Domakihub How to Install a Dual Flush Valve and Save Water
Swap out your old cistern flush mechanism for a dual flush valve in an afternoon — and cut toilet water use by up to 67%.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
A dual flush valve gives you two flush volumes — a short flush (roughly 3 litres) for liquid waste and a full flush (6 litres) for solid waste — cutting household water consumption dramatically compared with a standard 9-litre single-flush cistern. For holiday rentals and hotels across Greece, where water is metered and summer usage peaks, this is one of the fastest-payback plumbing upgrades you can make. The video above shows the full process; the steps below walk you through it with the specific details you need before you pick up a spanner.
What a Dual Flush Valve Actually Is
A dual flush valve (also called a flush tower or siphon-free flush mechanism) sits inside the cistern and replaces the old siphon or flapper. It consists of a cylindrical tower sealed at the bottom by a rubber seal, held shut by water pressure. Pressing the small button gives a partial flush; pressing the large button opens the seal fully for a complete flush. Most universal kits sold in Greece fit cisterns with a 3-inch (76 mm) outlet, though some older Ideal Standard or Geberit concealed cisterns use different sizes — check your existing outlet diameter before buying.
Dual flush valves are a DIY-friendly replacement. No soldering, no gluing, and no need to remove the toilet pan. The only tools required are an adjustable spanner, a sponge, and a small bucket. The full job typically takes 45 to 90 minutes for someone doing it for the first time.
When to Call a Licensed Plumber
Fitting a flush valve inside an open cistern is within the capability of a competent homeowner or property manager. However, if your toilet has a concealed in-wall cistern (built behind tiled walls, common in modern Greek apartments and hotel bathrooms), accessing it correctly without damaging the tile work requires a plumber who knows that specific cassette system — usually Geberit, Viega, or Grohe. Similarly, if you discover the water supply isolation valve is corroded and cannot be turned off, or if you find cracks in the cistern itself, stop and call a professional. Forcing a corroded valve can cause a flood.
For straightforward open-cistern toilets — the kind where you lift a lid and can see the mechanism — the instructions below apply directly.
How to Install a Dual Flush Valve: Step by Step
- 1
Shut off the water and empty the cistern
Locate the isolation valve on the cold-water pipe feeding the cistern — it is usually directly beneath or behind the toilet. Turn the screw slot to a horizontal position (a quarter turn) to close it. Now press the flush to empty as much water as possible from the cistern, then sponge out the remaining water into a bucket. Dry the base of the cistern so you can see the bottom outlet clearly.
- 2
Remove the old flush mechanism
Unscrew the large plastic nut on the underside of the cistern that holds the existing flush valve or siphon in place — it turns anti-clockwise when viewed from below. If it is stiff, grip it with an adjustable spanner using a cloth to protect the plastic. Lift the old siphon or tower straight out through the top of the cistern. Also remove the fill (inlet) valve if it is being replaced at the same time, which is good practice.
- 3
Fit the new dual flush valve
Drop the new valve tower through the cistern outlet hole from above. Thread the large plastic nut onto the threaded shank protruding through the bottom of the cistern and hand-tighten it, then add a final quarter-turn with the spanner — do not over-tighten or you will crack the cistern base. Position the overflow tube so it points downward into the pan. Most kits include a washer between the valve base and the cistern floor; make sure it seats flat.
- 4
Connect the push-button assembly
Fit the button actuator plate to the cistern lid (or to the wall plate if your kit includes one). Thread the connecting rods or cables down to the valve tower and clip them into the dual-flush mechanism following the kit's diagram — the short cable or rod connects to the small (half-flush) button and the long one to the large (full-flush) button. Adjust the rod lengths so each button travels freely and returns cleanly. Refit the cistern lid.
- 5
Restore water and test both flush modes
Slowly reopen the isolation valve. Watch the cistern fill and check the base seal and the bottom nut for any drips — tighten the nut a fraction if needed. Once filled, test the half-flush first: press the small button, hold briefly, and release. The flush should stop before the cistern is fully empty. Then test the full flush: press and hold the large button until the cistern empties completely. Adjust the water-level float on the fill valve so the water stops approximately 25 mm below the top of the overflow tube.
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FAQ
- Can I install a dual flush valve in any toilet cistern?
- Most open-cistern toilets accept a universal 3-inch dual flush valve kit. However, older Greek siphon-style cisterns with a narrow 2-inch outlet, and concealed in-wall cassette cisterns (Geberit, Grohe, Viega), require model-specific parts. Measure your outlet diameter and check the cistern depth against the kit specifications before purchasing.
- How much water does a dual flush valve actually save?
- A standard single-flush toilet uses around 9 litres per flush. A dual flush system uses approximately 3 litres on a half-flush and 6 litres on a full flush. In a household that averages five flushes per person per day — roughly 80% of which can use the half-flush — the saving is around 60–67% of toilet water use, which in Greek summer conditions and for holiday rentals with high occupancy translates to a meaningful reduction on the water bill.
- My new valve drips or the cistern keeps running — what is wrong?
- The most common cause is debris on the rubber seal at the base of the valve tower. Shut off the water, empty the cistern, lift the valve tower out, and clean the seal and the seating surface with a damp cloth. If the seal is deformed or scratched, replace it — replacement seals cost a few euros and are widely available. A running cistern can also be caused by the water level being set too high, so the water spills into the overflow: lower the float on the fill valve until the water level sits at least 25 mm below the overflow tube rim.