Glow-worm F14 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F14 fault code mean?
The F14 fault code on Glow-Worm boilers has two distinct meanings depending on your model range, so it is important to identify which boiler you have before attempting any diagnosis. On Flexicom CX and most modern combi and system models, F14 points to a gas valve control fault. The boiler's PCB has detected that the gas valve is not responding correctly to its commands — it may be failing to open, close, or modulate as required. This is a safety-critical issue: a misbehaving gas valve can cause the boiler to run dangerously hot, fail to ignite at all, or behave erratically. The PCB responds by locking the boiler out entirely, leaving you with no heating or hot water until the fault is resolved by a qualified engineer. On older CXI, HXI, and SXI models, F14 instead signals that the central heating flow temperature has climbed above 95°C. This overheating condition is typically triggered by a circulation problem — most often a scaled-up plate heat exchanger, a failing pump, or a faulty flow thermistor — rather than a gas valve issue. Real-world engineer reports confirm that a blocked plate heat exchanger is a particularly frequent culprit on models such as the 30CXi. In both cases the boiler will lock out as a protective measure. Do not repeatedly reset the boiler without understanding the underlying cause, as this could lead to unsafe operation or mask a worsening fault.
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
- Faulty gas valve (Flexicom CX and modern models) Common
The gas valve is the component that controls how much gas reaches the burner. When the PCB detects that the valve is not opening, closing, or modulating as commanded — whether due to an internal mechanical failure, a seized actuator, or an electrical fault within the valve itself — it logs F14 and shuts the boiler down. This is the most common root cause on Flexicom CX models, though an engineer will need to confirm whether the valve itself or its associated wiring is at fault before ordering parts.
- Blocked or scaled plate heat exchanger (CXI, HXI, SXI models) Common
On older CXI-series boilers, limescale accumulation inside the plate heat exchanger reduces water flow through the heat exchanger. As a result, the water in the boiler cannot be cooled efficiently, temperatures rise beyond 95°C, and the F14 overheating lockout triggers. This problem is more prevalent in hard-water areas and on boilers that have never had a chemical inhibitor added to the system.
- Wiring harness or connector fault Sometimes
Corroded, loose, or damaged wiring between the PCB and the gas valve (or between the PCB and the flow thermistor on CXI models) can cause the PCB to register an F14 code even when the valve or sensor itself is still serviceable. An engineer will check connector integrity and resistance values before condemning the valve or board outright.
- PCB (printed circuit board) failure Sometimes
The PCB governs the gas valve and monitors all sensor inputs. If the board develops a fault — due to age, moisture ingress, or a power surge — it may incorrectly report an F14 code or genuinely fail to send the correct signals to the gas valve. PCB replacement is expensive, so engineers typically rule out the valve and wiring first.
- Circulation pump failure (CXI, HXI, SXI models) Sometimes
If the pump stops moving water around the system effectively, heat builds up in the heat exchanger and flow temperatures rise sharply. A seized or weak pump is a secondary but real cause of the overheating variant of F14 on older models, particularly on boilers that are ten or more years old.
- Faulty flow thermistor Rare
On CXI and similar models, the flow thermistor reports the central heating flow temperature to the PCB. A thermistor that is sending inaccurate high readings — due to a wiring fault or a failed sensor — can cause a false F14 overheating code even when actual water temperatures are within safe limits.
How to fix it
- Check your gas supply is working DIY safe
Before assuming the boiler is at fault, confirm that other gas appliances in your home — a gas hob, for example — are functioning normally. If you have no gas elsewhere, contact your gas supplier. If other appliances are working, the issue is specific to the boiler.
- Do not attempt multiple resets Gas Safe engineer
Unlike some minor faults, F14 — particularly the gas valve variant — indicates a potentially serious safety issue. Repeatedly resetting the boiler without a proper diagnosis can mask a worsening problem or, in the worst case, lead to unsafe gas delivery. A single reset attempt is acceptable to see if the fault clears, but if it returns immediately or within a short period, stop and call an engineer.
- Note your boiler model before calling for help DIY safe
Because F14 means different things on different Glow-Worm ranges, having your exact model name and number to hand (usually on a label inside the boiler casing or in your documentation) will help an engineer prepare the correct parts before attending. Mention whether you have a Flexicom CX or an older CXI, HXI, or SXI model.
