Understanding the H3 Fault on IDEAL ISAR Boilers
Although the IDEAL ISAR range is now considered outdated, many of these boilers are still in operation across the UK.
One of the most common issues Gas Safe engineers encounter is the H3 fault, which is usually linked to the DHW (Domestic Hot Water) thermistor.
This guide is intended for Gas Safe registered engineers who are carrying out diagnostic and testing work on these older systems.
Fault Code: H3 – DHW Thermistor
The H3 fault typically indicates a failed or out-of-range DHW temperature sensor, located on the plate heat exchanger.
This sensor monitors the water temperature and adjusts the burner output accordingly. When it fails or reads incorrectly, the boiler will not deliver stable hot water and will display the H3 error.
Step-by-Step: Testing the DHW Thermistor
Tools Required
- Multimeter set to the Ohms (Ω) scale
- Standard hand tools
- Replacement DHW thermistor (if required)
Procedure
- Isolate the cold mains supply and ensure the boiler is safe to work on.
- Locate and remove the DHW thermistor from the plate heat exchanger (accessible from the underside of the boiler).
- Set your multimeter to measure resistance (Ω).
- Record the resistance reading and compare it to the expected values based on water temperature.
Expected Thermistor Resistance Readings
| Water Temperature | Resistance Range (Ohms) |
|---|---|
| 25°C | 9,700 – 10,300 Ω |
| 60°C | 2,400 – 2,600 Ω |
| 85°C | 1,000 – 1,100 Ω |
If your readings fall outside these ranges or if the thermistor shows an open circuit, it should be replaced.
Final Steps and Verification
After replacing the DHW thermistor:
- Reset the boiler using the control panel.
- Run a hot water demand test to verify correct operation.
- Confirm that the H3 fault code clears and stable DHW temperature is restored.
Important Reminder
All internal diagnostics and component replacements must be performed by Gas Safe registered engineers only.
Unqualified work on gas appliances is dangerous and illegal under UK law.
Stay Professional with C4UK
At C4UK, we work with Gas Safe engineers across Surrey and London, providing trusted maintenance, servicing, and compliance certification — including:
- Gas Safety Certificates (CP12)
- Boiler servicing and breakdown repairs
- EICR and EPC testing
👉 Learn more about our professional certification services
Contact Certificates4UK Ltd
Need a reliable Gas Safe engineer for boiler repairs, safety checks, or certification? We’re here to help.
We cover:
Molesey • Walton • Kingston • Richmond • Wimbledon • Surrey • South West London
Before replacing any parts or booking costly work, let one of our qualified engineers carry out a professional inspection to save you time and money.
Booking Options:
- 🌐 Book Online – Simple, quick, and available 24/7
- 📞 Call: 07869 520044 or 07596 076700
- 📧 Email: info@certificates4uk.co.uk
📍 Proudly serving Molesey, Surrey, and London, we specialise in:
Boiler repairs, Gas Safety Certificates (CP12), Installation, plumbing, EICR testing, EPC inspections, and full property compliance.
⭐ With over 14 years of experience and 120+ five-star reviews, Certificates4UK Ltd is the trusted choice for safe, professional, and cost-effective heating services.
Leave a Reply