Glow-worm F5 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F5 fault code mean?
The F5 code means your Glow-Worm boiler has detected that internal temperatures have climbed beyond safe limits, causing it to lock out and shut down completely. Until the underlying problem is resolved, the boiler will not fire up, leaving you without heating or hot water. On most modern Glow-Worm models the fault points to overheating caused by restricted water flow, limescale, or a circulation problem. On some older models — notably the 30si — F5 is more specifically associated with a fault on the return thermistor (temperature sensor) rather than a true overheat event. Either way, the boiler is protecting itself from damage and the cause must be investigated before it will restart.
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
- Restricted water circulation / faulty pump Common
If the central heating pump is worn, seized, or running at the wrong speed, water cannot circulate freely around the system. Heat builds up in the heat exchanger faster than it can be carried away, triggering the overheat lockout. This is one of the most frequent reasons for an F5 on modern Glow-Worm boilers.
- Limescale build-up in the heat exchanger Common
In hard-water areas, limescale deposits coat the inside of the heat exchanger and act as an insulating layer. Heat transfer becomes inefficient, water temperatures spike, and the boiler shuts down on overheat. Gurgling or kettling noises before the lockout are a classic sign.
- Faulty flow or return thermistor Common
The thermistor is a temperature-sensing resistor that reports water temperature to the PCB. If it has drifted out of calibration or failed entirely, the boiler may receive a false high-temperature reading and lock out even when actual temperatures are fine. On older models such as the 30si, a faulty return thermistor is often the primary diagnosis for F5.
- Airlocks in the heating system Sometimes
Air trapped in the system — often after draining work or a recent radiator change — can create hot spots because water flow is partially blocked. These localised temperature peaks can push the boiler into overheat shutdown.
- Low system pressure Sometimes
If the system pressure has dropped below around 0.8 bar there may not be enough water volume circulating to absorb heat effectively, which can cause overheating. Check the pressure gauge on the boiler front.
- Faulty pressure relief valve (PRV) Rare
A PRV that has stuck closed prevents the system from shedding excess pressure safely. This can contribute to elevated temperatures and trigger the F5 lockout. Conversely, a PRV weeping constantly may indicate existing overpressure events.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler fascia. For most Glow-Worm boilers the working pressure should sit between 1.0 and 1.5 bar when cold. If it reads below 0.8 bar, use the filling loop beneath the boiler to top up to around 1.2 bar, then close the filling loop valves fully. If the pressure is already within range, move on to the next step.
- Bleed your radiators to remove trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve slightly. Hold a cloth underneath to catch any drips. When water flows steadily with no air spitting out, close the valve. Work your way back towards the boiler. After bleeding, recheck system pressure and top up again if it has dropped.
- Attempt a boiler reset (once or twice only) DIY safe
Locate the reset button — on most Glow-Worm models it is a dedicated button on the front panel, sometimes held for 3 seconds. If the boiler restarts but locks out again within a short time, do not keep resetting it. Repeated resets without fixing the root cause can cause further damage. Two reset attempts is the sensible limit.
- Check for a frozen or blocked condensate pipe DIY safe
In cold weather, the plastic condensate pipe that runs outside can freeze and cause a secondary lockout. Pour warm (not boiling) water along the external section of pipe to thaw it. This is unlikely to be the primary cause of F5 but is worth ruling out if the weather is freezing.
- Have a Gas Safe engineer inspect and test the thermistors Gas Safe engineer
A registered engineer will use a multimeter to check the resistance values of both the flow and return thermistors against the manufacturer's specification. A thermistor reading outside its expected range at a known water temperature confirms a faulty sensor. Replacement is a straightforward parts-and-labour job.
- Have the pump inspected and replaced if necessary Gas Safe engineer
An engineer will check the pump is running, verify its speed setting, and measure the flow rate through the system. A pump that is seized, noisy, or delivering insufficient flow will need replacing. This is one of the more common repairs for a persistent F5.
- Arrange a system flush or powerflush if limescale or sludge is suspected Gas Safe engineer
If the engineer finds evidence of scale or black sludge — discoloured water at the bleed points, kettling noises, or blocked radiators — a chemical flush or full powerflush will be recommended. Inhibitor should be added afterwards to slow future corrosion and scale build-up.
- Have the PRV and associated components checked Gas Safe engineer
If the pressure relief valve is suspect, the engineer will inspect it for signs of weeping or seizure and replace it if necessary. This is not a homeowner task as it involves pressurised gas and water components.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If none of the homeowner checks resolve the F5, or if the boiler locks out again quickly after a reset, stop resetting and book a Gas Safe registered engineer. You can verify a company's registration at gassaferegister.co.uk. Attempting gas or sealed system repairs yourself is illegal and can void your boiler warranty and home insurance.
Parts you may need
- Flow or return thermistor (NTC sensor) · from £18
- Central heating pump (e.g. Grundfos UPS2 or equivalent) · from £75
- Pressure relief valve (PRV) · from £22
- Plate heat exchanger (if scaled beyond recovery) · from £120
- System inhibitor (e.g. Fernox F1, 1 litre) · from £14
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–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F5 fault myself?
The homeowner checks — topping up system pressure, bleeding radiators, and attempting a reset — are safe to try and cost nothing. However, if those steps do not clear the fault, or the boiler locks out again shortly afterwards, you will need a Gas Safe registered engineer. Replacing thermistors, pumps, or carrying out a powerflush requires professional tools and qualifications. Attempting gas-side work yourself is illegal in the UK.
Why does my Glow-Worm 30si show F5 when it is not obviously overheating?
On the 30si and certain other older Glow-Worm models, F5 is specifically linked to the return thermistor rather than confirmed overheating. The sensor itself may have failed and be sending a false high-temperature signal to the PCB. A Gas Safe engineer can test the sensor's resistance value quickly and replace it if it is out of specification — this is usually one of the cheaper F5 repairs.
How much does it cost to fix a Glow-Worm F5 fault?
Most F5 repairs fall in the range of £120–£380 including parts and labour. A straightforward thermistor swap or pump replacement sits at the lower end; a powerflush, which can be needed when heavy limescale or sludge is the cause, typically costs £300–£500 and sits above the typical range. If the heat exchanger is irreparably scaled and needs replacement, costs can reach £600 or more — in which case your engineer may suggest weighing up a new boiler, especially if yours is over 10 years old.
How can I stop the F5 fault from coming back?
Annual servicing by a Gas Safe registered engineer is the single most effective preventive measure — it catches limescale, pump wear, and sensor drift before they cause a lockout. In hard-water areas, fitting a scale reducer on the cold feed and maintaining a good concentration of corrosion inhibitor in the system (check it at every service) will significantly slow heat exchanger scaling. Keeping system pressure topped up to around 1.2 bar and bleeding radiators regularly also helps the pump circulate water efficiently.