Common problems guide

The most common Glow-worm Essential problems

The faults most likely to send a Glow-worm Essential 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.
39 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 filling loop, which is usually a flexible silver hose with one or two valves underneath your boiler.Open the valve(s) slowly while watching the pressure gauge on the front of the boiler.Wait for the needle to reach the green zone, typically between 1.0 and 1.5 bar.Carefully close the valve(s) tightly and check for any leaks.

Est. cost
£80-150
Parts
Pressure relief valve, Expansion vessel, Pressure sensor
Full guide for F22
02
F.22 High DIY-safe

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

What to try first

Locate the filling loop, which is usually a silver braided hose or a small plastic key underneath the boiler.Open the valves on the filling loop slowly to allow water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close the valves tightly and check the display to see if the error 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 stopped working because it can no longer accurately read the temperature of the water flowing through the system.

Est. cost
£120-180
Parts
Flow temperature sensor (NTC)
Full guide for F.00
04
F.01 High Engineer

Your boiler is struggling to accurately measure the temperature of the water returning to it, so it has stopped working to prevent potential overheating.

Est. cost
£120-180
Parts
Return NTC thermistor
Full guide for F.01
05
F.10 High Engineer

Your boiler has stopped working because it cannot correctly measure the temperature of the water flowing through the system due to a faulty electrical connection or sensor.

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

Your boiler is currently unable to correctly measure the temperature of the water returning to it, so it has shut down to prevent potential overheating.

Est. cost
£120-200
Parts
Return temperature sensor, Wiring harness
Full guide for F.11
Parts most often replaced

on the Glow-worm Essential

Pressure relief valveExpansion vesselPressure sensorFlow temperature sensor (NTC)Return NTC thermistorNTC Flow Temperature Sensor

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 39-code list for the Glow-worm Essential

Related guides

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