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Bathroom Tap Replacement: A Trade Guide to Isolation Valves, Flexi Connectors, and Quarter-Turn Service Valves
Bathroom Tap Replacement: A Trade Guide to Isolation Valves, Flexi Connectors, and Quarter-Turn Service Valves
Summer is bathroom season. Homeowners are finally tackling that dripping basin tap, tradespeople are booked solid on bathroom refurbs, and the trade counter gets a spike in isolation valves, flexi hoses, and tap tails. If you're doing bathroom tap replacements — or explaining to a customer what's involved — this is the job breakdown you need. Covers everything from shutting off the supply to leak testing the finished install.
Shop related products at APM Electricals
Before You Touch the Tap — Shut Off the Water
Step one on any tap replacement is isolation. Under a basin, you'll typically find two isolation valves on the supply tails — one hot, one cold. These are usually 15mm compression × ½" BSP and sit directly on the pipework below the tap.
Two types you'll encounter:
- Quarter-turn ball type — a lever or keyslot that turns 90° to open or close. Visual indicator: lever in line with pipe = open; lever across pipe = closed. Fastest to operate, most common on modern installs.
- Screwdriver slot type — flat-blade slot that turns a quarter turn. The slot across the pipe = closed. Slightly fiddlier but the standard type fitted with most flexi tap connectors.
If there's no under-sink isolation valve — typical in older properties — you're either isolating the branch at a nearby gate valve or going back to the property stopcock. That's a good reason to always fit isolation valves when you're in there.
Turn the valve off, then open the tap to bleed off pressure. If the tap still runs after closing the valve, the valve has failed and you'll need to go further upstream. Check the valve actually works before you disconnect anything.
Under-Sink Isolation Valves — Types, Sizes, and What to Buy
Standard under-sink isolation valves come in two orientations:
- Straight (inline) — pipe comes up vertically and runs straight through to the tap tail. Less common under a basin but used where the supply comes from the wall behind.
- Angle (90°) — pipe comes up from the floor and the valve turns the flow 90° upward toward the tap tail. Most common configuration under a bathroom basin.
Sizing: 15mm compression × ½" BSP covers the vast majority of domestic basin tap replacements. The compression end connects to your 15mm copper supply pipe; the ½" BSP threaded or flat-face outlet connects to the flexible tap connector.
On material: standard brass is fine for most installs. DZR (dezincification-resistant) brass is the right choice for aggressive water supplies — particularly relevant in West London where water hardness is high. DZR valves hold up better long-term in hard water areas. Embrass Peerless and Pegler both offer DZR variants in their service valve ranges.
Flat-face vs threaded outlet: If you're using a flexi tap connector with a swivel nut, you need a flat-face outlet on the isolation valve. The flat face ensures the hose washer seats properly against a smooth surface — no thread damage, no leak risk. Most modern service valves come flat-face as standard.
For a full breakdown of isolation valve types, sizes, and where to use each one, see our dedicated guide: Isolation Valves and Service Valves: Which to Use and Where.
Flexible Tap Connectors — Getting the Right Hose
Flexi tap connectors are the link between the isolation valve and the tap tail. They give you the flexibility to align the hose regardless of whether the supply pipe and tap tail are perfectly in line — which they rarely are.
Key specs to match:
- Length — 300mm suits most under-basin installations. Go to 450mm or 600mm if the tap tail is unusually high or the isolation valve is positioned low. Measure before you order.
- Bottom connection — 15mm compression end (to fit standard copper or plastic supply pipe), or a swivel nut onto a ½" BSP flat-face isolation valve outlet.
- Tap tail connection — ½" BSP is the standard for most pillar taps and monobloc basin mixers. Some older or continental taps use 3/8" BSP — check the tap spec before buying.
Stainless steel braided hose is the trade standard: EPDM inner liner for compatibility with both hot and cold water, stainless braid for burst pressure tolerance. WRAS approved braided hose is required where the flexi connects to the potable supply.
Best option for most bathroom tap replacements: the Embrass Peerless Flexible Tap Connector c/w Isolating Valve 15mm × 3/4" × 300mm — combines the flexi hose and isolation valve in one unit, which is ideal when you're replacing a tap that has no existing under-sink isolation.
For more on spec-ing the right connector: Flexible Tap Connectors: How to Choose the Right Size and Length.
Removing the Old Tap
With the supply isolated and pressure bled off:
- Disconnect the flexi connectors from the isolation valve outlets — swivel nuts, no need for grips, just an adjustable spanner.
- Unscrew the flexible connectors from the tap tails (½" BSP swivel nut at the top).
- Get under the basin with a basin wrench (cranked tap spanner). This is not optional for taps with the back-nut more than 200mm above the supply entry — a standard spanner won't reach. The basin wrench swivels to grip the back-nut in tight spaces.
- Remove the back-nut. If it's seized — common on older installs with no service history — penetrating oil and patience. Top-hat washers (the rubber seating washer between the tap body and the basin surface) often disintegrate on older taps; have replacements to hand.
- Lift the tap out. Clean the basin surface before fitting the new one.
Fitting the New Tap
Seat the new tap into the basin hole with the top-hat washer in place, thread the tap tails through, fit the back-nut from below and hand-tighten. Snug with a basin wrench — don't overtighten on a ceramic basin.
Reconnecting the water supply:
- For threaded connections (tap tails and BSP fittings), use PTFE tape on the male thread — 3–5 wraps in the direction of the thread. For malleable iron or steel fittings, jointing compound or hemp-and-compound is more appropriate. See our PTFE tape guide for full detail.
- For compression connections (15mm compression onto supply pipe), no PTFE — the seal is made by the olive, not the thread. Hand-tighten the compression nut then a quarter to half turn with a spanner. Overtightening distorts the olive.
Reconnect the flexi connectors: swivel nut onto the tap tail (hand-tight plus a quarter turn with a spanner), then onto the flat-face isolation valve outlet (same). Don't cross-thread the swivel nuts — they're brass and will damage easily.
Browse the full range of plumbing fittings and compression fittings at APM.
Torque and Leak Testing
Open the isolation valves slowly — quarter turn at a time. Watch all connections as pressure builds. Check:
- Both swivel nut connections at the tap tails
- Both swivel nut connections at the isolation valves
- Both compression connections at the supply pipe (if you removed and refitted the isolation valves)
Any weeping at a compression joint: close isolation valve, half-turn tighter on the compression nut, retest. Any weeping at a swivel nut: close isolation valve, check the washer is seated, retighten. If a flexi connector continues to leak after two retightens, the washer is probably damaged — replace the hose.
Run hot and cold fully and check for leaks at all joints including the tap body itself. Run the tap for two minutes to flush installation debris before closing up under the basin.
Get the Parts from APM in Acton
Isolation valves, flexi tap connectors, PTFE tape, compression fittings, and basin wrenches — all in stock at the trade counter. Come in and pick what you need, same day.
APM Plumbing & Electrical
24 Western Avenue, Acton, London W3 7TZ
020 8702 8080
www.apmi.uk
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