Symptoms of Faulty Boiler Thermistor

There are many different components in your boiler. Like any other piece of electronic, any piece can easily fail. The Thermistor is one of them. If you notice your boiler not being able to detect when to stop heating water or has problems with heat regulation, it is possible that your thermistor is not working. There are many problems that contribute to the failure of the thermistor. These include;

Limescale build up

Lime-scale builds up when you have hard water, the minerals in that water can cause problems for your boiler. The scale looks like white particles and can get attached to the heat exchanger. The symptoms of lime-scale build-up are noises. You might notice uncommon noises coming from your boiler. Normally you would expect to hear kettling or even whistling. Kettling is when your boiler makes banging or popping noises when turned on.

Boiler lockouts

Your boiler is normally fitted with an NTC thermistor. The flow pipe thermistor monitors the temperature of the water exiting the boiler. If the water temperature is too high, then it will lock out. The water temperature becomes too high when the heat exchanger develops lime-scale. The thermistor recognizes the water temperature is oddly high and shuts the boiler down.


Most boiler leaks are caused by corrosion. Corrosion occurs because the boiler is weakened due to the frequent exposure to water at varying temperatures. Corrosion weakens the pies and tanks over time, eventually leading to leaks. Failure to address corrosion issues as they happen could lead to extensive damage throughout your boiler. Usually, corrosion will prevent itself in a valve or pipe that is easily replaceable.

Excess heat

Usually, boilers overheat due to the failure of safety controls that maintain the gas and water supply, the temperature and the pressure. When the boiler starts to overheat, the interior components of the boiler are put under immense pressure and are likely to suffer from irreparable damage. This is a very dangerous condition for you and your family.

Dirty water when bleeding radiators

Bleeding radiators is when you let out air that has become trapped inside. Trapped air causes the radiators to have cold spots, reducing their efficiency. If you have bled radiators then you will notice that after the air is released, water is released. If the water being released is black, then that is a sure sign that your boiler thermistor has a problem. If it is not repaired quickly, the water can lead to blockage of your boiler.

See similar post: My Boiler is Making Banging Noises

Blocked or frozen condensate

If it is particularly cold outside, with visible frost, and your boiler is refusing to turn on, it could be a sign that your condensate is frozen or blocked. Frozen pipes are dangerous as they can cause major damage to your boiler. One of the symptoms is lack of running water out of the faucet. At times there may be an odd smell coming from your drain. If the pipes are blocked, the only way for the odour to escape is back up in the direction of your property.

Regardless of what the cause is, your thermistor needs to be replaced. It is important that this is done by a qualified person. Get in touch with us today and we can assist you to replace the thermistor and get your boiler working properly again.

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