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Glow-worm boilers are a popular brand within the UK, particularly for families looking for cheap and quality heating. As with any boiler, however, they too may occasionally malfunction. Whether it’s fault flashing codes, a lack of hot water, or unusual noises.
Being able to tell what’s happening in your boiler can help you avoid a considerable amount of stress (and money). The majority of problems can be easily diagnosed using Glow-worm’s in-built fault codes, which appear on the display screen.
In this guide, we will discuss the most common Glow-worm boiler issues, understand the fault codes, and provide quick fixes where applicable. We will also guide you on when you can fix a problem yourself and when you will require a Gas Safe engineer.
How Long Do Glow-worm Boilers Last?
A Glow-worm boiler, if serviced properly, can last for around 10–15 years. Annual services maintain their efficiency and also safeguard your warranty. However, as the boiler gets older, parts get used up, and faults are more likely to occur.
Common Glow-worm Boiler Problems
Even the most efficient boilers encounter problems now and then. The most common issues Glow-worm users experience are:
1. Glow-worm Boiler Keeps Losing Pressure
Low pressure is the most frequent problem. It normally occurs due to a water leak in the system, bleeding radiators, or a faulty pressure relief valve.
Quick fix:
- Look at the pressure gauge at the front of your boiler. It should be between 1.0 and 1.5 bar.
- If it’s lower than 1, you can re-pressurise the system with the filling loop (normally a silver flexible hose beneath the boiler).
- If pressure continues to fall, you might have a leak and will require a Gas Safe engineer.
2. Glow-worm Boiler No Hot Water or No Heating
If your Glow-worm boiler is not heating or giving you hot water, a number of things might be at fault: a frozen condensate pipe, a faulty pump, a faulty diverter valve, or even an electrical fault.
Quick fix:
- Check your thermostat settings first.
- If it’s freezing outside, the condensate pipe (the white plastic pipe leading outside) could be blocked with ice. Pour warm water over it to clear.
- If the problem persists, call a Gas Safe engineer.
3. Boiler Keeps Locking Out
Glow-worm boilers often go into “lockout mode” when they detect a fault to protect themselves. You’ll see an error code on the display.
Quick fix:
- Try resetting the boiler using the reset button.
- If the lockout continues to occur, record the fault code and contact an engineer.
4. Strange Noises (Banging, Gurgling, or Whistling)
Strange sounds are usually due to air that has become trapped in the system, limescale deposits (called “kettling”), or inadequate water pressure.
Quick fix:
- Bleed your radiators to eliminate trapped air.
- Inspect boiler pressure and fill up if low.
- If kettling, you might require a system flush by a professional.
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Glow-worm Boiler Fault Codes and Fixes
One of the biggest advantages of modern Glow-worm boilers is that they display error codes when something goes wrong. These codes can feel overwhelming at first, but they’re there to guide you.
Here’s a breakdown of the most common ones:
1. F1 Fault – Ignition Failure
Cause:
The boiler isn’t firing up properly. This could be due to:
- A frozen or blocked condensate pipe
- Issues with the gas supply
- Faulty ignition components
Quick Fix:
- Check the condensate pipe (the white plastic pipe going outside) first. If it’s frozen, thaw it carefully with warm water or a hot water bottle.
- Reset the boiler.
- If it still won’t ignite, the fault could be with the ignition electrode or spark generator — call in a Gas Safe engineer.
2. F3 Fault – Fan Error
Cause:
The fan within the boiler, which serves to expel dangerous gases safely from your house, isn’t functioning correctly.
Quick Fix:
- Switch off your boiler straight away.
- Don’t try to fix this yourself. A Gas Safe engineer will need to service or replace the fan.
3. F4 Fault – Ignition Fault (Advanced)
Cause:
Another ignition fault, but typically more advanced than F1. The possible causes are:
- Faulty ignition electrode
- Blocked burner
- Faulty spark generator
Quick Fix:
This is not DIY. A professional will have to clean or change the ignition components.
4. F9 Fault – Low Water Pressure / Circulation Problem
Cause:
Glow-worm boilers are highly sensitive to water pressure. The code may indicate:
- The system pressure is low
- The pressure sensor is dirty
- The diverter valve or plate heat exchanger is sludged up
Quick Fix:
- Check the pressure gauge. It should be between 1 and 1.5 bar. If lower, repressurise using the filling loop.
- Clean the pressure sensor if you’re confident.
- If still unresolved, call an engineer. They may flush the system or check for a faulty diverter valve.
Blocking vs Locking Error Codes
Glow-worm boilers display two types of errors:
- Blocking Errors: Temporary issues that can sometimes be cleared by a reset.
- Locking Faults: More severe failures that cause the boiler to shut down entirely.
Blocking Fault Codes
- F1 – Ignition/gas supply issue
- F2 – Flame fault
- F4 – Heat exchanger issue
- F9 – Low-pressure/diverter valve
- F11 – Gas valve/control board fault
- F12 – Wire or PCB fault
These might reset, but if the issue continues to recur, contact an engineer.
