Common problems guide

The most common Glow-worm Energy System problems

The faults most likely to send a Glow-worm Energy System into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
41 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
F22 High DIY-safe

Your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.

What to try first

Locate the external filling loop, which is usually a flexible silver hose connecting two pipes underneath the boiler.Open the valves at both ends of the hose to allow mains water to enter the system.Monitor the boiler's pressure gauge until the needle reaches the green zone (typically between 1.0 and 1.5 bar).Close both valves tightly and remove the hose if required by your specific boiler setup.

Est. cost
£0-£100
Parts
Filling loop, Pressure relief valve, Expansion vessel
Full guide for F22
02
F.22 High DIY-safe

Your boiler has stopped heating because the water pressure inside the system has dropped too low to operate safely.

What to try first

Locate the external filling loop, usually two small valves connected by a flexible silver hose under the boiler.Slowly open the valves to allow mains water into the system until the pressure gauge on the front reads between 1.0 and 1.5 bar.Close both valves tightly and check the boiler display to see if the fault code has cleared.

Est. cost
£80-150
Parts
Pressure relief valve, Expansion vessel, Pressure sensor
Full guide for F.22
03
F.00 High Engineer

Your boiler has lost the ability to accurately measure the temperature of the water flowing through it, so it has shut down to prevent overheating.

Est. cost
£120-200
Parts
Flow temperature sensor (NTC), Wiring harness
Full guide for F.00
04
F.01 High Engineer

Your boiler has identified a communication problem with the sensor that monitors the temperature of the water returning to the system, causing the boiler to shut down as a safety precaution.

Est. cost
£120-220
Parts
Return NTC thermistor, Wiring harness
Full guide for F.01
05
F.10 High Engineer

Your boiler has stopped working because it is unable to accurately measure the temperature of the water flowing through the system due to a faulty electrical connection.

Est. cost
£120-200
Parts
Flow temperature sensor, Wiring harness
Full guide for F.10
06
F.11 High Engineer

Your boiler has detected an electrical fault with the sensor that monitors the temperature of the water returning to it, meaning it cannot accurately judge how much heat to provide.

Est. cost
£120-220
Parts
Return temperature sensor, Wiring harness
Full guide for F.11
Parts most often replaced

on the Glow-worm Energy System

Filling loopPressure relief valveExpansion vesselPressure sensorFlow temperature sensor (NTC)Wiring harness

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Glow-worm shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 41-code list for the Glow-worm Energy System

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →