Domakihub How to Fit an Isolation Valve: A Step-by-Step Guide
Take control of your water supply with a simple quarter-turn valve that stops flooding and costly call-outs.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
An isolation valve lets you shut off water to a single tap, toilet, appliance, or section of pipework without cutting supply to the entire property — a must-have in any Greek home, holiday rental, or hotel room where a quick maintenance stop can prevent a soaked floor. The video above walks through the physical fitting process; this guide gives you the ordered steps, the right tools, and the judgement calls so you know exactly what to expect before the plumber arrives or if you are tackling it yourself. Fitting an isolation valve is well within DIY reach on existing supply pipes, provided you are comfortable cutting copper or plastic tubing and working with compression or push-fit fittings.
What Is an Isolation Valve and Why Does Every Property Need One?
An isolation valve — sometimes called a service valve or ball valve — is a compact inline fitting with a quarter-turn slot-head or lever that opens or closes water flow through a pipe. When the slot runs parallel to the pipe the valve is open; turned 90 degrees it is fully closed. They are typically installed directly behind toilets, under sinks, before washing machines, and before water heaters, giving you or a plumber a fast shut-off without racing to the street stopcock.
In Greece, many older apartments and holiday villas were plumbed without individual isolation points, so a burst fill valve or leaking tap connection means shutting down the whole building while repairs are made. Retrofitting isolation valves costs very little — valves run from €2 to €8 each — and can prevent thousands of euros in water damage, especially in properties left unoccupied between rental seasons.
Valve Types: Compression, Push-Fit, or Soldered?
Compression isolation valves are the most common choice for DIY or quick plumber call-outs. Olive rings grip the outside of the pipe as the cap nut is tightened, creating a watertight seal without heat. They work on both 15 mm copper pipe (the standard Greek domestic supply diameter) and on cross-linked polyethylene (PERT) pipe found in newer builds. Push-fit valves, made by brands such as John Guest or Speedfit, are even faster to install — the pipe simply clicks into the body — but require that the pipe end be clean, round, and fully inserted to the depth mark.
Soldered (capillary) valves give the most permanent and compact joint but require a gas torch, flux, and solder, and disturbing existing solder joints introduces risk. Unless you are already confident with torch work, stick to compression or push-fit on a retrofit project. If you have plastic pipes with a flexible or corrugated section, always check the valve manufacturer's compatibility list before purchasing, as some push-fit bodies are rated for rigid pipe only.
How to Fit an Isolation Valve: 5 Steps
- 1
Shut Off the Water and Relieve Pressure
Locate the nearest upstream stopcock — typically under the kitchen sink, in a utility cupboard, or at the building mains entry. Close it fully clockwise, then open the downstream tap or appliance to drain residual pressure and water from the pipe section you will be cutting. Place a towel or small bucket under the work area because some water will remain in the pipe even after draining.
- 2
Mark and Cut the Pipe
Measure the valve body length (usually 60–80 mm end to end for a standard 15 mm compression valve) and add 10 mm on each side for the pipe to seat inside the fitting. Mark both cut points on the pipe with a permanent marker. Use a pipe slice or rotary pipe cutter for a clean, square cut — a hacksaw works but leaves burrs that must be filed smooth with a deburring tool or fine-grit sandpaper. A clean cut is essential: an angled or burred end will not seal.
- 3
Prepare the Pipe Ends
For compression valves, slide a cap nut onto each pipe end (threaded end facing outward), then push an olive over each end. Make sure the olives seat squarely and sit roughly 10 mm from the cut end. For push-fit valves, insert a pipe support insert (also called a liner or stiffener) into plastic pipe ends to prevent the pipe from collapsing under the compression of the fitting body. Wipe the pipe ends clean and dry — grease or lime scale prevents a good seal.
- 4
Insert the Valve and Tighten
Slide the valve body into the gap, ensuring the flow direction arrow (if present) faces toward the appliance or tap — not toward the mains supply. For compression fittings, hand-tighten the cap nuts onto the valve body, then use two spanners: one to hold the valve body still and one to tighten the cap nut a further one-and-a-quarter turns. Do not over-tighten; this can split olives or crack plastic valve bodies. For push-fit valves, firmly push each pipe end into the fitting until it clicks and cannot be pulled back out.
- 5
Restore Water and Check for Leaks
Set the isolation valve to the open position (slot parallel to the pipe) before restoring the mains supply — otherwise you will have no flow to the appliance and may not notice a slow leak at the joint. Slowly open the upstream stopcock and watch every joint for drips for at least two minutes. If a compression fitting weeps, close the mains, dry the joint, and tighten the cap nut by an additional quarter-turn. Never leave a wet joint hidden behind a panel: the damp will eventually cause mould and structural damage in masonry walls common in Greek construction.
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FAQ
- Can I fit an isolation valve on plastic PERT or PEX pipe?
- Yes, both compression and push-fit isolation valves are compatible with PERT and PEX pipe in 15 mm or 20 mm diameters, which are the most common sizes in Greek residential builds. Always insert a pipe support liner before fitting a push-fit valve on plastic pipe, as the liner keeps the pipe round and prevents the body from cutting into the pipe wall under pressure. Check the valve packaging for a compatibility mark or manufacturer data sheet if you are unsure.
- Do I need a licensed plumber to fit an isolation valve in Greece?
- Fitting an isolation valve on an internal supply pipe (cold or hot domestic water, not gas) is generally considered minor plumbing maintenance and does not legally require a licensed installer in Greece under the current framework. However, any work that connects to or modifies the building's main entry point, gas supply, or requires a pressure test certification — for example after a full bathroom renovation — should be carried out or signed off by a licensed plumber (ydravlikos). When in doubt, a qualified plumber can fit a standard isolation valve in under 30 minutes.
- How do I know if the isolation valve is open or closed?
- On a standard slotted isolation valve, the slot on the head indicates flow direction. When the slot is parallel (in line with) the pipe, the valve is open and water flows. When the slot is perpendicular (at 90 degrees, forming a cross with the pipe), the valve is closed. Some lever-type valves use a colour indicator or a printed open/closed label on the body. Always confirm by checking water flow at the downstream tap before assuming the valve position.