
- • Failed cable/rod connection
- • Cracked plastic button housing
- • Corroded metal lever
Quick DIY Fix:
Remove cistern lid and inspect button/lever connection. If plastic button broken, replace button assembly (£8-25). If cable disconnected, reattach to flush valve.
Noisy Cistern Filling
Loud vibrating, humming, or whistling sound when cistern refills after flushing. Disturbs household, especially at night.
Most Likely Cause:
- • Partially closed isolation valve
- • Worn inlet valve washer vibrating
- • High water pressure (over 3 bar)
- • Loose pipework resonating
Quick DIY Fix:
Fully open isolation valve. Install quiet-fill inlet valve (£20-40) like Fluidmaster 400UK or Geberit AP110. Consider pressure-reducing valve if water pressure exceeds 3 bar.
Quick Diagnostic Flow
✓ Can you hear water running?
→ Check flush valve seal (dual flush) or siphon diaphragm (lever)
✓ Is the flush weak or incomplete?
→ Check water level, clean/replace siphon, verify isolation valve fully open
✓ Does the cistern keep filling?
→ Replace inlet/fill valve, check float mechanism isn't stuck
✓ Is it making noise when filling?
→ Install quiet-fill valve, check water pressure, secure loose pipes
✓ Is the button/lever broken?
→ Replace button assembly or lever mechanism
4 Methods to Identify Your Flush Valve or Siphon
Once you know the problem, you need to identify the exact part to replace. Use these four professional methods to pinpoint your specific valve or siphon model.
Method 1: Measurement Method
The most accurate method - measure your existing valve to identify compatible replacements. UK flush valves come in standardized sizes.
Critical Measurements
1. Outlet Diameter (Bottom)
- • 50mm (2") - Most common, standard toilets
- • 38mm (1.5") - Compact/space-saving toilets
- • 63mm (2.5") - High-performance toilets
- • 42mm - Some Geberit and Siamp models
2. Height (Cistern to Pan)
- • Standard: 160-280mm adjustable
- • Tall: 280-340mm for raised cisterns
- • Compact: 130-160mm for close-coupled
3. Thread Type
- • External thread - Screws onto cistern base
- • Internal thread - Push-fit with locking nut
- • Bayonet fitting - Twist-lock mechanism
How to Measure
- 1.Turn off water and flush toilet empty. Sponge out remaining water.
- 2.Remove cistern lid and locate the flush valve (center) or siphon (under button/lever).
- 3.Measure outlet diameter at the bottom where valve enters cistern. Use digital calipers or tape measure.
- 4.Measure height from cistern base to top of valve mechanism. Note if adjustable.
- 5.Photograph thread type and compare to manufacturer specifications.
- 6.Check button/lever connection type - cable, rod, or direct mount.
⚠️ Professional Tip:
Most UK homes have 50mm outlet valves. If your measurement is close to 50mm, it's almost certainly a 50mm valve (manufacturing tolerances vary). When in doubt, buy a universal adjustable valve like the Fluidmaster PRO550UK which fits 38-63mm outlets.
Method 2: Brand Identification
Look for manufacturer logos, model numbers, or distinctive design features molded into the plastic. Most valves have branding somewhere.
Where to Look for Branding
Top of Valve
Geberit, Wirquin, and Siamp typically mold their logo on the top of the flush valve body, visible when lid is removed.
Side of Valve Body
Macdee, Fluidmaster, and Thomas Dudley often have model numbers molded into the side of the valve body.
Flush Button
Armitage Shanks, Ideal Standard, and Roca typically brand the flush button plate itself.
Top 10 UK Brands by Recognition Features
1. Geberit
Blue or grey plastic, "Geberit" logo on top, model number starts with AP (AP110, AP140, etc.)
2. Macdee
Distinctive "M" logo, model numbers like METRO, KAYLA, WIREX. Often white plastic with green seal.
3. Fluidmaster
Red and white branding, model numbers 400UK, PRO550UK, 510UK. Distinctive red adjustment dial.
4. Wirquin
"Wirquin" molded on valve body, Jolly range (Jolly 1, Jolly 2, Jolly 3). Often blue plastic.
