Mixer Tap Leaking: Cartridge or O-Ring?

Spout drip or seep at the joints: the test that decides it

The one question that sorts almost every leaking mixer is this: does the water come out when the tap is off, or only when it is on? Dry the tap completely, including the underside of the spout and the base, and leave it off for a few minutes.

If a drip forms at the end of the spout with the lever or handles closed, water is getting past the valve, and at a drop a second that adds up to thousands of litres a year. That is the cartridge in a single-lever mixer, or one of the two headgear valves in a mixer with separate hot and cold handles. Nothing on the outside of the tap will stop it.

If the tap stays dry when off but water appears round the base of the spout, under the lever or from a seam in the body once you run it, a seal around a moving joint has failed. That is an O-ring, and it is the cheaper, easier repair.

• Drip from the spout, tap off: cartridge or headgear valve.
• Water round the swivel spout collar when running or when the spout is moved: spout O-rings.
• Water under the lever or from the cap on top when running: cartridge body seal or the O-ring round the cartridge.
• Water under the sink, not on top: tap connectors or the base fixing. See a kitchen mixer tap leaking at the base.

Identify the mixer before you take it apart

Mixers come in three main designs, and they come apart differently. A single-lever mixer has one handle and one large cartridge that controls both flow and temperature. A two-handle mixer has separate hot and cold handles, each with its own small valve, feeding one spout. A thermostatic mixer, common on bath-shower mixers, has a temperature dial with its own thermostatic cartridge and a separate flow control.

Find the maker. The brand is usually printed on the lever, the body or the cap on top, and sometimes on a tag attached to the flexible hoses under the sink. Most large tap makers publish exploded diagrams of their taps with part numbers online, and the correct part number saves a wasted trip. Generic cartridges exist for many taps, but a mixer is less forgiving of a near-match than an old pillar tap.

Replacing a single-lever mixer cartridge

Close both isolation valves under the sink, or the stopcock, and open the lever in the middle position to drain the tap. Put the plug in.

The lever is held by a small grub screw, usually under a red and blue indicator cap on the front or back of the lever, or under a cap on top. Prise the cap off with a fingernail or a thin blade and undo the screw with a small hex key. Lift the lever off. Underneath is a decorative shroud that unscrews by hand, then a brass nut holding the cartridge down.

Undo that nut with a spanner and lift the cartridge straight out. Note which way round it sat: there are locating lugs on the base that line up with holes in the tap body. Measure the cartridge across its base. Most are 35 mm or 40 mm, but the height, the spline pattern on the stem and the base seal differ between makers, so take the old cartridge with you or order by part number.

Wipe out the seat in the tap body, check for grit, fit the new cartridge with its lugs located, and tighten the nut firmly but not hard. Over-tightening distorts the base seal and can make a new cartridge stiff or leaky. Refit the lever, open the isolation valves slowly, and run hot and cold fully before checking for drips.

A cartridge seated even slightly crooked will leak from the top of the tap. If water appears under the lever straight after the repair, isolate again and reseat it rather than tightening the nut further.

Changing the O-rings on a swivel spout

On many kitchen and some basin mixers, the spout is a separate piece that swivels on the tap body, sealed by two or three rubber O-rings. Every time the spout is moved, the rings wear, and hard water speeds that up. The first sign is usually water collecting round the collar at the base of the spout when the tap runs.

With the water isolated, look for a grub screw at the back of the spout collar, or a threaded collar at its base. Undo it and pull the spout straight up with a gentle twist. The O-rings sit in grooves on the tap body or inside the spout. Cut the old ones off, take them to a merchant or match them from a mixed O-ring kit by inside diameter and thickness, and roll the new ones into place.

Smear the new rings lightly with silicone grease made for taps before refitting the spout. Do not use petroleum jelly or general grease, which can make rubber swell and fail early. Push the spout back on squarely, refit the grub screw, and run the tap.

Two-handle and thermostatic mixers

A two-handle mixer that drips from the spout has a failing valve on one side. To find which, close the isolation valve for the hot supply only and watch. If the drip stops, the hot valve is the culprit; if not, try the cold. Each side is either a compression headgear with a rubber washer, repaired as in how to stop a tap leaking, or a quarter-turn ceramic cartridge. Water from under a handle on one side is the spindle O-ring, covered in a tap leaking from the handle.

Thermostatic bath and shower mixers are more particular. A drip from the bath spout or the shower head usually points to the flow cartridge or the diverter, not the thermostatic element. The inlets of these valves often carry small check valves and filters that clog with scale, so a thermostatic mixer that has started to leak and run at an odd temperature at the same time is best left to a plumber with the maker's parts.

When to stop and call someone, and what it costs

Stop if the cartridge nut will not move without the tap body turning, if the isolation valves do not shut the water off, or if the tap body is cracked or corroded around the joint. Stop too if a thermostatic mixer will not deliver a steady temperature after you have finished, because that is a scald risk in a bath.

As a typical guide at merchants, a spout O-ring kit costs a few pounds, a generic single-lever cartridge roughly £5 to £25, and a branded one £25 to £70 or more, depending on the maker. A replacement thermostatic cartridge is usually the dearest part. Where the mixer is cheap and the cartridge is not stocked, a new tap can make more sense than a repair.

Our leaking tap repair visit carries the £60 call-out, at any hour. The repair price is agreed with you on site before any work starts, whether that is a cartridge, a set of O-rings or a new mixer.