Shower is too hot. Cartridge issue?

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Hi. Our shower runs too hot and the temperature is not controllable. It’s only 4 years old though the installer can’t tell me what model it is, unfortunately. That rather complicates the identification of spare parts. The shower is fed by a pump common to a hand basin. That basin has adequate cold flow so my suspicion is that either the shower’s thermostatic cartridge has failed or the cold inlet to the the mixer is blocked somehow.

The indents on the ring around the thermostatic unit (11 o’clock & 5 o’clock positions) suggest a tool like the attached (in silver) is needed to remove the cartridge. I don’t have that - it’s from an out-of-stock eBay listing.

How would you proceed, what would you try - and how? Thanks
 

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1. The filters / non-return valves (if fitted) are likely to be behind the rather flat hexagons above and below the temperature control.
2. Turn off the hot and cold supplies, undo the hexagons, and clean / de-scale the internals.
3. The shower will have some form of adjustment to allow the hot temperature to be set to a normal 38 degrees, with an override provided some form of push in button or lever. Usually takes the form of a stop which fits over the (larger) splines of the cartridge which interacts with a stop within the handle / knob. You adjust the shower temperature to 38 via the smaller splined shaft, then fit the splined stop so that the stop within the handle abuts it until pressed.
 
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Thanks @oldbuffer . I’ve tried removing the hex nut it it’s stuck fast and I’m just rounding off the corners so I’ve stopped that effort. I’ve not tried WD40 on that yet though.

I’ve managed to remove the retaining ring in front of the cartridge without that special tool, just tapping a flat bladed screwdriver with a hammer. The cartridge should theoretically pull straight forward now - but it’s not moving. It does rotate a little with torque from some grips but no movement forward/back yet. Any tips on ideas to try and get it free, please. I appreciate your input here, thank you .
 
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If you're certain that the ring you removed is the only thing holding in the cartridge then by gripping the splined shaft with mole grips and judiciously moving the shaft up, down, and left, right it should tease out. Once out you may find a similar large nut at its far end, this will need to be unscrewed in order to fully dismantle the cartridge, however a soaking in kettle descaler (your location isn't shown so I'll assume a hard water area) overnight may suffice to free-up the mixing valve shuttle and restore operation.
The wax-stat may have failed (similar looking to the old car thermostat wax-stat). Not sure where or if you can buy one individually, however you can test its operation by submerging it into a cup of warm (<60C) water to see if it expands by a few millimetres.

If not dismantled then turn the temperature knob from full hot to cold. No water supply is needed for this test. The adjustment should suddenly feel looser as you turn towards the hot setting. If it's loose all the way then that's a sign that the shuttle is stuck in the 'hot' position.

Down the centre of the adjustment shaft should be a small grub screw, flat blade slot or allen key operated. You'll need H&C water supplies for this test. Screwing it in (clockwise) should increase the shower temperature, and vice versa. Try unscrewing it one turn to see if this cools your shower.
Alternatively re-fix the temperature adjustment knob and rapidly turn it from hot to cold to hot, etc. several times. This may free-off a stuck shuttle.

MM
 
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Thanks @MeldrewsMate - the cartridge was removed just by wiggling with some grips and edging it forward ever so slowly. The cartridge is imaged here. There is indeed a nut at the end. I can’t shift that. Maybe soaking in descaler will help that. We are in north Hampshire - yes, a hard water area. Our water softener is VERY old and I suspect needing to be replaced.
 

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Down the inside of the splined shaft there may be an adjustment screw as I mentioned previously.
The temperature sensitive element is the brass bulb seen at the opposite end with the large nut. Soak the whole assembly in descaler overnight, then immerse the whole cartridge in hot water (but no hotter than 60C). You may see the temperature control bobbin move, maybe only 2 or 3 mm, towards the splined end. Cooling it rapidly in cold water, and repeating this hot cold regime may be sufficient to bring this cartridge back to life. Confirm that the bobbin moves smoothly between the hot-cold cycles before refitting it, and clear any scale from the valve body too...use ONLY silicone grease on the rubber 'o'rings. If the bobbin doesn't move then you'll have to source a new cartridge or remove that large tight nut to replace the thermal element (the wax capsule). Having no manufacturer's name will make sourcing these spares an order of magnitude harder!
Once fitted and before the water is switched on, fit the temperature control knob and turn the temperature setting towards HOT. The shaft should be easy to turn.
This next stage is important.
Slowly turn the temp towards COLD until resistance is felt, this will approximate to room temperature, so assume 20C. Using the knob graduations as a guide turn it back 20C towards HOT. This will approximate the 38C safety setting; ie the point where the temperature adjustment will stop unless the safety button is depressed.
Turn on water supplies - to check for leaks and sillies - before replacing all the shiny bits.
Set the shower to its safety stop position and turn it on. Use the adjustment screw and a decent thermometer to adjust the flow temperature to 38C.
Replace the knob, and check for satisfactory operation.
 

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