Is Your Boiler Thermistor Faulty? Warning Signs You Shouldn’t Ignore
Has your boiler started showing erratic temperature readings or begun switching on and off without warning? A faulty boiler thermistor could be the root cause of these heating system problems.
Thermistors serve as your boiler’s temperature monitoring system, and when they malfunction, they disrupt your entire heating operation. Many homeowners miss these early warning signs until they face more serious complications – often leaving them without proper heating or hot water during the coldest months.
At National Boiler Spares, we know how important it is to identify these issues before they escalate. Our technical specialists have seen how small components like thermistors can cause significant disruption when they fail. In this guide, we’ll show you the clear signs of thermistor failure, explain their crucial role in your heating system, and provide practical steps to take when you suspect a problem.
To find the part you need, use the search bar at the top of this page. Click here to return to the top. You can search using the unit’s part number or its GC (Gas Council) Number – a unique identifier used for gas appliances, including boilers.
“If your boiler keeps overheating or refusing to fire, it could be a faulty thermistor—sort it before it trips more serious faults.” explained Kerem Demir – Technical Specialist at National Boiler Spares.
Radiant Combi Temperature Thermistor 73507La
What happens when a boiler thermistor fails?
When a thermistor fails, it creates significant boiler malfunctions that affect your entire heating system. Knowing what goes wrong and why helps you spot problems early before they lead to costly repairs or complete system failure.
How a thermistor controls boiler temperature
Your boiler depends on thermistors—small but essential electronic components that measure temperature through changes in their electrical resistance. As the temperature increases, the thermistor’s resistance decreases, sending vital signals to your boiler’s control board.
These small components act as the temperature monitoring system for your boiler, constantly feeding temperature data to the PCB (Printed Circuit Board). This continuous monitoring allows the boiler to maintain ideal water temperatures—typically between 60-80°C for central heating and 35-65°C for domestic hot water in combi boilers.
“Thermistors play a crucial role in safe boiler operation,” Debbie Clarke – Technical Specialist at National Boiler Spares explains “They prevent dangerous overheating and ensure consistent temperatures throughout your system. Without properly functioning thermistors, your boiler simply cannot determine when to fire up or shut down safely.”
Flame staying on briefly – As noted in one review, the boiler “would not keep flame on for more than 5 seconds at a time”
Impact on combi boilers and heating systems
For combi boilers, thermistor problems create particular challenges because these units handle both heating and hot water through one system. A faulty thermistor often triggers complete system shutdown as a safety measure.
Beyond immediate heating problems, ongoing thermistor issues force your boiler to work inefficiently. This increases energy consumption and accelerates wear on critical components like the heat exchanger and pump.
“Thermistors are inexpensive but essential—don’t waste money chasing faults when a simple sensor swap can sort it.” explained Jaspreet Dhillon – Mechanical Engineer at National Boiler Spares.
Customer experiences often tell us more about service quality than any technical specification. At National Boiler Spares, we’ve helped countless homeowners solve their boiler thermistor problems. Here’s what some of them have to say about their journey from heating frustration to reliable warmth.
‘Engineer said it was the flow thermistor – he was right’
Many of our customers come to us following a professional diagnosis from their heating engineer. One reviewer described the persistent cutting out of their system, noting that “The engineer said it was the flow thermistor” – precisely the type of specific component issue we specialise in resolving.
Another customer shared the classic symptoms of thermistor failure that many homeowners experience: “It would not keep flame on for more than 5 seconds at a time.” After fitting our replacement part, their heating system returned to full, reliable operation.
‘Quick delivery, easy install, problem solved’
Trevor’s feedback demonstrates our commitment to prompt, effective solutions: “I was advised to contact NBS by my heating engineer;
Our technical specialist points out: “Thermistor faults typically require straightforward fixes with the right parts. We maintain extensive stock of both common and rare components, ready for next-day delivery to minimise your downtime.”
Ideal Domestic Hot Water (DHW) Thermistor Kit 170996
How NBS helps you fix boiler thermistor faults
Finding the right thermistor for your boiler is straightforward with the right support. At National Boiler Spares, we’ve built a practical support system designed to get your heating back up and running with minimal hassle and downtime.