5. Thomas Dudley
Vantage, Turbo 88, Hydroflo ranges. "TD" or "Thomas Dudley" marking on siphon body.
6. Siamp
Optima 50, Optima 49 models. "SIAMP" logo prominent on valve top. Often grey/blue plastic.
7. Armitage Shanks
"AS" logo on flush button. Premium toilets. Model numbers on cistern interior.
8. Ideal Standard
"Ideal Standard" on button plate. Often proprietary valves specific to toilet model.
9. Flomasta
Screwfix own brand. "Flomasta" molded into plastic. Budget-friendly universal valves.
10. Viva
"Viva" logo on valve. Sky range popular. Often stocked at B&Q and Wickes.
Method 3: Flush Type Identification
Identify whether you have a dual flush valve, single flush siphon, or lever siphon. This narrows down compatible replacement options significantly.
Dual Flush Valve
You have this if:
- ✓ Two buttons on cistern lid (large/small flush)
- ✓ Push-down button mechanism
- ✓ Central valve in middle of cistern base
- ✓ Installed after 2001 (building regs)
How it works: Cable or rod connects button to valve. Small button releases 3-4 liters, large button 6-9 liters. Seal lifts to allow water through.
Common problems: Perished seal causing constant running, stuck button, broken cable.
Replacement options: Universal dual flush valves (Fluidmaster, Macdee, Wirquin), brand-specific valves (Geberit, Siamp), or seal-only kits.
Lever Siphon
You have this if:
- ✓ Side-mounted lever handle
- ✓ Pull-up or push-down lever action
- ✓ Tall vertical siphon under lever
- ✓ Installed before 2001 (typically)
How it works: Lever lifts piston creating suction. Water siphons out through diaphragm until air breaks the siphon. Full flush every time (9-11 liters).
Common problems: Perished diaphragm, split siphon body, broken lever arm connection.
Replacement options: Universal siphons (Fluidmaster, Dudley Turbo 88, Macdee), diaphragm-only kits, or complete siphon replacement.
Pneumatic/Pressure
You have this if:
- ✓ Air-actuated button (no cable/rod)
- ✓ Push-button on wall or cistern
- ✓ Geberit concealed cistern (in wall)
- ✓ Commercial/modern installation
How it works: Button activates pneumatic air tube. Air pressure triggers valve mechanism. Common in concealed cistern installations.
Common problems: Air tube leak, failed pneumatic actuator, valve seal failure.
Replacement options: Brand-specific valves only (Geberit AP110, Wirquin pneumatic). Rarely universal compatibility.
Method 4: Visual Valve Type Recognition
Learn to recognize different valve and siphon types by their visual appearance. This helps when no branding is visible.
Still Not Sure Which Valve You Have?
Use our comprehensive 67-manufacturer library to visually identify your exact flush valve or siphon. Each entry includes photos, model numbers, compatible replacements, and direct purchase links.
Browse Manufacturers Library (Guide 2) →Need Professional Help in Hampshire or Wiltshire?
If identification proves too difficult (especially for pneumatic or concealed cistern systems), Rosebourne Plumbing provides expert flush valve identification and replacement services across Hampshire and Wiltshire.
Service Coverage
Andover, Marlborough, Swindon, Newbury, Hungerford, and surrounding areas
Response Times
Same-day service available. Emergency callouts within 2 hours.
Pricing
Standard service from £125. Emergency service from £285.
Related Guides in This Series
Continue your journey with these comprehensive guides covering installation, troubleshooting, and stockists.
Installation Guide
Three complete DIY installation guides with step-by-step instructions. Seal replacement, lever siphon, and dual flush valve installation.
Read Installation Guide →Troubleshooting Guide
Comprehensive troubleshooting for every common problem. Diagnostic flowcharts, repair vs. replace decisions, and emergency fixes.
Read Troubleshooting Guide →UK Stockists Directory
Complete directory of 52+ UK retailers. Trade counters, DIY retailers, online specialists, and manufacturer direct sales.
Browse Stockists Directory →