Guide to Boiler Thermistors

A thermistor is a vital component of your boiler and is important for your boiler to heat water to the correct temperature.

If you are not an expert in this area, it might be confusing to understand what a boiler thermistor is. However, it is essential to be informed about the components of your boiler to ensure that it is not malfunctioning.

We’ve put together this guide to boiler thermistors to help you understand everything you need to know.

What Is a Boiler Thermistor and What Does It Do?

A boiler thermistor is essentially a type of resistor that is used in your boiler. A resistor, in simple terms, is a component that can be added to a circuit and connects with two other parts through its two terminals.

A resistor creates electrical resistance in the circuit and this can be used to control different aspects of the circuit, such as current flow, voltage or other signals.

A thermistor is a specific kind of resistor that is controlled through heat. Hence the name combines ‘thermal’, which refers to heat and resistor to form the word ‘thermistor’.

In a boiler, a thermistor connects to the temperature setting of the boiler on one side or terminal. This setting is known as the potentiometer.

From the other terminal, the thermistor is connected to the central component of the boiler that controls many of its functions. This is known as the printed circuit board or the PCB.

The thermistor is designed to create a certain amount of electrical resistance at any given temperature. This provides a signal to the PCB to modify the boiler’s controls, such as the amount of gas being burned, to change the temperature if required.

In other words, the thermistor is used to control the temperature of the water in the boiler and ensure that it is not too high or too low.

There are two kinds of thermistors based on how they function.

NTC Thermistors

NTC thermistors, or negative temperature coefficient thermistors, give a decrease in resistance with an increase in temperature.

PTC Thermistors

PTC thermistors or positive temperature coefficient thermistors increase resistance with an increase in temperature and vice versa.

Where to Fit a Boiler Thermistor

A boiler thermistor can either be touching the water inside the boiler or sensing the heat of the water through a pipe. The boiler thermistor is usually fitted into the pipes through which the water flows out and returns to the boiler.

How to Control the Temperature

The boiler thermistor controls the temperature of the boiler water. On one terminal, it connects to the temperature setting or the potentiometer.

Depending on the temperature, it provides a given amount of resistance. This signals to the PCB to reduce or increase the amount of gas being combusted in the boiler to adjust the temperature of the water.

Additional Functions of a Boiler Thermistor

The boiler thermistor has a few additional functions to protect and maintain the boiler.

One of these is the frost protection function. The thermistor signals to the boiler to turn on automatically if the temperature falls below a certain point to avoid frosting of water inside the boiler.

The thermistor also keeps the pump running for a certain period after the boiler has been switched off. This function ensures that water keeps circulating inside the boiler until the temperature is lowered to a certain level.

This ensures no residual heat that may cause noise and build-up of limescale over time.

Testing and Thermistor Faults

A thermistor provides predictable readings of electrical resistance for given temperatures.

Hence, to test the thermistor, you need first to check the temperature of the water near the resistor. Then, after removing the PCB wires from the thermistor, you need to check the resistance provided by the thermistor using a multimeter.

Compare the readings you get from the test with the readings provided by the expected manufacturer to ensure that the thermistor is functioning correctly.

If the readings show a vast difference from the given chart, then the thermistor is faulty.

Symptoms of a Faulty Boiler Thermistor

If your boiler thermistor provides very unpredictable readings with sharp rises and falls, it might be faulty. Similarly, if the resistance reading does not change smoothly and proportionately to the change in temperature, it might be defective.

There might be external symptoms of your thermistor being faulty as well, such as damage to the wiring, build-up of limescale or leakage of water.

Temperature Sensors

Temperature sensors can be used with NTC thermistors to reduce the time taken to detect the temperature. The temperature sensor is an additional component that can be clipped to the thermistor to provide instant temperature readings.

Temperature sensors are more energy efficient and can provide more precise readings.

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