Replace a thermostatic shower cartridge

Turn off the water, swap the worn cartridge for the correct match, then test for leaks and safe temperature.

SC-01Turn off the water and test it's offTurn off the isolating valves for the shower first — these are usually small screwdriver-slot valves behind an access panel, under the bath, or in the loft. If there are none, turn off the main stopcock, usually under the kitchen sink, and check it actually turns before you rely on it. Run the shower until no more water comes out, to drop the pressure in the pipework before you open anything up.

SC-02Protect the area and get your tools togetherLay a towel in the shower tray or bath to catch drips and protect the surface, and put the plug in so small screws don't go down the drain. Get an Allen key set, a flat screwdriver, an adjustable spanner, and a torch to see into the recessed valve housing.

1 each · Allen key set

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1 each · Adjustable spanner

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SC-03Remove the temperature handleMost thermostatic handles hide a small grub screw under a cap on the front or underside. Pop the cap off with the flat screwdriver, loosen the grub screw with the correct Allen key — usually 2mm or 2.5mm — and pull the handle straight off. Don't force it; a stuck handle usually just needs a firmer straight pull, not levering sideways.

SC-04Take off the faceplateUnscrew or unclip the decorative faceplate sitting against the tiles. Some pull straight off, others have a ring you turn anticlockwise to release. Keep any screws in a small dish so they don't disappear.

SC-05Note the temperature limit stop positionBehind the handle there's a small ratchet ring that limits how far the handle turns towards hot, to stop scalding. Take a photo of its current position and count the notches back from full hot. You'll need to set the new cartridge to the same position or the limiter won't work properly afterwards.

SC-06Undo the retaining nut and pull out the old cartridgeUse the adjustable spanner or the correct socket to undo the large retaining nut around the cartridge. Some valves use a small U-clip instead — ease this out with the flat screwdriver rather than levering hard. Pull the old cartridge straight out; limescale can make it stiff, so wiggle it gently rather than forcing it sideways.

SC-07Check the new cartridge is the right matchCompare the old and new cartridges side by side — body shape, pipe spacing and model number should match exactly. Wipe out the housing with a cloth and check the O-rings inside aren't torn or missing. If the housing itself is scored or pitted, a new cartridge may still leak and the whole valve could need replacing instead.

1 each · Cloth or rag

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SC-08Fit the new cartridgeSmear a little silicone plumber's grease on the new cartridge's O-rings to help it seat and to protect the rubber. Push it into the housing in the same orientation as the old one, lining up the locating lug or flat edge. Refit the retaining nut or clip by hand, then just a quarter-turn more with the spanner — thermostatic cartridges have plastic bodies and crack if overtightened.

1 each · Thermostatic shower cartridge (matching brand and model)

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1 tube · Silicone plumber's grease

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SC-09Reset the temperature limit stopFit the limit stop ring back on in the same position you photographed and counted in step SC-05. This sets the maximum temperature the shower can reach before the thermostatic element cuts the flow, so it's worth getting right rather than guessing.

SC-10Refit the faceplate and handleRefit the faceplate or ring, then push the handle back on and retighten the grub screw. Snap the small cap back over the grub screw to finish.

SC-11Turn the water back on and check for leaksTurn the isolating valves or stopcock back on slowly, not all at once. Run the shower on hot and cold for a few minutes, checking around the retaining nut and the pipe joints behind for drips. A slight leak at the retaining nut usually means it needs another quarter-turn, not a full retighten.

SC-12Test the temperature limiter worksTurn the handle fully towards hot and check the water stops getting hotter at a safe point rather than climbing dangerously. If it's not right, repeat step SC-09 and adjust the limit stop by one notch at a time until it feels safe, especially if children or elderly people use this shower.

SC-13Tidy upWipe down the tiles and fittings, and dry the shower tray or bath. Most councils don't recycle mixed plastic-and-metal cartridges, so the old one usually goes in general waste — check your local council's rules if unsure. Put your tools away and give the valve one last look for drips before you finish.

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