Replacing the thermostat cartridge in a (built-in) shower mixer tap
In this blog post, we explain how to replace a thermostat cartridge in a shower or bath mixer. A thermostat needs to be maintained and disassembled annually for descaling. The amount of limescale in Dutch water has increased dramatically in recent years. Water consumption is now so high that filtering drinking water isn't fast enough, and many additives like lime and bleach are added to purify the water.
Replacing a thermostat is a serious job and should be carried out precisely according to a few steps, preferably by a certified plumber with the necessary experience. We'll also briefly explain what a thermostat does and what its function is.
This manual is suitable for :
comparable built-in shower thermostatic mixer box systems with two or three outlets (push buttons; (rain shower / hand shower / bath tap outlet) and a round or square temperature knob with 38 °C safety stop.
Duration : ± 30 min. working time, 1 person.
Required tools & materials
|
Tools |
Remark |
|
Water pump pliers or socket wrench 30 mm |
For the cartridge retaining nut |
|
Allen keys (usually 2 mm & 3 mm) |
For the adjustment screw in the temperature knob |
|
Flat screwdriver |
To carefully lift the cover plate |
|
Hair dryer or heat gun (optional) |
Makes it easier to loosen cemented plates |
|
Silicon grease |
For O-rings and seals |
|
Thermometer |
For the zero point calibration |
|
Old toothbrush / soft brush |
To clean calcium in the valve housing |
|
Bucket & cloth |
Collect residual water / keep workplace dry |
Safety measures
1. Turn off the water supply
Close cold and warm main connection or the service taps in the meter cupboard.
2. Release pressure
Press one of the buttons to release the line pressure.
3. Check temperature
Check that the tap is completely depressurized; no more water should come out.
What exactly is a thermostat cartridge?
A thermostat cartridge (also called a thermostatic element or "cartridge") is the heart of a thermostatic shower or bath faucet . It is a compact, replaceable module—usually made of brass with a plastic casing—that mixes hot and cold water in a single motion. and continuously corrects so that the set exhaust temperature remains the same.
Above, you see a complete built-in thermostatic mixer tap. The thermostat cartridge is located in the bottom center. On both sides, you see a gold-colored round screw. These are the adjusting knobs for hot (left) and cold (right) water supply. These allow you to adjust the amount of hot or cold water. Never overtighten these adjusting screws. Also, make sure you turn off the water supply before adjusting these. This should definitely be done by a professional.
How does a shower thermostat work?
|
Element |
Function |
|
Thermal sensor (wax element or bimetal spring) |
Expands with heat ↔ contracts with cold. This change in length drives the rest of the mechanism. |
|
Slide/piston (shuttle) |
Moves by sensor force and thus opens or squeezes the hot/cold water channels. |
|
Feather(s) |
Serve as a counterforce so that the sensor is always in the middle of its stroke. |
|
O-rings & filters |
Keep the mixing section leak-tight and trap dirt particles. |
|
Mechanical 38°C safety stop |
A cam or catch that prevents you from unintentionally turning above ± 38 °C; you can only go hotter after pressing it. |
When the sensor detects the outflowing water getting warmer than the setpoint, it expands → the piston moves toward the hot inlet and squeezes it, while the cold inlet opens. When the water gets colder, the exact opposite happens.
What are the main functions of a shower thermostat cartridge?
1. Constant temperature
Lower water pressure because someone else turns on a tap? The cartridge corrects this within fractions of a second so you don't even notice.
2. Safety against burns
If the cold water supply fails, the cartridge almost completely shuts off the hot side; conversely, it stops the entire flow if the hot water fails.
3. Comfort & water/energy savings
Adjusting once is enough; no more fiddling around with turning until it's nice and toasty → less waste while "getting up to temperature".
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4. Adjustable zero point
Because hot and cold water temperatures differ per home, you can calibrate the cartridge (hence the adjustment screw under the knob).