“Always test resistance at room and operating temperatures—guesswork with thermistors leads to misdiagnosis.” commented Amir Hussain – Technical Specialist at National Boiler Spares.
“When tracking down a potential thermistor issue, first look for erratic temperature readings on your boiler display,” Lewis Chapman – Technical Specialist at National Boiler Spares explains. “You can also use a multimeter to test resistance values—a properly functioning thermistor should show decreasing resistance as temperature increases. If readings don’t change or show ‘infinity,’ you’re almost certainly dealing with a faulty unit.”
Our expert emphasises proper installation technique as well: “Always apply a small amount of thermal paste when fitting a new thermistor. This ensures proper temperature transfer between the component and the pipe surface, giving you accurate readings and reliable operation.”
Boiler thermistor FAQs answered
Homeowners frequently ask questions about boiler thermistors. Here are clear, practical answers to the most common queries about these vital temperature sensors.
What is the difference between flow and return thermistors?
Flow thermistors monitor water temperature as it leaves the boiler‘s heat exchanger, controlling when the burner ignites or shuts down. Return thermistors measure the temperature of water returning to the boiler after circulating through your radiators. Working together, these components maintain optimal system efficiency by keeping the temperature difference within safe operating parameters.
“Replace in pairs if you’ve got both flow and return thermistors—if one’s failed, the other may not be far behind.” commented Lewis Chapman – Technical Specialist at National Boiler Spares.
While thermistor replacement is relatively straightforward for those with basic DIY skills, we recommend professional installation unless you’re confident in your abilities. The main challenge lies in accessing the component, which varies significantly between boiler models. Always ensure you completely disconnect power before attempting any replacement work.
How much does a boiler thermistor cost?
Thermistor prices typically range from £15-£40 depending on your specific boiler model. At NBS, we offer refurbished options that provide substantial cost savings without compromising on quality or reliability.
Conclusion
Faulty boiler thermistors create widespread disruption to your heating system, causing temperature fluctuations and frequent system shutdowns. Identifying these problems early prevents more serious complications and helps you avoid costly repairs.
At National Boiler Spares, we maintain an extensive inventory of thermistors ready for immediate dispatch. Our next-day delivery service means minimal heating disruption during the coldest months. We’re particularly proud of our refurbished options, which deliver substantial cost savings while maintaining the quality and reliability you need.
Our customers value our understanding of the urgency behind heating problems. When your home is cold, you need solutions quickly. That’s why we’ve built our technical support team to guide you through diagnosis and help identify the exact replacement part for your specific boiler model.
Proper thermistor function sits at the heart of your boiler’s efficiency and safety systems. Without accurate temperature readings, your heating system simply cannot operate reliably or efficiently. Trust NBS to provide the genuine parts and expert guidance needed to maintain optimal performance throughout the heating season.
Need expert assistance? Contact our technical team via WhatsApp click here.
FAQs
Q1. What are the common signs of a faulty boiler thermistor? Common signs include inconsistent water temperature, frequent boiler cut-outs, short cycling (boiler turning on and off repeatedly), specific error codes on modern boilers, and the flame staying on for only brief periods.
Q2. Can I test a boiler thermistor at home? Yes, you can test a thermistor using a digital multimeter set to measure resistance. Disconnect the thermistor, connect the multimeter probes to its terminals, and check the reading at room temperature. The resistance should decrease when you gently warm the thermistor with your fingers.
Q3. How much does it typically cost to replace a boiler thermistor? Boiler thermistor prices generally range from £15 to £40, depending on your specific boiler model. Some suppliers offer refurbished options that can provide significant cost savings without compromising on quality.
Q4. Is it necessary to use thermal paste when installing a new thermistor? Yes, thermal paste is essential when installing a new thermistor. This heat-conductive compound ensures proper thermal transfer between the thermistor and the component it’s monitoring, leading to accurate temperature readings and efficient system operation.
Q5. What’s the difference between flow and return thermistors in a boiler? Flow thermistors monitor the water temperature as it leaves the boiler’s heat exchanger, controlling when the burner ignites or shuts down. Return thermistors measure the temperature of water coming back into the boiler after circulating through the radiators. Together, they help maintain optimal system efficiency.
