Intro

On most jobs, a small leak at a service valve eats time fast. The correct way to connect a flexi to isolation valve is simple, but many fittings get paired wrong. That crushes washers, kinks hoses, and causes drips. In this guide, you’ll see exactly what to buy, how to fit it, and how to test it. We’ll cover the right valve style, which faces seal, when to use PTFE, and how tight to go. Follow these steps and you’ll avoid the classic flexi-to-valve headaches.

Quick Answer

Use an isolation valve with a 1/2" BSP male flat face (tap-connector style). The flexi’s captive washer seals on that flat face—no PTFE on those threads. Point the valve’s arrow toward the outlet, support plastic pipe with inserts, avoid kinks, and tighten the flexi nut hand-tight plus 1/4–1/2 turn. Flush and test for 10 minutes.

Table of Contents

• Key Takeaways
• Pick the Right Valve for a Flexi
• Tools, Materials, and Prep
• How to Connect a Flexi to an Isolation Valve (Step-by-Step)
• Avoid These Common Mistakes
• Testing and Commissioning
• When You Need Adaptors or Special Parts
• FAQ
• Conclusion

Key Takeaways

• Use a tap-connector style isolation valve with a 1/2" BSP male flat face.
• The seal is the flexi’s washer, not the threads. No PTFE on that joint.
• Tighten: hand-tight + 1/4–1/2 turn. Don’t crush the washer.
• Keep bend radius over 50 mm and avoid torsion in the flexi.
• Flush 30–60 seconds and pressure test for 10 minutes.

Pick the Right Valve for a Flexi

The cleanest, leak-free setup pairs a flexi with a valve that has a flat sealing face. Look for a 15 mm x 1/2" BSP tap-connector isolation valve, or an isolation valve with a 1/2" BSP male flat face. That face lets the flexi’s captive washer seal properly.

• Valve type: 1/4-turn ball isolation valve, full-bore if you can. It keeps flow up.
• Outlet thread: 1/2" BSP parallel (G1/2) with a flat face. This is key.
• Inlet side: 15 mm compression to copper or plastic (use inserts for plastic).
• Flow direction: follow the arrow on the valve body toward the outlet.
• Format: straight or angled. Choose based on pipe route and space.

If you only have a compression isolation valve (cone seat), don’t clamp a flexi washer onto that cone. It will leak or cut the washer. Use a proper tap-connector style valve or a 1/2" BSP male flat-face adaptor.

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Tools, Materials, and Prep

Have everything ready so you’re fast and tidy.

• Two spanners or adjustable wrenches (typical 17–24 mm across flats)
• Pipe cutter and deburring tool
• Compression olives (15 mm) and nuts (usually supplied with the valve)
• Pipe inserts/support sleeves for PEX/MLCP (per manufacturer)
• 1/2" BSP tap-connector isolation valve
• Flexi hose with captive washer (1/2" or 3/8" as needed)
• PTFE tape only for tapered male threads if used elsewhere (not for washer joints)
• Cloths, bucket, and tissue for leak checks

Shut off the water. Open a low tap to drain. Plan the valve handle position so it’s reachable after tiling, boxing, or vanity install.

How to Connect a Flexi to an Isolation Valve (Step-by-Step)

1. Isolate and Drain: Shut the feed. Open a nearby tap for 30–60 seconds to relieve pressure.
2. Cut and Prep Pipe: Cut square. Deburr inside and outside. For plastic, fit the correct insert.
3. Fit the Valve (Inlet): Slide nut and olive on the pipe. Push the valve on fully (15–22 mm insertion depending on valve). Hand-tighten the nut.
4. Align and Tighten: Hold the valve body with one spanner. Tighten the compression nut with the other. From finger-tight, go about 1/2–1 turn. Don’t overtighten.
5. Check Flow Direction: Arrow goes toward the flexi/tap. Adjust before final nip-up.
6. Prep the Flexi: Confirm the captive washer is seated and undamaged. Replace if cracked.
7. Connect the Flexi to the Valve: Offer the flexi nut to the 1/2" BSP male flat face. Thread by hand to avoid cross-threading.
8. Tighten the Flexi Nut: Hand-tight, then add 1/4–1/2 turn with a spanner. The washer makes the seal. No PTFE here.
9. Set the Hose Path: Keep a smooth bend. Aim for a bend radius over 50 mm. No twists.
10. Restore Water and Test: Close downstream taps. Open the isolation valve slowly. Check for weeps for 10 minutes.