- If you smell gas at any point, act immediately DIY safe
Do not attempt any further checks. Open doors and windows to ventilate the property, avoid operating any electrical switches, turn off your gas supply at the meter, leave the building, and call the National Gas Emergency Service on 0800 111 999. Do not re-enter until cleared to do so.
- Gas valve inspection and testing — engineer required Gas Safe engineer
A Gas Safe registered engineer will use specialist test equipment to check the gas valve's electrical resistance, actuator response, and mechanical operation. They will also inspect the wiring harness and connectors. On Flexicom CX models this step usually determines whether a new valve, a replacement wiring loom, or a PCB is needed.
- Plate heat exchanger descale or replacement — engineer required Gas Safe engineer
On CXI, HXI, and SXI models where overheating is the cause, an engineer will remove and inspect the plate heat exchanger. If it is scaled but otherwise intact, a chemical descale (similar in principle to descaling a kettle, but carried out with specialist products and procedures) can restore flow. Heavily blocked or corroded exchangers may need replacing entirely. The engineer may also check system inhibitor levels and recommend a powerflush if the whole system is sludged.
- Pump testing and replacement if required — engineer required Gas Safe engineer
Where a weak or seized pump is contributing to overheating on older models, the engineer will test pump operation and flow rates. A failed pump will need replacing; this is a straightforward job for an engineer and often resolves recurring F14 faults on ageing CXI boilers without the need for more expensive parts.
- PCB fault diagnosis — engineer required Gas Safe engineer
If the gas valve, wiring, pump, and heat exchanger have all been assessed and found satisfactory, the engineer will turn attention to the PCB. Board replacement is costly, and on an older boiler it is worth having a frank conversation with your engineer about whether investing in a new PCB makes financial sense compared with replacing the boiler altogether.
- Call a Gas Safe registered engineer DIY safe
F14 requires professional diagnosis in virtually all cases. Use the Gas Safe Register search tool at GasSafeRegister.co.uk to find a locally registered engineer. Provide them with your boiler model, the F14 code, and any symptoms you have noticed — unusual noises, fluctuating temperatures, or how long the fault has been showing.
Parts you may need
- Gas valve (Flexicom CX / combi models) · from £210
- PCB (printed circuit board) · from £230
- Wiring harness / loom · from £55
- Plate heat exchanger (CXI series) · from £140
- Circulation pump · from £90
- Flow thermistor (NTC sensor) · from £25
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 £240–£550, depending on the underlying cause.
Frequently asked questions
Can I reset a Glow-Worm F14 fault myself?
You can try a single reset — hold or press the reset button as described in your boiler's handbook — but do not repeat this more than once or twice. F14 relates to either a gas valve control fault or an overheating condition, both of which are safety-critical. If the fault returns after one reset, the boiler is telling you something is genuinely wrong, and a Gas Safe engineer should carry out a proper diagnosis rather than you continuing to reset and run the boiler.
My boiler is a Glow-Worm 30CXi and it keeps showing F14 — what is usually wrong?
On the 30CXi and other CXI-series models, F14 almost always means the boiler has overheated rather than there being a gas valve problem. The most frequently reported cause on these older models is a scaled-up plate heat exchanger that restricts water flow and causes temperatures to climb above 95°C. A Gas Safe engineer can descale or replace the exchanger and check that the circulation pump and flow thermistor are also working correctly. If your 30CXi is over ten years old and has had no inhibitor treatment, a powerflush of the whole system is often recommended at the same time.
How much does it cost to fix a Glow-Worm F14 fault in the UK?
Most people pay somewhere between £240 and £550 to resolve an F14 fault, depending on what needs doing. A gas valve replacement — the most common repair on Flexicom CX models — typically costs £140 to £300 for the part plus £100 to £200 in labour. A plate heat exchanger descale on CXI models is cheaper, usually around £80 to £120 in labour alone. If the PCB turns out to be the root cause, the board itself can cost an additional £150 to £300 plus fitting, which may push the total beyond £550. On boilers that are ten years old or older, it is worth asking your engineer whether repair or replacement represents better value.
Is the Glow-Worm F14 code dangerous?
It can be, which is why the boiler locks itself out automatically. On Flexicom CX models, a misbehaving gas valve poses a risk of incorrect gas delivery to the burner. On CXI models, temperatures exceeding 95°C can damage internal components and, in extreme cases, stress the system further. In either scenario, do not attempt to bypass the lockout. If you smell gas at any point, turn off the gas supply at the meter and call the National Gas Emergency Service on 0800 111 999 immediately.