Why will my bath shower mixer not switch to the shower?
What a diverter actually does, why limescale and worn O-rings stop it, and how to service or replace one without touching the tap body.
- Difficulty
- beginner
- Time
- 40 min
- Read
- 3 min
Short answer
A diverter is a small plunger or cartridge that blocks the spout so water goes up the hose instead. When it will not switch, or water pours from both, it is nearly always scale on the plunger or a perished O-ring. Both are serviceable without removing the tap.
Bath shower mixers have one of two diverter designs: a lift-up knob on the spout that pulls a plunger, or a lever on the body that turns a small cartridge. Both work by sealing the spout outlet, and both fail in the same way.
What you'll need
- Descaler or white vinegar
- An Allen key set and a screwdriver
- Replacement O-rings or a diverter cartridge
Step by step
- Watch what actually happens when you switch.Nothing at the head means the diverter is not sealing the spout. Water from both spout and head means it is sealing only partly. A stuck knob means it is seized.
- Turn the water off at the isolating valves.There should be a valve on each supply under the bath or in the adjacent cupboard. Turn both off and open the tap to prove they have shut.
- Check the shower head first.A heavily scaled head raises the back-pressure so much that the diverter cannot hold the water up the hose. Descale it before dismantling the tap.
- Remove a spout-type diverter.The knob usually unscrews, or the whole spout unscrews from the tap body. Inside is a plunger with a rubber or O-ring seal on a spring.
- Descale and inspect the plunger.Soak it in descaler, work it by hand, and look at the seal. A flattened, hardened or split seal is the cause and is a pence-priced part.
- Service a lever-type cartridge instead.Remove the lever's grub screw, pull the handle, and withdraw the ceramic or brass cartridge. Descale it and replace its O-rings, greasing them with silicone tap grease.
- Use silicone grease, not oil.Petroleum-based grease swells and destroys rubber O-rings. Silicone tap grease is the only correct lubricant here.
- Reassemble and test at low then full flow.A diverter that holds at a trickle but drops at full flow usually still has a partly perished seal.
- Replace the whole diverter if parts are unavailable.Many are model-specific. Photograph it and take the old part to a merchant; a universal replacement rarely fits a branded mixer.
Tips
- Descale the shower head before dismantling anything — it fixes a meaningful share of diverter complaints on its own.
- Keep the diverter switched to bath when not in use. Leaving it up holds the seal compressed and shortens its life.
- Photograph the order of parts as you remove them. Diverter plungers have springs and washers that go back one way only.
Common mistakes
- Forcing a seized diverter knob — The stem is thin brass and snaps inside the spout, which turns a ten-minute O-ring job into replacing the whole tap.
- Greasing O-rings with petroleum jelly or oil — Hydrocarbon grease swells nitrile and EPDM seals until they no longer fit or seal. Only silicone-based tap grease is safe.
If it doesn't work
Water comes from spout and head together
Cause: Diverter seal worn or scaled, not sealing fully — Fix: Descale and replace the plunger seal or cartridge O-rings.
Diverter drops back to bath by itself
Cause: Weak spring or back-pressure from a blocked head — Fix: Descale the head first; replace the spring or plunger if it persists.
Knob will not lift at all
Cause: Seized on limescale — Fix: Soak the spout in descaler in situ if you can; do not force the knob.
Works only at low flow
Cause: Partly perished seal — Fix: Replace the seal — descaling alone will not restore a hardened one.
Questions people ask
Why does my diverter drop back down when I turn the flow up?
Usually a blocked shower head. The restriction raises the pressure under the diverter until it pushes the plunger back down. Descale the head first.
Can I buy a universal diverter?
Generic spout diverters exist and fit many older taps, but branded mixers usually use their own cartridge. Take the old part with you rather than measuring.