Glow-worm F20 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F20 fault code mean?
The F20 fault code means your Glow-worm boiler has detected an abnormal rise in primary circuit temperature — typically above 95°C — and has shut itself down as a precaution. Some Glow-worm models display the message 'Safety switch off temperature limiter' on the LCD alongside F20, which confirms this interpretation. The boiler will not restart on its own; it requires a manual reset. Because the root cause is excessive heat build-up rather than a gas or combustion issue, the fault is not immediately dangerous, but it should not be ignored: repeated overheating can damage internal components, particularly the heat exchanger. Once the boiler has cooled, a single reset attempt is reasonable, but if the fault returns you need a Gas Safe registered engineer to investigate.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Circulation pump failure or seizure Common
The pump is responsible for moving heated water around the system. If it has seized, is running too slowly, or is airlocked, heat cannot escape the heat exchanger fast enough and temperatures rise rapidly. You may notice the pump is silent or unusually noisy when the boiler fires.
- Blocked or scaled primary heat exchanger Common
Limescale accumulation — particularly in hard-water areas — or sludge from an unprotected system can restrict flow through the heat exchanger. The boiler then struggles to transfer heat into the water circuit, causing localised overheating.
- Low or incorrect system pressure Common
The system pressure should sit between 1.0 and 1.5 bar when cold. If pressure has dropped significantly, there may be insufficient water in the circuit to absorb heat properly, which can trigger the overheat limiter.
- Trapped air in the central heating system Sometimes
Air locks in radiators or pipework reduce effective water circulation. If water cannot flow freely through all parts of the circuit, hot spots can develop and push temperatures above the safety threshold.
- Radiator valves closed or system flow restricted Sometimes
If most or all thermostatic radiator valves are closed simultaneously, the boiler has very limited capacity to dissipate heat. This is sometimes overlooked but can cause rapid overheating, especially on modern condensing boilers.
- Faulty overheat thermostat or NTC thermistor Sometimes
The sensors that monitor primary circuit temperature can drift out of calibration or fail entirely. A faulty thermistor may report a falsely high temperature and trigger F20 even when the actual water temperature is normal.
- Faulty PCB Rare
In rare cases the printed circuit board misinterprets temperature sensor signals or fails to regulate the burner correctly, causing genuine or phantom overheat events.
How to fix it
- Let the boiler cool down before doing anything else DIY safe
Switch the boiler off at the controls and allow it to cool for at least 10–15 minutes. Attempting a reset on a boiler that is still extremely hot can mask the underlying problem and may cause further lockouts immediately.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, top up the system using the filling loop in accordance with your boiler's manual, then re-check the gauge. Do not over-pressurise — stop adding water once the needle reaches around 1.2–1.3 bar.
- Check that radiator valves are open DIY safe
Walk around your home and confirm that the thermostatic radiator valves (TRVs) on at least several radiators are open rather than fully closed. A system where every valve is shut gives the boiler nowhere to push heat and is a straightforward cause of F20.
- Bleed the radiators to remove trapped air DIY safe
Using a radiator bleed key, open each radiator's bleed valve slightly until water (rather than air) runs out, then close it. Start on the ground floor and work upward. After bleeding, re-check the boiler pressure and top up if it has dropped below 1.0 bar.
- Attempt a single manual reset DIY safe
Once the boiler has cooled and you have completed the checks above, press and hold the reset button for 3–5 seconds as described in your manual. Monitor the boiler closely as it restarts and watch for the F20 code returning. If the boiler fires up and runs normally, keep an eye on it over the next hour or so for any recurrence.
- Do not reset more than once if the fault returns DIY safe
If F20 reappears shortly after the reset, switch the boiler off and leave it off. Repeatedly resetting an overheating boiler risks damaging the heat exchanger and other internal components. At this point the fault requires professional investigation.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will measure pump speed and head pressure, check whether the pump is seized or airlocked, inspect the heat exchanger for scale or blockages, and test the overheat thermostat and NTC thermistor with appropriate instruments. Depending on findings, they may power-flush the system, replace the pump, replace sensors, or advise on heat exchanger replacement. Do not open the boiler casing or attempt to adjust any internal components yourself.
Parts you may need
- Circulation pump (replacement) · from £85
- NTC thermistor / temperature sensor · from £25
- Overheat thermostat / safety cut-out · from £30
- Inhibitor and system cleaner (power flush chemical) · from £35
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
Is the Glow-worm F20 fault dangerous?
Not immediately — the boiler shuts itself down before temperatures reach a genuinely hazardous level, so there is no immediate risk to your home. However, if the underlying cause is not fixed, repeated overheating will eventually damage the heat exchanger, which is one of the most expensive components to replace. Switch the boiler off and get it looked at promptly rather than continuing to reset it.
Can I fix the F20 fault myself without an engineer?
There are a few checks any homeowner can safely carry out: topping up system pressure via the filling loop, bleeding radiators to remove trapped air, and making sure radiator valves are open. If one of those issues was the culprit, a single reset may clear the fault for good. Beyond that — anything involving the pump, heat exchanger, sensors, or internal components — you need a Gas Safe registered engineer. Opening the boiler casing yourself is not safe and could invalidate your warranty.
How much does it cost to repair a Glow-worm F20 fault?
Most F20 repairs fall in the £120–£350 range, covering the engineer's call-out, diagnosis, and the most common fixes such as pump replacement, thermistor replacement, or a system flush. Simple jobs like bleeding the system or repressurising are at the lower end; replacing a pump with parts and labour typically sits in the middle of that range. A full primary heat exchanger replacement is a bigger job and can cost £400–£600 or more depending on the model — but this is only necessary in more severe or long-neglected cases.
Why does my Glow-worm boiler keep showing F20 after I reset it?
A fault that returns quickly after resetting usually means the root cause — most often a sluggish or seized pump, a blocked heat exchanger, or a faulty temperature sensor — is still present. The boiler is simply detecting the same overheating condition each time it fires. Stop resetting and book a Gas Safe engineer: continued resets without fixing the cause can lead to heat exchanger damage, which is a much more costly repair than sorting the original problem early.