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+===== Repairing a Cold Shower: Remeha W21 ECO =====
+(translated by a GenAI program, NL version is original text)
+=== Introduction ===
+Our Remeha W21 ECO central heating boiler (21 stands for 21 kilowatt power) provides both central heating and domestic hot water.
+It has been doing this for at least 20 years. (although we replace the expansion vessel every 5 years due to defects, not a costly job)
+However, this morning it was an unpleasant surprise: an ice-cold shower.
+Brief description of how this high-efficiency (HR) combi boiler works.
+When there is a demand for hot water (signaled by the flow sensor), a 3-way valve changes the water flow from the central heating to a heat exchanger, which then delivers the hot tap water.
+When there is no hot water demand, the three-way valve will be positioned towards hot water preparation to be able to react quickly to tap demand.
+If the 3-way valve no longer works, or the flow sensor is malfunctioning, no hot tap water will flow.
+Also, a prankster who sets the thermostat to "frost protection" will cause a cold shower.
+Our indoor temperature is 25 degrees Celsius, it is summer, the central heating is currently not heating the house, so hot water demand only comes from tap water.
+=== Initial Diagnosis ===
+We already know a lot:
+  * The system has worked for years without failure.
+  * Wiring is properly installed.
+  * The communication from the boiler to the thermostat is in order, which means that almost everything in the system is working.
+  * Thermostat seems to be okay (no empty battery?)
+  * Boiler has mains power etc.
+  * Cold water in the shower can also be caused by someone setting the Chronotherm thermostat to "frost protection", then the tap water is no longer heated, "user error" is ruled out.
+=== Problem Identification ===
+The display of our Chronotherm4 thermostat showed a flashing error message.
+A quick reset of the boiler did not solve the problem.
+In fact, the boiler immediately showed a fault again, with error codes 3 and 6 flashing on the display.
+{{ :nl:w21ecoimg_0560.jpg?600 |}}
+==== Checking all the standard things ====
+A visual inspection gave no clarity:
+  * water pressure in the central heating system, manometer reads 1.5 bar (which is within the margins)
+  * all plugs and terminals were in place
+  * all indicator lights were lit as they should be
+  * All fuses checked
+  * no stench
+  * no leaks
+  * no gas smell
+  * fan is OK, starts running during reset, also circulation pump is OK, pump makes noise (speed is adjustable via button)
+This was followed by an online search, which led me to the technical documentation of Remeha.
+{{ :blog:remeha-cv-ketels-w21-w28-eco-technische-informatie.pdf |}} local copy.
+=== Error Codes ===
+From the technical documentation, it appeared that the error codes have the following meaning:
+<WRAP center round info >
+  * **3 1.** Short circuit supply temperature sensor (upper sensor)
+  * **3 2.** Short circuit return temperature sensor (lower sensor)
+  * **3 6.** Supply temperature sensor not connected or defective
+  * **3 7.** Return temperature sensor not connected or defective
+</WRAP>
+<WRAP center round alert 60%>
+Before you start screwing and measuring: remove the mains power! Do this first!
+</WRAP>
+{{ :undefined:sensor_w21eco_img_0549.jpg?600 |}}
+The sensors are connected to the X3 connector of the main circuit board.
+{{ :nl:selection_001.png?600 |}}
+{{ :nl:w21ecoimg_0558.jpg?600 |}}
+<WRAP center round info >
+**Checklist and Explanation:**
+. **What is a Thermistor?**
+  * An electronic component whose resistance changes with temperature.
+. **What does NTC mean?**
+  * Negative Temperature Coefficient: as the temperature increases, the resistance decreases.
+. **What does "12K" mean for an NTC?**
+  * The thermistor has a resistance of 12 kilo-ohms at a reference temperature of 25°C.
+</WRAP>
+**How does the resistance vary with temperature?**
+  * Below is a table showing the resistance of a 12kOhm NTC at different temperatures:
+<datatables>
+^ T [°C] ^ R [Ω]      ^
+|  -15   |    76020   |
+|  -10   |    58880   |
+|  -5    |    45950   |
+|  0     |    36130   |
+|  5     |    28600   |
+|  10    |    22800   |
+| 15     |    18300   |
+| 20     |    14770   |
+| 25     |    12000   |
+| 30     |    9805    |
+| 35     |    8055    |
+| 40     |    6653    |
+|45      |   5522     |
+|50      |   4609     |
+|55      |   3863     |
+|60      |   3253     |
+|65      |   2752     |
+|70      |   2337     |
+|75      |   1994     |
+|80      |   1717     |
+|85      |   1467     |
+|90      |   1266     |
+|95      |   1096     |
+|100     |   952      |
+</datatables>
+<WRAP center round tip 60%>
+ **What do I measure in case of a fault (open or closed connection)?**
+   - With an **open connection** you measure infinite resistance (no continuity).
+   - With a **closed connection** (short circuit) you measure almost 0 ohms.
+</WRAP>
+{{ :nl:mm.jpg?600 |}}
+===== The Power of a Wiring Diagram =====
+What a relief to have a complete wiring diagram at hand!
+In my opinion, this should be standard with every device. In the past, this was common; every device came with a diagram, which made it possible for the user to identify the location of a possible fault themselves.
+===== The Solution =====
+After thorough investigation, it was determined that the upper sensor (supply) was defective, as the measurement indicated infinite resistance.
+This concerns an NTC sensor with the specifications **12K/007**. The official name is **Remeha temperature sensor** for all boilers **44698 (S44698)**.
+During measurements, it was observed that the sensor is supplied with **5 Volts**, and it turned out that it was interrupted.
+By applying a little force and wiggling the wiring, the sensor fell apart.
+By short-circuiting the sensor wires and then restarting, the expected error code **3 1.** was generated.
+This is what a new one looks like:
+{{ :blog:ntc_12k_remeha_7713001.gif?300| }} also called 12K3007.
+After a short online search, I found several suppliers, including WarmteService, which is fortunately nearby. Within an hour I already had a new sensor in hand.
+When testing, the new sensor showed a resistance of about 8.5 kOhms at the current outside temperature (it is summer after all).
+To my surprise, the sensor was accompanied by no less than 10 information sheets, with explanations and diagrams for installation in various Remeha boilers. I was really impressed by this excellent service.
+==== Closing Remarks ====
+With the help of the wiring diagram and some online research, I was able to quickly trace and fix the problem.
+This adventure illustrates how a mix of technical insight, methodical working and the use of basic tools (such as a Phillips screwdriver and a size 17 wrench) can be of invaluable value in tackling everyday challenges.
+Thanks to a modest time investment (30 minutes) and the purchase of a new sensor (priced online between 5 and 25 euros), we were able to prevent a cold shower (in the form of purchasing a completely new boiler).
+The service surrounding the Remeha sensor was excellent: prompt delivery and accompanied by all necessary information. Although they did not sell us a new boiler now, they certainly made an impression. When the time comes for a new boiler, I will definitely remember this positive experience.
+All in all, I see this as a win-win scenario!
+Love to all from #OETELX