Glow Worm
The Glow Worm Energy 25c is a 25.2 kW natural gas combi boiler with a seasonal efficiency of 99.6% and a domestic hot water flow rate of 12.1 litres per minute at a 30°C temperature rise.
The Glow Worm Energy 25c is a wall-hung natural gas combi boiler designed for domestic heating and hot water in UK properties. This appliance provides a maximum central heating output of 18.3 kW and a domestic hot water output of 25.2 kW. It features a condensing design for high efficiency and is equipped with integrated controls for user operation. This page details its technical specifications, installation requirements, and common fault codes for heating engineers and homeowners.
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Full technical specifications extracted from the manufacturer brochure. Empty sections are hidden.
Diagram on page 9 shows connections for heating flow, hot water connection, gas connection, cold water connection, heating return, and flue pipe connection.
Heating water quality must be checked and treated if necessary (pH value between 6.5 and 8.5). Filling and supplementary water hardness must be checked and treated if system volume exceeds three times nominal volume or guideline values are not met. Use of suitable additives (Fernox, Sentinel) is recommended.
The Glow Worm Energy 25c is a natural gas condensing combi boiler, providing a central heating output range of 5.0 to 18.3 kW and a domestic hot water output of 25.2 kW. Its seasonal efficiency is rated at 99.6%. The appliance delivers a domestic hot water flow rate of 12.1 litres per minute at a 30°C temperature rise. Physical dimensions are 700mm height, 390mm width, and 280mm depth, with a dry weight of 32 kg. Key connections include 19.05mm flow/return pipes, a 12.7mm gas pipe, and a 21.5mm condensate drain. The integrated pump provides an available head of 70 kPa. Electrical supply is 230V/50Hz, with a maximum power consumption of 66W. Flue options include concentric and twin systems, supporting lengths up to 25 metres horizontally or vertically.
Every fault code listed in the manufacturer brochure, with likely causes and engineer actions. Each code has a permalink — paste a URL like https://www.boilertag.com/boilers/glow-worm/energy-30c#fault-f.00 to jump straight to the entry.
| Code | Meaning | Details |
|---|---|---|
| F.00 | Fault: Flow temperature sensor Show causes & actionsLikely causes
| Details |
| F.01 | Fault: Return temperature sensor Show causes & actionsLikely causes
| Details |
| F.10 | Short circuit: Flow temperature sensor Show causes & actionsLikely causes
| Details |
| F.11 | Short circuit: Return temperature sensor Show causes & actionsLikely causes
| Details |
| F.12 and F.91 | Short circuit: Cylinder temperature sensor Show causes & actionsLikely causes
| Details |
| F.13 | Short circuit: Domestic hot water cylinder temperature sensor Show causes & actionsLikely causes
| Details |
| F.20 | Safety switch-off: Overheating temperature reached Show causes & actionsLikely causes
| Details |
| F.22 | Safety switch-off: Low water pressure in the boiler Show causes & actionsLikely causes
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| F.23 | Safety switch-off: Temperature difference too great (NTC1/NTC2) Show causes & actionsLikely causes
| Details |
| F.24 | Safety switch-off: Temperature rise too fast Show causes & actionsLikely causes
| Details |
| F.25 | Safety switch-off: Flue gas temperature too high Show causes & actionsLikely causes
| Details |
| F.27 | Safety switch-off: Fault in flame detection Show causes & actionsLikely causes
| Details |
| F.28 | Fault: Ignition unsuccessful when starting up Show causes & actionsLikely causes
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| F.29 | Fault: Flame loss Show causes & actionsLikely causes
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| F.32 | Fan frost protection function active: Fan speed outside the tolerance values Show causes & actionsLikely causes
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| F.33 | Fan frost protection function active: Air pressure switch fault Show causes & actionsLikely causes
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| F.49 | eBUS fault: Voltage too low Show causes & actionsLikely causes
| Details |
| F.61 | Fault: Gas valve control system Show causes & actionsLikely causes
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| F.62 | Fault: Gas valve switch-off control Show causes & actionsLikely causes
| Details |
| F.63 | Fault: EEPROM Show causes & actionsLikely causes
| Details |
| F.64 | Fault: Electronics/sensor/analogue-to-digital converter Show causes & actionsLikely causes
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| F.65 | Fault: Electronics temperature too high Show causes & actionsLikely causes
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| F.67 | Value sent back by ASIC is incorrect (flame signal) Show causes & actionsLikely causes
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| F.68 | Fault: Unstable flame (analogue input) Show causes & actionsLikely causes
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| F.70 | Invalid product code (DSN) Show causes & actionsLikely causes
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| F.71 | Fault: Flow/return temperature sensor Show causes & actionsLikely causes
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| F.72 | Fault: Deviation in the water pressure sensor/return temperature sensor Show causes & actionsLikely causes
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| F.73 | Fault: Water pressure sensor not connected or has short-circuited Show causes & actionsLikely causes
| Details |
| F.74 | Fault: Electrical problem in the water pressure sensor Show causes & actionsLikely causes
| Details |
| F.75 | Fault: Pressure sensor Show causes & actionsLikely causes
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| F.76 | The safety cut-out in the primary heat exchanger is defective Show causes & actionsLikely causes
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| F.77 | Fault: Condensate or smoke Show causes & actionsLikely causes
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| F.78 | Interruption to DHW outlet sensor at external controller Show causes & actionsLikely causes
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| F.83 | Fault: Burner temperature sensor Show causes & actionsLikely causes
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| F.84 | Fault: Flow/return temperature sensor Show causes & actionsLikely causes
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| F.85 | Fault: Temperature sensor Show causes & actionsLikely causes
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| F.86 | Fault: Underfloor heating contact Show causes & actionsLikely causes
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| F.87 | Fault: Electrodes Show causes & actionsLikely causes
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| F.88 | Fault: Gas valve Show causes & actionsLikely causes
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| F.89 | Fault: Pump Show causes & actionsLikely causes
| Details |
| Connection | No communication between the PCB and the user interface Show causes & actionsLikely causes
| Details |
Patterns that group across the manufacturer's fault-code table. Each entry links to the full code listing further down the page.
Triggered by fault code F.00.
Triggered by fault code F.01.
Triggered by fault code F.10.
Triggered by fault code F.11.
Triggered by fault code F.12 and F.91.
Diagnostic checklists from engineer-submitted Glow Worm faults, with step-by-step checks and safety notes.
The Glow Worm Energy 25c displaying fault code F.32, with the fan cycling erratically and making a struggling humming noise, indicates an…
The Glow Worm Energy 25c displaying an F.28 fault code indicates an ignition failure.
Common questions from engineers and homeowners about the Glow Worm Energy 30c.
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Specifications on this page are extracted from the manufacturer brochure. Always cross-check against the latest datasheet before commissioning or quoting. Last updated 28 June 2026.