Controlling a bathroom heater – a design for a simple circuit with a thermostat and safety feature

A warm hello

I’m looking for someone who, as a hobby, could help me with this.
With winter approaching, I’ve finally decided to tackle the faulty heater controller on my bathroom radiator.
After taking the controller apart, it turned out that the circuit board, although very simple in design, is completely corroded and burnt. Admittedly, after cleaning it and removing the (probably) damaged components, it turned out that none of the tracks were broken and it could probably have been salvaged, but I’ve come up with another idea.

Since I want to use an external thermostat to control the temperature in the bathroom anyway, I don’t really need this controller, but…
I could, in fact, connect the heater directly to the mains and fit a thermostat along the way, but there’s still the matter of controlling the temperature of the heating medium (water).

Theoretically, during a very cold winter with a window open in the bathroom, a situation could arise where the water in the radiator would boil and the radiator itself would be damaged by the pressure build-up inside (at best, only the heater itself )

To get to the point, I need a simple circuit that would switch off the heater when the temperature of the fluid inside reaches 90 degrees. There are five wires coming out of the heater itself – three for powering the heater and two from the capillary tube measuring the temperature of the fluid. It would be great if the circuit included two LEDs to indicate the device’s operating status (power supplied from the thermostat and the heater’s operation).

Whilst I do have some manual skills and preparing the circuit board wouldn’t be a problem, I have absolutely no knowledge of designing this type of circuit, which is why I’m asking all hobbyists for help

I’m also not sure if this is the right section; I hope no one will take offence at this. Thank you in advance, and have a lovely evening.

smolam wrote:

and two from the capillary tube measuring the refrigerant temperature.

Does it close these wires when a certain temperature is reached, or does it open them? Or does it change its resistance depending on the temperature?

BILGO wrote:

smolam wrote:

and two from the capillary tube measuring the temperature of the working fluid.

Does it short-circuit these wires when a certain temperature is reached, or does it open the circuit? Or does it change its resistance depending on the temperature??

To be honest, I’ve no idea; I’ll dig out my multimeter tomorrow and have a look.
It’s a heating element for a ladder-type bathroom radiator, model Enix GR-19 900W. As soon as I have any further information, I’ll post it on the forum.

OK, I’ve checked what I could, and here’s what I found:
1. There’s continuity, so I assume the circuit isn’t open
2. The capillary tube has two wires: a thicker one (≈0.8 mm) which is white, and a thinner one (≈0.3 mm) which is red.
3. When I connect the positive lead to the red wire and the negative lead to the white wire, the multimeter reads 105–110 kohm
4. When I connect the positive lead to the white wire and the negative lead to the red wire, the multimeter reads 20–25 kΩ

I hope this clarifies things a bit (the heating element is cold)

That looks like a thermocouple.
It would be useful to measure the voltage and temperature as accurately as possible.
We could then determine the type and characteristics required for control.

Best regards