Time on site for a clean retrofit is often 20–40 minutes, including testing.

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Avoid These Common Mistakes

When you connect a flexi to an isolation valve, these errors cause most call-backs:

• Sealing on the Wrong Surface: Don’t clamp a washer onto a compression cone. Use a flat-faced 1/2" BSP outlet.
• PTFE in the Wrong Place: Don’t tape the threads that pull a washer joint together. The seal is the washer.
• Missing Pipe Inserts: Plastic pipe without inserts leads to ovaled pipe and leaks.
• Overtightening: Crushing the washer or deforming the olive makes slow weeps. Stick to hand-tight plus 1/4–1/2 turn on the flexi nut.
• Twisting the Flexi: Torsion weakens the braid and stresses the joint. Hold the hose body while tightening.
• Too-Tight Bends: Keep radius over 50 mm. Use an angled valve if space is tight.
• Hidden Handles: Don’t bury the isolation handle. Leave 20–30 mm clearance so a screwdriver or fingers can turn it.
• No Flush: Always flush debris for 30–60 seconds before final sign-off, especially on mixers.

Testing and Commissioning

A neat install still needs a proper test.

• Visual Check: Dry all joints. Place tissue around the compression and flexi joints. It shows weeps fast.
• Working-Pressure Test: Open the valve slowly. Pressurize to normal line pressure (commonly 3–5 bar in homes). Observe for 10 minutes.
• Operate the Valve: Turn it 90° off and on once to confirm movement and no stem weep.
• Recheck After Warm-Up: Run hot water if connected to a mixer. Thermal expansion can reveal a loose joint. Recheck after 10–15 minutes.
• Backflow Protection: Many regions require check valves on mixer feeds. Fit single or double checks as required by local code.

Contractors often report that a 10-minute hold test and a quick next-day check save re-visits.

When You Need Adaptors or Special Parts

Sometimes you’re not working with a standard 1/2" setup.

• 3/8" Monobloc Tails: Use a 3/8" x 1/2" adaptor, or a flexi with a 3/8" female end. Don’t force threads.
• Space Is Tight: Use an angled isolation valve or a street elbow rated for potable water. Keep the flexi radius over 50 mm.
• Low Flow Complaints: Swap to full-bore 15 mm valves. A narrow bore can drop flow by a noticeable margin.
• Corroded Threads: Replace, don’t fight them. A new £5–£15 valve is cheaper than a call-back.

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FAQ

Do I need PTFE tape between the flexi and the isolation valve?

No, not when the flexi seals with a captive washer onto a flat-faced 1/2" BSP male. The threads only pull the parts together. Use PTFE only on tapered male threads that rely on thread sealing, not on washered joints.

Which way does the isolation valve face?

Follow the arrow on the body. It points from the supply toward the outlet (toward the flexi/tap). If there’s no arrow, the flat face for the flexi should be on the outlet side.

Can I connect a flexi straight to a compression isolation valve cone?

Not recommended. The cone isn’t a flat sealing face. It can cut the washer and leak. Use a tap-connector isolation valve with a 1/2" BSP male flat face or a suitable adaptor.

How tight should I make the flexi nut?

Go hand-tight, then 1/4–1/2 turn with a spanner. Stop if the nut bottoms out or the washer is clearly compressed. Overtightening can crush the washer and cause a slow weep.

Do I need check valves on mixer feeds?

Often, yes. Many local codes require single or double check valves to prevent backflow, especially where hot and cold mix. Check your regional rules and the tap manufacturer’s instructions.

Conclusion

Here’s the bottom line: pair the flexi with a flat-faced 1/2" BSP isolation valve, let the washer do the sealing, and avoid PTFE on that joint. Keep bends gentle, inserts in plastic, and test under working pressure for 10 minutes. Next steps:

• Stock 15 mm x 1/2" tap-connector isolation valves and spare flexi washers.
• Use hand-tight + 1/4–1/2 turn on flexi nuts, then tissue-test every joint.
• Document the job, note valve locations, and add photos for your records.

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