Is your Glow-worm boiler displaying fault code F81? Don’t panic – you’re not alone. This is one of the most common fault codes reported by Glow-worm boiler owners, and in many cases, it can be resolved quickly without expensive repairs.
Fault code F81 indicates: Overheating of hot water circuit (35 Store). This comprehensive guide walks you through everything you need to know about this error, from understanding what causes it to step-by-step solutions you can try yourself, plus when it’s essential to call a Gas Safe registered engineer.
Whether you’re a hands-on DIY enthusiast or simply want to understand the issue before contacting a professional, this guide provides clear, actionable advice to get your heating system back up and running safely.
In This Guide
At a Glance
Key Takeaways
- Fault code F81 means: Overheating of hot water circuit (35 Store)
- This fault causes your boiler to shut down as a safety precaution to prevent damage or unsafe operation
- Some causes can be resolved with simple DIY fixes like repressurising or resetting your boiler
- Other causes require professional diagnosis and repair by a Gas Safe registered engineer
- Never ignore this fault code – addressing it promptly prevents more serious and costly problems
- Regular boiler servicing can prevent many fault codes from occurring in the first place
What Causes Glow-worm Fault Code F81?
Understanding what triggers fault code F81 is the first step toward resolving it. Based on the fault description “Overheating of hot water circuit (35 Store)”, here are the most common underlying causes:
1 Overheating Issues
The boiler temperature has exceeded safe operating limits:
- Pump not circulating water properly
- Air trapped in the system
- Blocked heat exchanger
- Closed or partially closed valves
- System sludge buildup
2 Other Contributing Factors
Additional issues that can trigger this fault code:
- Recent power cut or boiler reset
- Extreme weather conditions
- System modifications or repairs
- Age and wear of boiler components
- Poor installation or maintenance history
How to Fix Glow-worm Fault Code F81
Before calling an engineer, try these troubleshooting steps. Important: Only attempt fixes you feel comfortable with. If unsure, contact a Gas Safe registered engineer.
1 Reset Your Boiler
You can try this yourself:
- Locate the reset button (usually marked with a flame symbol)
- Press and hold for 2–3 seconds
- Release and wait for the boiler to restart
- Important: Only reset ONCE. If the fault returns, do not keep resetting – call an engineer
Still Having Issues?
If the fault code returns after trying these steps, or you’re not comfortable attempting DIY fixes, it’s time to call a professional.
Gas Safe registered engineers • Same-day emergency callouts • Serving across Kent
When You MUST Call a Gas Safe Engineer
Some causes of fault code F81 require professional diagnosis and repair. Never attempt these repairs yourself:
Do NOT Attempt DIY For:
- Gas valve faults or replacements
- Ignition electrode or spark generator replacement
- Fan motor or air pressure switch issues
- PCB (printed circuit board) faults
- Heat exchanger problems
- Pump replacement or electrical work
- Sensor replacement or wiring repairs
- Flue system work
- Any work involving gas connections
- Any fault you’re unsure about
Why use a Gas Safe registered engineer?
- Legal requirement for gas work in the UK
- Proper diagnostic equipment to identify the exact fault
- Access to genuine manufacturer parts
- Work is guaranteed and insured
- Maintains your boiler warranty
- Ensures safe operation for your family
Preventing Future Glow-worm Fault Codes
Regular maintenance is the best way to prevent fault codes and extend your boiler’s lifespan:
Annual Boiler Service
Book a service every 12 months with a Gas Safe registered engineer. A service includes full safety checks, combustion analysis, component inspection and cleaning, flue gas testing, pressure testing, filter cleaning, and fault code history review. An annual service prevents the vast majority of common faults, maintains your warranty, and keeps efficiency high – saving you money on your energy bills.
Insulate Your Condensate Pipe
If your boiler has an external condensate pipe, have it insulated. This simple investment prevents winter breakdowns caused by frozen pipes – and can save you the cost of an emergency callout.
Monitor System Pressure
Check your boiler pressure gauge monthly. It should be 1.0–1.5 bar when cold. If it drops regularly, you may have a small leak that needs addressing.
Bleed Radiators Correctly
When bleeding radiators, always check and top up boiler pressure afterward. Many pressure faults occur because homeowners bleed radiators but forget to repressurise.
Keep Vents Clear
Ensure air vents and flue terminals are clear of obstructions. Don’t block boiler vents with storage or furniture.
Frequently Asked Questions
Fault code F81 indicates: Overheating of hot water circuit (35 Store). This means your boiler has detected a problem and shut down as a safety precaution. The boiler will not operate until the underlying issue is resolved and the fault is cleared.
It depends on the cause. Some issues like low pressure or a frozen condensate pipe can be resolved with DIY fixes. However, problems involving gas components, electrical parts, sensors, or the fan require a Gas Safe registered engineer. When in doubt, always call a professional – incorrect DIY repairs can be dangerous and invalidate your warranty.
A Gas Safe registered engineer will diagnose the exact issue and advise whether a repair or replacement is needed. They have specialist testing equipment that can pinpoint the problem accurately. In many cases a simple repair is all that’s needed, but your engineer will explain all options and let you decide.
If fault code F81 returns repeatedly after resets or attempted fixes, there’s an underlying problem that hasn’t been properly diagnosed. Common reasons include a leak causing pressure loss, an intermittent electrical connection, a component at end of life, system blockage or sludge buildup, or an incorrect previous repair. Do not keep resetting – persistent faults require professional diagnosis.
No. Your boiler will not operate when this fault code is active – it’s locked out as a safety precaution. Do not attempt to bypass safety features or force the boiler to run. Resolve the fault properly, either through DIY fixes (if appropriate) or by calling a Gas Safe engineer.
Most common faults can be diagnosed and repaired on the same visit, often within a couple of hours. Simple fixes like repressurising or resetting take just minutes. More involved repairs may take longer, but your engineer will give you an honest assessment of the timescale before starting any work.
Related Glow-worm Fault Codes
If you’re experiencing other issues with your Glow-worm boiler, these related fault codes might help:
Need Professional Help With Your Boiler?
Our Gas Safe registered engineers are boiler specialists serving across Kent.
✓ Same-day emergency callouts
✓ All major brands serviced
✓ Genuine manufacturer parts
✓ Experienced, qualified engineers