How to Test a Thermistor and Troubleshoot Boiler Issues

Testing a thermistor is a crucial step in ensuring your boiler operates efficiently and safely. You can use a multimeter to measure the resistance of the thermistor.

First, turn off the power to the boiler and let it cool down. This is an important safety precaution to avoid any potential electrical shocks.

A thermistor's resistance changes in response to temperature fluctuations. This property makes it an ideal component for temperature-sensing applications.

To troubleshoot boiler issues, start by checking the thermistor's continuity with a multimeter. If the thermistor is faulty, it may not provide accurate temperature readings.

A faulty thermistor can cause the boiler to malfunction or fail to heat properly. This can lead to costly repairs and even safety hazards.

Broaden your view: How Thermistor Is Used to Measure Temperature

There are two main types of thermistors: NTC and PTC.

NTC thermistors decrease in electrical resistance as the temperature increases.

A PTC thermistor, on the other hand, shows an increase of electrical resistance when the system water increases in temperature.

Here's a quick comparison of the two:

Let's take a closer look at the two types of thermistors.

There are two main types of thermistors: NTC and PTC.

An NTC thermistor is a type of thermistor whose resistance decreases when the temperature it's exposed to increases. This is a crucial characteristic to check when testing an NTC thermistor.

Here are the two types of thermistors:

The key difference between these two types is how their resistance changes with temperature.

PTC thermistors are unique in that they show an increase in electrical resistance when the system water increases in temperature.

At room temperature or below, a PTC thermistor measures a very low resistance with an ohmmeter.

They are less common than NTC thermistors, with characteristics that are identical to look at but behave differently.

A PTC thermistor's resistance can rise to 1600 Ohms from a starting point of 800 Ohms as the temperature increases.

If a PTC thermistor's resistance does not increase steadily when heat is applied, it is faulty and should be discarded.

Related reading: Thermistor and Temperature

A thermistor is a crucial component in a combi boiler, responsible for monitoring the temperature of the water and controlling the heating elements accordingly. It's used for overheating protection, temperature sensing, flow rate monitoring, efficiency control, safety control, fault detection, energy management, hot water demand control, circulating pump control, and system balancing.

A thermistor can also indicate when the temperature gets too cold, triggering the boiler to ignite when the resistance reading drops below a certain level. This is known as frost protection.

Here are some key functions of a thermistor in a combi boiler:

A thermistor plays a crucial role in a combi boiler, serving as a safety device to shut down the system in the event of an over-temperature condition. The thermistor monitors the temperature of the water in the boiler and controls the heating elements accordingly.

The thermistor is also used for temperature sensing, measuring the temperature of the water and adjusting the heating elements to optimize efficiency. This is especially important for ensuring the system performs well and doesn't waste energy.

Discover more: Temperature Control Using Thermistor

One of the key functions of a thermistor in a combi boiler is to monitor the flow rate of water through the system. This helps to adjust the heating elements to ensure the water is heated evenly and efficiently.

The thermistor can also detect faults in the heating system and trigger an alarm to alert the user. This is a vital safety feature that can prevent damage to the boiler and ensure the user's safety.

Here are some of the key functions of a thermistor in a combi boiler:

The thermistor can also be used for frost protection, igniting the boiler when the resistance reading drops below a certain level. This helps to prevent the boiler from freezing in cold temperatures.

If your boiler is malfunctioning, it's essential to identify the root cause. Scale or magnetite deposits can affect heat transfer and sensitivity, causing issues with your boiler's thermistor.

System water ingress can cause an open circuit or infinity, leading to a faulty boiler. I've seen this happen when a leak is not addressed promptly.

Loss in manufacturers calibration can also affect the thermistor's performance. This is often due to wear and tear over time.

Damaged terminals, connections, or wiring faults can also cause problems with your boiler's thermistor. These issues can be tricky to diagnose, but checking the wiring is a good place to start.

Here are some common symptoms of a faulty boiler thermistor:

To test a thermistor, you'll need to electrically isolate the boiler or system controls to prevent any stray resistances from affecting the results.

First, ascertain the system water temperature near the thermistor using a thermal laser or clamp on thermal sensor.

Remove the thermistor wires at the p.c.b end to prevent any potential interference.

Set your multimeter on the ohms scale for resistance readings.

For PTC thermistors, the black lead from the multimeter must go on the metal casing of the sensor and the red lead from the multimeter must go on the terminal of the sensor.

Resistance readings must be cross-referenced to the manufacturer's specification to ensure correct calibration.

A closed circuit of less than 1 ohm indicates a faulty thermistor.

No resistance or an infinite reading also indicates a faulty thermistor.

Here's a table showing example NTC resistance values:

To check if a thermistor is working properly, start by measuring its resistance at room temperature using a multimeter. Set the multimeter to the ohmmeter and let the test leads touch the two pins of the thermistor. The reading on the multimeter is the resistance value of the thermistor at room temperature.

If the reading is zero or infinity, it indicates that the thermistor has been damaged. You can also use a thermal effect to check the thermistor's functionality. Place an electric soldering iron close to the thermistor and observe the resistance displayed by the multimeter. If the resistance has changed from the normal temperature resistance value and returned to the normal value when the iron is moved away, the thermistor is still effective.

For a more thorough test, you can perform a low temperature check. Clamp the two pins of the thermistor with a multimeter clamp and put the thermistor into the refrigerator. For a negative temperature coefficient thermistor, the resistance value displayed by the multimeter should be larger than the resistance value at room temperature; for a positive temperature coefficient thermistor, the resistance value displayed by the multimeter should be lower than the resistance value at room temperature.

Check this out: Negative Temperature Coefficient Ntc Thermistor

You can also check the thermistor's appearance for any signs of damage or quality issues. A normal thermistor should have clear printing on the shell, no cracks or expansion of the shell, and no rust on the pins. If you notice any of these issues, the thermistor may have quality problems.

Here's a summary of the steps to check a thermistor's functionality: