Quick answer
A mixer that delivers only hot or only cold has a failed thermostatic cartridge or a seized diverter. Modern mixer cartridges are brand-specific and often model-specific, so identifying the make before ordering parts saves a second trip. If the shower ran fine and then changed suddenly, suspect the cartridge; if it has never mixed properly since installation, suspect hot and cold reversed at the wall.
Common causes
- Failed thermostatic or ceramic mixing cartridge
- Scale build-up seizing the diverter or the cartridge internals
- Hot and cold reversed at installation, so the mixer works backwards
- One supply restricted, so the mixer cannot balance against it
- Failed thermostatic wax element in a thermostatic valve
- Debris from supply work lodged inside the cartridge
What you can do now
- 1Check both hot and cold arrive with similar pressure at another nearby fitting
- 2Note the mixer brand and any model marking before sourcing parts
- 3Try the full range of the handle slowly rather than snapping between ends
- 4Check whether the isolation valves feeding the mixer are both fully open
- 5Do not force a seized diverter — the stem shears and turns a cartridge swap into a wall repair
When to call a professional
Mixer cartridges are typically a thirty-minute swap for a plumber who carries the common brands. Call rather than forcing anything, since the shear point on a seized diverter is inside the wall. If the mixer has never balanced since it was installed, ask for the hot and cold feeds to be checked at the wall — reversed connections are a surprisingly common installation error.
Typical cost in Johannesburg
Cartridge R450–R950 supplied
Indicative ranges only. We provide written, itemised quotes before any work.
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