How to Replace a Kitchen Sink Mixer Tap: A Step-by-Step Trade Guide

Knowing how to replace a kitchen sink mixer tap is a core skill every plumber carries on every job. Whether it's a dripping monobloc that's been reported three times or a full kitchen refit, swapping a kitchen tap is a clean, profitable task when you've got the right kit and a clear process. This guide covers exactly that — from isolating the supply to testing the new tap, with the tools and products that make the job go smoothly.

What Type of Kitchen Tap Are You Replacing?

Before you start, identify the existing tap type. Most kitchen sinks in the UK run either a monobloc mixer (single-hole, single body, hot and cold on one lever or two handles) or a separate pillar tap set (two holes, separate hot and cold). You're replacing like-for-like unless the customer wants a full sink upgrade.

The most common replacement is a monobloc — and for good reason. A single-hole, quarter-turn mixer gives you full flow control from one lever, WRAS approval on the better models, and a clean look that suits a working kitchen. The KeyPlumb Contract Monobloc Kitchen Sink Mixer Tap (HS17N, £37.60) covers that brief exactly: WRAS-compliant, chrome finish, standard 3/8" tail fittings, and solid enough for a trade kitchen or residential job.

For customers who want lever operation — easier for older hands or anyone with grip issues — the KeyPlumb Contract Lever Kitchen Sink Mixer Tap (HS37N, £44.89) fits the same single-hole cutout. Both are available for same-day collection from APM at 24 Western Avenue, Acton, W3 7TZ.

Tools and Materials You'll Need

• Adjustable basin wrench (essential for reaching tap back-nuts under the sink)
• Adjustable spanner or slip-joint pliers
• Flexible tap connectors — 3/8" x 15mm, 300mm or 450mm to suit tail length
• PTFE tape
• Isolation valves (if not already fitted on the hot and cold supply)
• Small towels or container for residual water in supply pipes
• Headtorch — under-sink visibility is always poor

If isolation valves aren't already fitted, fit them before you start. A quarter-turn angle isolation valve on each supply tail means you can isolate the tap without draining the system and lets the customer turn the tap off if there's a problem before the next visit. It's good practice on every tap job.

Step-by-Step: How to Replace a Kitchen Sink Mixer Tap

1. Isolate the Water Supply

Turn the isolation valves clockwise (or close the stopcock if no isolation valves are fitted). Turn the existing tap on to release pressure and check flow has stopped completely. Put a small towel or tray under the supply connections — there's always residual water in flexible connectors.

2. Disconnect the Flexible Tap Connectors

Use a spanner to undo the compression nuts at both the tap tail end and the isolation valve end. If the flexi hoses are old, replace them — they're cheap, and old fibre-washer flexi hoses are a common source of under-sink leaks. Have new 3/8" x 15mm flexible tap connectors ready before you start.

3. Remove the Old Tap

Under the sink, find the back-nut (or back-nuts on a two-hole pillar set) that clamps the tap body to the sink deck. This is where a basin wrench earns its keep. The back-nut is usually an oversized plastic or brass nut on a threaded tail. Undo it, then lift the old tap out from above. Clean any sealant residue from the sink surface.

4. Prepare the New Tap

Unbox the new tap and check what sealing method is used. Some monoblocs come with a rubber gasket that seats directly on the sink deck — use it. If not, or if the sink surface is uneven, run a thin bead of silicone around the base before fitting. Thread the hot and cold tails down through the sink hole.

5. Fit the Back-Nut and Secure the Tap

From underneath, thread on the back-nut or mounting bracket by hand first. Then tighten with a basin wrench until the tap is firm and doesn't rotate. Don't overtighten on ceramic or composite sinks — firm, not brutal. Check the tap spout is pointing in the right direction and the body is aligned straight before you fully tighten down.

6. Connect the Flexible Tap Connectors

New flexi hoses should come with fibre washers pre-fitted. If not, fit them — never use a bare brass-on-brass connection. Wrap the tap tail threads lightly with PTFE if specified, but most modern tap tails use a flat-faced fibre washer compression connection that doesn't need PTFE. Tighten by hand first, then half a turn with a spanner. Hand-tight plus half a turn is almost always enough — compressing the washer too hard causes cracking over time.

Connect the other end to the isolation valve — 15mm compression. Hot to hot, cold to cold. The hot supply is typically on the left, cold on the right, though this varies by installation.

7. Open the Isolation Valves and Test

Slowly open each isolation valve in turn. Check immediately around every connection point for weeps. Run both hot and cold, check operation of the lever or handles, and confirm the aerator flow is clean. If there's a minor weep at a flexi connector, tighten a quarter turn more and recheck. Never over-tighten plastic compression bodies.

Monobloc vs Lever: Which Tap to Specify?

Both the KeyPlumb monobloc and lever tap fit a standard 35mm sink hole. The choice comes down to customer preference and budget:

• Monobloc (HS17N, £37.60): Single lever controls flow and temperature simultaneously. Clean, minimal look. Most common choice for modern kitchens and trade installs. WRAS-approved.
• Lever (HS37N, £44.89): Separate lever handles for hot and cold. Easier to operate for customers who find single-lever motion difficult. Also suits traditional kitchen styles. Same single-hole fit.

Both are contract-grade — built to handle daily kitchen use without the failures you get on budget internet taps. For a full range of kitchen tap options, see the Kitchen Taps & Mixers collection at APM.

Common Issues on Kitchen Tap Replacement Jobs

Corroded back-nut that won't shift: Apply penetrating oil and leave for 10 minutes. If it still won't move, cut the old flexi hoses clear, use a strap wrench or flat-blade jammed against the nut, and work it slowly. Don't use a hammer and screwdriver — you'll damage the underside of the sink.

Flexi connector too short to reach the tap tail: Modern taps often have longer tails than older ones. 450mm flexi connectors give more reach than the standard 300mm. Keep both in the van.

No isolation valves fitted: You'll need to close the stopcock on the incoming mains (for cold) or the central heating primary flow (for hot, if it's an indirect system). Fit isolation valves while you're in there — it's a minor add-on that prevents the next plumber from having the same problem.

Low flow after fitting: Remove and clean the aerator on the spout end. Debris from the supply pipe often lodges in the aerator on first use. It's a 30-second fix once you know to check for it.

Where to Pick Up Kitchen Taps in West London

APM Electricals stocks trade plumbing products at 24 Western Avenue, Acton, London W3 7TZ. If you're on a job in West London, you can pick up KeyPlumb kitchen taps, flexi connectors, and under-sink fittings same-day. Call ahead on 020 8702 8080 to confirm stock. Visit apmi.uk/collections/kitchen-taps-mixers to see the full range before you come in.

Need to sort the sink waste at the same time? Check the Sink Waste Kits collection for 1½" and 2" basket strainers to suit most kitchen sinks.