Domestic hot water thermistor
The F7 error code on Glow-Worm boilers indicates domestic hot water thermistor.
Glow-Worm F7 means: domestic hot water thermistor. On this component the documented failure modes are the water thermistor has failed — short circuit or open circuit or wiring, connector or harness of the water thermistor damaged or loose. You can safely reset the boiler once — if F7 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does F7 Mean
The F7 code is shown by the boiler’s control board when it detects domestic hot water thermistor. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows F7 and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 8 different manufacturers.
| Cause |
|---|
| The water thermistor has failed — short circuit or open circuit |
| Wiring, connector or harness of the water thermistor damaged or loose |
Safety Warning
F7 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
F7 DHW thermistor fault F8 Tank thermistor fault F9 Water pressure sensor fault Faulty sensor connection Check wiring F10 Central heating return thermistor fault Thermistor cable defective/broken, thermistor faulty Chec
How to Fix It
Measure the resistance of the water thermistor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range.
Unplug and re-seat the connector of the water thermistor, then check the harness for chafing, corroded pins and breaks in continuity.
Refer to the Glow-Worm service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Glow-Worm models that can display fault code F7:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Glow-Worm models | — | Possible |
Frequently Asked Questions
Measure the resistance of the water thermistor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range. According to the technical documentation, this is the first check on the list for F7.
F7 is not classified as a high-severity fault, so the boiler can be reset once. If F7 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.