Fitting an under-sink water filter yourself
Yes, in most cases. If your cold supply already has an isolation valve and you are feeding an existing tap, this is a competent DIY job that takes an afternoon and needs no specialist tools.
There are two situations where you should stop, and both are about the tap rather than the filter: a new hole in a stone or composite worktop, and replacing your existing mixer with a three-way tap.
What you need
An adjustable spanner, a proper tube cutter — not scissors and not a knife, because a square cut is the whole basis of a sound push-fit joint — PTFE tape, food-safe silicone grease, a bucket and more towels than you expect.
A torch matters more than you would think. Under-sink cupboards are dark, and every leak you fail to spot is one you will find later on the cupboard floor.
The mistakes people actually make
Not fitting isolation valves. If the kit does not include them, buy them. Without valves either side of the housing, every cartridge change for the next ten years means shutting the water off to the whole house.
Overtightening the housing. These are hand-tight plus a nip. People reef on them with the spanner and crack the bowl, which then fails under mains pressure at an inconvenient moment.
Skipping the flush. The first water out of a new carbon cartridge is grey with carbon fines and tastes of nothing good. Ten litres to waste, and then it is fine.
Not greasing the O-ring. The housing will seize, and in six months you will be trying to open a sealed plastic vessel with a strap wrench. Food-safe silicone grease, every time.
Forgetting to check the next day. Compression fittings sometimes settle and need a small nip up. Kitchen paper under each joint overnight will tell you.
When to stop and call someone
There is no isolation valve on the cold feed. Fitting one means cutting into live pipework, which is a different job with a different risk profile.
The worktop is granite, quartz, composite or solid surface and you need a new hole. Get it drilled professionally. A chipped stone worktop is not repairable and the replacement costs many times the plumber's fee.
You are fitting a three-way tap. That means removing a working mixer and connecting three supplies in a confined space, usually lying on your back in a cupboard. It is doable but it is a much bigger job than the filter itself.
The pipework is old copper, or you suspect lead. Disturbing old pipework has a way of turning a two-hour job into a two-day one.
Step by step
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Check you have an isolation valve
Look at the cold feed to your kitchen tap. There should be a small valve with a screwdriver slot or a lever. If there is not one, stop — fitting one is the first job, and it is the point at which many people decide to call a plumber.
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Turn off and drain
Close the isolation valve, then open the cold tap to release the pressure and drain the standing water. Put a bucket and towels underneath before you do anything else.
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Mount the housing
Fix the bracket to the side or back of the cupboard, not to the pipework. A full housing is heavy, and hanging it on the pipes works the joints loose over months.
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Connect the feed
Fit the supplied adapter or tee to the isolation valve, and run push-fit tubing from there to the housing inlet. Cut the tube square with a tube cutter and push it fully home — a tube cut at an angle will hold for a fortnight and then let go.
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Connect the outlet
Run tubing from the housing outlet to your filter tap, or to the three-way tap's filtered inlet. Keep the runs tidy and avoid tight bends that kink the tube.
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Fit the cartridge
Grease the O-ring with food-safe silicone grease, seat the cartridge, and hand-tighten the housing. Use the spanner only to nip it up — overtightening cracks housings.
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Open up slowly
Open the isolation valve about a quarter turn and listen. Check every joint with a dry finger before opening it fully.
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Flush it through
Run at least ten litres through the new cartridge and discard it. The first water carries fine carbon dust and will look grey. This is normal and it clears.
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Check again the next day
Put a sheet of kitchen paper under each joint and look at it in the morning. A slow weep shows up on paper long before it shows up as a puddle.