Glow-worm F26 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F26 fault code mean?
The F26 fault code on a Glow-Worm boiler indicates that the temperature difference between the flow (water leaving the boiler) and the return (water coming back) is too narrow. In a healthy system, water leaves the boiler hot, circulates around the radiators or underfloor heating, gives up its heat, and returns noticeably cooler. When the boiler detects that the returning water is almost as hot as the water it just sent out, it interprets this as a serious circulation problem — almost as though the water is barely making it past the first radiator before looping back. To protect the heat exchanger from overheating, the boiler locks out and displays F26. This code is seen across several Glow-Worm ranges including the Betacom4 and Ultracom3 series.
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 return thermistor (NTC sensor) Common
The return thermistor monitors the temperature of water arriving back at the boiler. If this sensor has drifted out of calibration or failed entirely, it can feed the PCB an inaccurate reading — making the controller think the return temperature is far higher than it actually is. This is the single most common trigger for F26 and is a relatively affordable fix.
- Poor circulation — blockage or airlock Common
Sludge build-up, a partial blockage in the system pipework, or a stubborn airlock can all severely restrict water flow. When water moves too slowly, it barely loses any heat before returning, causing the flow-to-return differential to collapse. A system that has never been power-flushed or chemically cleaned is particularly vulnerable.
- Pump running at incorrect speed or failing Sometimes
The central heating pump controls how quickly water circulates through the system. If it is set to the wrong speed, seized, or beginning to fail, flow rate drops and the temperature differential narrows. Some pumps have a speed selector switch that can be knocked inadvertently during servicing.
- Undersized or restricted pipework Sometimes
If the heating pipework is narrower than the system requires, or if valves (including thermostatic radiator valves) have been closed on too many radiators simultaneously, flow resistance rises sharply. The result is a low flow rate that mimics the symptom profile of F26.
- Irregular or low gas supply Rare
An insufficient gas supply can cause the burner to underperform, affecting the heat output and the way temperatures are balanced across the system. This can occasionally nudge the boiler into an F26 lockout, though it more commonly produces other fault codes first.
How to fix it
- Check that the gas supply is active DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier — there may be a supply interruption or your meter prepayment may need topping up. Do not attempt to inspect or adjust any gas pipework yourself.
- Top up system pressure if it is low DIY safe
Check the pressure gauge on the boiler. Glow-Worm boilers typically need a cold pressure of 1.0–1.5 bar. If it reads below 1.0 bar, use the filling loop (usually a braided flexible hose with two isolating valves beneath the boiler) to add water until the gauge reads around 1.2 bar, then close both valves. Low pressure can worsen circulation issues that contribute to F26.
- Attempt a boiler reset DIY safe
On most Glow-Worm models, turn the CH control knob to the reset position (marked with a power symbol), hold for a few seconds, then return it to your desired temperature setting. Wait to see whether the boiler fires and runs without the fault returning. Only attempt a reset two or three times at most — repeated lockouts on the same fault mean the underlying problem is still present and needs professional diagnosis.
- Check that radiator valves are not all closed DIY safe
Walk around your home and confirm that thermostatic radiator valves (TRVs) and manual valves on radiators are open on at least several radiators. A system where nearly every radiator is closed forces water through a very restricted circuit, causing the differential to collapse.
- Have a Gas Safe engineer test the return thermistor Gas Safe engineer
An engineer will use a multimeter to check the resistance of the return NTC sensor against the manufacturer's temperature-resistance table. A sensor reading outside tolerance needs replacing. This is a straightforward repair and one of the most cost-effective resolutions to F26.
- Have the system checked for circulation problems and the pump inspected Gas Safe engineer
The engineer will assess water flow rate, check the pump speed setting, and look for evidence of sludge or airlocks. If the system is heavily magnetite-contaminated (black sludge), a power flush or chemical flush may be recommended to restore proper circulation. A magnetic system filter can help prevent re-accumulation.
- Have the gas valve inspected if other causes have been ruled out Gas Safe engineer
Although F26 is primarily a temperature-differential fault rather than a gas valve fault, an engineer may test the gas valve as part of a broader diagnostic check if the boiler's behaviour suggests combustion anomalies are affecting system performance. Gas valve work must only be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the boiler locks out again after a reset, or if you have not been able to resolve the issue with the homeowner checks above, book a qualified Gas Safe engineer. You can verify any engineer's registration at gassaferegister.co.uk. Repeatedly resetting a locking-out boiler without fixing the root cause can mask a developing fault and risks damage to the heat exchanger.
Parts you may need
- Return NTC thermistor (sensor) · from £25
- Central heating pump (replacement) · from £60
- Magnetic system filter (e.g. Magnaclean) · from £55
- Gas valve · from £120
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
Can I reset a Glow-Worm F26 myself and will it fix the problem?
You can reset the boiler yourself — turn the CH control knob to the reset symbol, wait briefly, then set it back to your chosen temperature. If the boiler runs without locking out again, the fault may have been a transient blip. However, if F26 returns after one or two resets, there is an underlying issue that a Gas Safe engineer needs to diagnose. Repeatedly resetting without addressing the root cause is not recommended and can cause further wear.
How much does it typically cost to fix a Glow-Worm F26 fault?
For most households, the repair bill falls between £120 and £350. A thermistor replacement is usually at the lower end of that range, while resolving a circulation problem (including a power flush or pump replacement) tends to sit in the middle to upper portion. If the gas valve itself needs replacing, costs can reach £250–£350 including parts and labour. Prices are generally higher in London and the South East than in northern regions. If your boiler is over 10–12 years old and has had repeated faults, your engineer may discuss whether a replacement boiler represents better value.
Why does F26 mention a gas valve on some websites if it is really a temperature problem?
The confusion arises because different Glow-Worm models use fault codes slightly differently, and some older online resources mix up the F26 description with gas valve faults from other manufacturers. On current Glow-Worm boilers, F26 specifically relates to the flow-to-return temperature differential being too small — a circulation or sensor issue. That said, a Gas Safe engineer will carry out a full diagnostic check, which may include the gas valve as part of their broader assessment if the boiler's combustion behaviour is also in question.
How can I prevent the F26 fault from coming back?
The best preventive measures are an annual boiler service by a Gas Safe engineer, fitting a magnetic system filter (such as a Magnaclean) if you do not already have one, and having the system chemically treated to remove sludge. Keeping your boiler pressure in the 1.0–1.5 bar range and ensuring radiator valves are not all closed at the same time also helps maintain good circulation. A well-maintained, clean system is far less likely to trigger temperature-differential faults.