How to replace a shower cartridge in a mixer or thermostatic valve

Signs the cartridge is the problem

The cartridge is the working part of a mixer shower valve. It mixes the hot and cold, controls the flow, and on a thermostatic valve it holds the temperature steady when someone runs a tap elsewhere. When it wears or scales up, the shower tells you in a few recognisable ways.

• The shower drips or runs from the head after it has been turned off.
• The temperature swings, runs cold, or will not get hot even though the hot taps are fine.
• The handle is stiff, grinding or turns without changing anything.
• The flow is weak even after the head and hose have been cleaned.
• The shower will not turn off completely.

Different kinds of shower cartridge

A shower valve may have one cartridge or two. On a thermostatic bar mixer, one handle controls temperature through a thermostatic cartridge and the other controls flow through a separate flow cartridge, often a ceramic disc. A single-lever or sequential valve has one cartridge that does both. Concealed valves behind the tiles use the same kinds of cartridge, but reaching them means working through the trim plate.

Match the fault to the cartridge before buying. A drip when the shower is off usually points to the flow cartridge. Temperature problems usually point to the thermostatic one.

Identify the valve before buying anything

Cartridges are specific to the make and model, and ones that look alike often are not interchangeable. Look for a brand name on the handle, the trim plate or the underside of a bar mixer. The original instructions or the product code on the receipt make it easy. Failing that, take clear photos of the valve, the handles and the back of the handle once it is off, and compare them with the manufacturer's spare parts pages.

Buy the manufacturer's genuine part or one listed as a direct match. A cheap universal cartridge that nearly fits can leak or fail to control temperature properly, and on a thermostatic valve that is a scald risk.

Check which side the hot water comes in. By convention, bar mixers take hot on the left. If yours was plumbed the other way round, some makers sell a reversed-inlet cartridge, and fitting the standard one will give you cold where the hot should be.

Step by step: replacing the cartridge

Allow an hour for a bar mixer you can see and reach. Have a towel in the shower tray, cover the waste so small screws cannot drop into it, and lay the parts out in the order they come off.

• Isolate the water. Many bar mixers have small isolating valves in the wall elbows that turn with a flat screwdriver. If there are none, turn off the cold at the stopcock and the hot at the cylinder or boiler supply, or both at the stopcock for a combi or unvented system.
• Open the shower to release any pressure and let the water drain from the valve.
• Remove the handle. Prise off the cover cap and undo the screw underneath, or loosen the grub screw with a hex key, then pull the handle straight off.
• Remove any shroud or collar. Some unscrew by hand, some need a strap wrench, and some are held by a clip.
• Note which way the cartridge sits, then unscrew it anticlockwise with a spanner of the right size. Support the valve body with your other hand so it does not twist on the wall.
• Clean the inside of the valve body gently with a soft cloth. Do not scrape the seat with anything metal.
• Lightly grease the new cartridge's O-rings with silicone grease, not petroleum jelly, which can swell rubber seals.
• Screw the new cartridge in by hand first, then snug it with the spanner. Do not overtighten.
• Turn the water back on slowly and check for leaks round the cartridge before refitting the shroud and handle.
• Run the shower through its range and test that it turns fully off.

Set the maximum temperature on a thermostatic valve

A new thermostatic cartridge needs the temperature stop setting. Most valves have a button on the temperature handle that stops it turning past a set point, and the position of the handle on the spline sets where that point is. Run the shower at full hot, measure the water with a thermometer, and reposition the handle following the manufacturer's instructions so the stop sits at the temperature they recommend.

If anyone in the house is very young, elderly or has reduced sensation, take this step seriously. The Health and Safety Executive's guidance on scalding is written for care settings but explains why a correctly set thermostatic control matters. A thermostatic cartridge that has not been set can let water much hotter than intended through.

When to stop and call a plumber

Some cartridge changes are quick, and some are not. Stop if any of the following apply.

• The cartridge will not move with steady pressure. Scale can lock it into the body, and more force can crack the valve or loosen the pipe joints behind the wall.
• The valve is concealed and the trim plate does not give enough room to get a spanner on. Our guide to concealed shower valve replacement access explains the options.
• You cannot isolate the hot and cold without shutting off the whole house for longer than you can manage.
• The valve has no brand marking and you cannot find a matching part.
• Water is leaking behind the tiles or through a ceiling below.

If the shower will not turn off and you cannot isolate it, turn off the water at the main stopcock and switch off an electric shower or pump at the isolator. Our guide to shower dripping after switching off covers what to do next.

Descaling as well as replacing

In hard water areas such as London, limescale is the usual reason cartridges wear out early. When the cartridge is out, look for white scale in the valve body and around the inlet filters, which are small mesh screens where the hot and cold enter a bar mixer. Rinse these clean. It is also worth cleaning the head at the same time, and our guide on how to descale a shower head explains the method.

If the temperature problem comes back quickly with a new cartridge, the cause may be in the supply rather than the valve, such as unbalanced hot and cold pressures or a hot water system that is struggling. Our guide to thermostatic shower temperature problems explains how to separate the two.

Getting it done

Loyal Plumbers replaces shower cartridges, repairs mixer and thermostatic valves, and deals with concealed valves and leaks behind tiles across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Our call-out charge is £60. Call 020 4652 8585 or send an enquiry with the make of the shower if you know it, a photo of the valve, and what it is doing.