When should a thermostat cartridge be replaced?
· Temperature fluctuates or no longer gets hot enough.
· Button is hard to operate or is blocked.
· Tap is > 7-10 years old and there is a lot of limescale or dirt in it.
A new cartridge solves 90% of these types of complaints and can usually be installed in 10–30 minutes (see the previous manual).
In short: The thermostat cartridge is a temperature-controlled mixing valve in a miniature format. Thanks to its sensor element, it continuously regulates the hot/cold ratio, prevents sudden temperature fluctuations, and protects against scalding – the difference between a standard mixer tap and a modern, safe thermostatic valve.
TIP: If you've purchased and installed a new shower with thermostatic controls, it's important to turn it all the way to the cold and hot settings, the extreme settings, each time you shower. This prevents limescale from blocking the thermostat.
Dismantle – clear access path
1. Remove buttons
Pull the three black push buttons straight off (secured with an O-ring, so apply a little force). Some mixer taps have a different type of push button, such as a rotary knob; this can be loosened with an Allen screw in a hole at the bottom.
2. Remove temperature knob
O Find the small Allen screw at the bottom.
O Unscrew 1-2 turns and pull the knob off.
3. Loosen the cover plate
O First, carefully cut loose the silicone sealant all around.
O Pry the plate away evenly with a flat-head screwdriver, working from top to bottom.
Tip: Gently warm the sealant with a hairdryer – this reduces the risk of tile damage.
You are now looking at the brass valve block ; the thermostat cartridge is centrally located (large round nut).
Remove thermostat cartridge
1. Loosen the lock nut
O Place the water pump pliers or 30 mm socket wrench on the hexagonal nut.
O Turn to the left (counterclockwise) until the nut loosens.
2. Pull out the cartridge with some force, but pull straight.
O Grasp the brass stem or pull the plastic rim with two fingers.
O If it won't come loose, turn it half a turn left and right to break the O-ring loose.
3. Inspect & clean
O Check the valve body for limescale.
O Brush clean and rinse briefly with a plant sprayer.
Install new thermostat cartridge
1. Grease O-rings
O Leave a thin layer of silicone grease; this will facilitate later service.
2. Correct alignment
O Note the lugs/slots – the cartridge will only fit in one position.
3. Tighten the lock nut
O Hand-tighten + 1/8 turn with a wrench. Do not overtighten; brass is soft.
Calibrate temperature
1. Water open again and bleed (press the “hand spray” button).
2. Test temperature measurement
O Turn knob all the way to hot, then back until the mechanical 38°C stop clicks.
O Measure the outgoing temperature with a thermometer in the beam.
3. Adjust
O Are you not exactly at 38°C?
§ Pull off the knob.
§ Turn the serrated bushing on the cartridge slightly left/right (1 tooth = ± 2 °C).
§ Press button back and check again.
Assembly in reverse order
1. Cover plate replace (apply new sealant, allow to dry for 24 hours).
2. Three push buttons press (click-fit).
3. Temperature knob secure with the Allen screw (stop at the 38 °C marking).
Final inspection
· Leak-proof ? Look around the plate with a flashlight.
· Function ? Tests all three outputs, both cold and warm.
· Safety stop ? Temperature only rises after pressing/bypassing.
Common problems & solutions
|
Problem |
Possible cause |
Solution
|
|
Button feels stiff after installation |
O-ring dry |
Loosen again, grease lightly |
|
Temperature fluctuates |
Dirty filters in cartridge |
Remove cartridge, soak in vinegar for 15 minutes, rinse well |
|
Water continues to flow after turning off |
Diverter cartridge contaminated |
Same disassembly principle, but top instead of central |
Tip
Keep the old cartridge for the first few weeks; if the factory cartridge turns out to be defective, you have a temporary backup solution.
Good luck with the job! If you have any doubts about the seals or the condition of the valve body, contact a certified installer to be sure.