Locking Fault Codes
- F13 – Temperature sensor fault
- F14 / F15 – Flow or return sensor issues
- F22 – Low water pressure sensor fault
- F28 – Fault in flue gas sensor
- F16 – If the fault code F16 flashes up on your Glow-worm Ci model, this indicates a flame detection fault with your boiler.
For these, the boiler will not restart until the root fault has been corrected. A Gas Safe engineer is required.
What If No Fault Code Is Displayed?
Your boiler sometimes just behaves badly and displays no codes. Here are some typical situations:
1. Low Boiler Pressure
If radiators are not heating, the pressure could have fallen.
Normal range = 1–1.5 bar.
If less, repressurise on the filling loop (refer to the manual for your type).
2. Diverter Valve Fault
Signs: heating comes on but no hot water, or the reverse.
It requires a Gas Safe engineer.
3. Airlock in the System
Radiators not warming evenly? Air is probably trapped.
Solution: bleed your radiators with a radiator key.
4. Thermostat & Timer Problems
If heating comes on/off intermittently, your thermostat could be malfunctioning or clogged.
Clean dust within (after disconnecting the power supply).
If still defective, replace with a smart or modern thermostat.
How to Reset a Glow-worm Boiler?
If your Glow-worm boiler shows a fault code, a quick reset can often clear small glitches like ignition errors or low-pressure blips.
Steps to reset:
- Find the reset button: Usually on the front panel (some models use a dial).
- Press and hold: Hold for 5–10 seconds, or turn the dial to reset and back.
- Wait for restart: The boiler will ignite, and the fault code should clear.
- Check performance: If the boiler runs normally, the issue was temporary.
If the fault returns or the boiler won’t reset, don’t keep trying. Call a Gas Safe engineer, as repeated resets can cause damage.
How Much Does it Cost to Fix a Glow-worm Boiler?
Here’s what you’ll have to pay in the UK:
- Engineer call-out: £60–£80/hour
- Annual service: £80–£100
Part replacement prices:
- Fan: £150–£250
- Ignition electrode: £100–£150
- Diverter valve: £150–£300
- PCB (control board): £300–£400
If your boiler is older than 20 years and you’re seeing frequent repair bills, it can be more cost-effective in the long term to have it replaced. And with the ECO4 scheme, you can even get a free boiler grant and have your old boiler upgraded for nothing.
When to Call a Gas Safe Engineer
Whereas certain quick repairs (such as repressurising or bleeding radiators) are okay to attempt by yourself, anything that involves gas or internal boiler parts should be carried out by a professional. Call a Gas Safe-registered engineer if:
- The same error code keeps recurring.
- You detect gas or suspect there is a leak.
- Loud, strange noises are coming from your boiler.
- Your boiler continually loses pressure after being topped up.
How to Find a Reliable Heating Engineer
For safe, professional boiler repair, seek:
- Gas Safe Registration: Non-negotiable for boiler work on gas appliances.
- PAS 2030 & PAS 2035 Accreditation: Mandatory for ECO4 scheme installations.
- TrustMark Approval: Government-endorsed guarantee of quality.
- Good Customer Reviews: Look on Trustpilot, Checkatrade, or Google.
Prevention of Glow-worm Boiler Faults
Prevention is the best cure. Here’s how to minimise the likelihood of faults:
- Annual servicing: Get your boiler serviced annually by a Gas Safe engineer.
- Check pressure regularly: Maintain it between 1.0 and 1.5 bar.
- Bleed radiators: It keeps the system in good condition.
- Lag condensate pipes: Insulate pipes outside to avoid freezing during winter.
- Fit a magnetic filter: Traps sludge and safeguards your boiler.
The Bottom Line
Glow-worm boilers are trustworthy, but as with any heating system, there can be faults. Knowing what each Glow-worm boiler fault code means and the most frequent Glow-worm boiler issues will save you time and anxiety. Symptoms such as no hot water, F75 faults, or ignition lockouts (F28) occur, but several are easily diagnosed.
If unsure, always seek assistance from a Gas Safe engineer. Here at ECO Energy Services, all of our engineers are Gas Safe, TrustMark-approved, and PAS-accredited, so your boiler is in good hands.
FAQs
These are the signs that tell you what is wrong with a glow-worm boiler. Like F1 is usually for ignition failure, or F9 for low pressure. Each of them warns you fast.
Press and hold the reset button for 5-10 seconds. If the malfunction still occurs, call a Gas Safe engineer. Do not try restarting the reset.
It is usually caused by leaking, bleeding radiators, or faulty valves. The pressure should be between 1.0 and 1.5 bar. If it falls regularly, then get it checked.
You can try to increase pressure or bleed the radiators. But only do any gas or electrical work under the supervision of a competent engineer.
