Glow-worm Boiler Fault Codes Explained: F1, F22 and F75 (London Guide 2026)

A Glow-worm that has stopped working and is showing a code on its display has already done you a favour. It has told you which of its internal safety checks failed. The problem is that the codes are written for engineers, so F1, F22 and F75 mean very little to a homeowner standing in a cold kitchen at seven in the morning wondering whether this is a five-minute fix or a call-out.

This guide translates the codes we attend most often on London Glow-worm installations, across the Betacom, Flexicom, Ultimate and Energy ranges. For each one you get what the boiler is actually complaining about, the checks you can safely do yourself from outside the casing, and the clear point at which you stop and ring a Gas Safe registered engineer. Nothing in here asks you to open the boiler, touch a gas component or defeat a safety device, because none of that is legal or safe for anyone who is not on the Gas Safe Register. If you want the general principle before the brand specifics, start with our guide to boiler fault codes across every brand.

How Glow-worm Fault Codes Work

Glow-worm uses an F prefix for faults. Depending on the model and the display, the same fault appears as F.22 or F22, so treat the two as identical.

The single most useful thing to know about Glow-worm codes is where they come from. Glow-worm is part of the Vaillant Group, and the newer ranges are built on the Vaillant control platform. That is why F22 for low water pressure and F75 for no pressure change on pump start mean the same thing on a Glow-worm as they do on a Vaillant ecoTEC, and why an engineer who works on one is immediately at home with the other. Our Vaillant fault code guide covers those two codes in more depth and the detail carries across.

The corresponding warning is that this does not apply to every code or every range. Glow-worm has also sold boilers that predate that platform and use a different code set, and the low-numbered codes are exactly where the two sets diverge. The same F1 or F4 can point at an ignition fault on one range and a temperature sensor fault on another. So use the shared codes with confidence, treat the low-numbered ones as model-specific, and check anything you are unsure about against the manual for your exact model.

The last thing to hold on to is the general principle: a fault code names the check that failed, not the part that broke. F75 does not mean "your pump is dead". It means the boiler expected to see a pressure change when the pump ran and did not see one. That could be the pump, the sensor that reads the pressure, or the water in the system. Which of the three it is takes a meter and a set of eyes inside the casing.

F22: Low Water Pressure

What the boiler is saying: there is not enough water in the sealed heating circuit to fire safely, so I have shut down before I damage the heat exchanger by running it dry.

This is the friendliest code on the page, and the only one a homeowner can usually resolve without an engineer, because everything you need to touch is outside the casing.

What you can safely check and do yourself

• Read the pressure gauge on or near the boiler, or the digital pressure reading on the display. Below about 1 bar when the system is cold is low; below roughly 0.8 bar is usually a lockout.
• Top the system up through the filling loop. On most Glow-worm installations this is a short braided metal hose underneath the boiler, often with two lever handles that need opening together. Some models use a built-in filling arrangement instead, so if there is no braided hose in sight, check your model's user guide rather than hunting for one. Our step-by-step repressurising guide walks the whole process and shows the valve types.
• Stop at 1.2 bar. Do it with the boiler switched off and cool, open the valves slowly, and close them the instant the needle reaches 1.2. Releasing excess pressure is far more awkward than adding it.
• Reset once if the code has not cleared by itself when you switch the boiler back on.

When you stop and call an engineer

Call if the pressure will not rise at all when you open the filling loop, if the code persists at 1.2 bar, or if the gauge reads above 2.5 to 3 bar, in which case the answer is to release pressure rather than add it and that is not a homeowner job.

More importantly, call if F22 keeps coming back. A sealed system that is behaving should hold pressure for six to twelve months between top-ups. If you are topping up weekly, the water is going somewhere: a weeping radiator union, a hidden pipe joint, a failed expansion vessel, or a pressure relief valve discharging out through the wall. Repeated topping up masks the fault and pumps fresh oxygenated water into the system, which accelerates corrosion in the radiators and the heat exchanger. We rank the four causes by how often we actually see them in why your boiler keeps losing pressure.

F1: Ignition or Flame Fault (Betacom, Flexicom and Energy)

What the boiler is saying: I ran my ignition sequence and no flame was established, or a flame I did establish was lost, so I have stopped rather than keep trying.

On the Glow-worm ranges most people are looking this up for, F1 is an ignition or flame fault, and you can usually hear it happening: a run of clicks from the ignition electrodes, a pause, another attempt, and then the code. Because the complaint is that the boiler could not light, every productive check is about whether gas is actually reaching the appliance and whether the combustion and condensate paths are clear. All of those checks sit outside the casing, and in a London winter there is a genuinely good chance one of them fixes it without a call-out at all.

What you can safely check and do yourself

When you stop and call an engineer

If the gas is on, other appliances work, the meter is in credit and the condensate pipe is clear, then the remaining causes all sit inside the appliance. The gas valve, the ignition and flame sensing electrodes, the fan and the flue are Gas Safe territory without exception. Two resets is the limit. A lockout after failed ignition is the boiler doing precisely what it is designed to do, and repeatedly forcing it to retry a start sequence it has already judged unsafe is the one thing you should not do.

F75: No Pressure Change Detected When the Pump Starts

What the boiler is saying: I started the pump at the beginning of a demand and expected the water pressure to change as water began to move. It did not change, so I do not trust that the circuit is circulating and I have shut down.

F75 is one of the codes Glow-worm shares directly with Vaillant, and it is widely misread as "the pump has failed" by people who then find the pump is fine. The boiler is reporting a mismatch between two things: the pump running, and the water pressure sensor reporting a change. In practice, three causes cover the overwhelming majority of F75 call-outs.

• A faulty water pressure sensor. The sensor reads system pressure and reports it to the boiler's control board. In London's hard water areas it scales up and stops reading changes accurately, so the boiler sees a flat line even though the pump is doing its job. This is the most common single cause we find.
• A failed, seized or airlocked pump. If the pump genuinely is not moving water, the boiler is correct to shut down. Pumps seize after long summer idle periods, which is why the first cold week of October is always the busiest of the year, and sludge in an older London system is a frequent contributor.
• Very low system water. If pressure is already near zero there is little for the pump to act on, and F75 can appear alongside or instead of F22.

What you can safely check and do yourself

• Check the pressure gauge first. This is the only cause of the three you can address from outside the casing. If it reads below 1 bar, top up to around 1.2 bar through the filling loop exactly as described for F22 above.
• Reset once after topping up, and give the boiler a full start cycle.
• Listen. Stand near the boiler on a demand. A pump that hums but produces no flow sound, or a boiler that is silent where you would normally hear circulation, is a useful observation to pass on. Listening is fine. Opening anything to look is not.

The good news is that F75 is normally a same-visit fix. A water pressure sensor is an inexpensive part and a straightforward swap, and an engineer who knows the code before arriving will usually carry one.

F4 and the Codes That Vary by Range

This is the section where we are going to be honest rather than complete, because on Glow-worm that is the more useful thing to be.

F4 is a code we will not give a single definitive meaning for. Glow-worm has used more than one code set across its ranges, and the low-numbered F codes are exactly where those sets diverge. Depending on the range, a code in that group can relate to a temperature sensor, to flame detection, or to a safety shutdown. Publishing one answer would send a decent share of readers looking at the wrong component, and on a gas appliance that is not a harmless mistake.

What is consistent is what you should do:

The same applies to any other Glow-worm code not covered above. We have deliberately kept this page to the codes we can stand behind rather than reproducing a long list of half-remembered numbers, because a confident wrong answer about a gas appliance is worse than no answer.

Resetting a Glow-worm Safely

On most Glow-worm models the reset is a dedicated button or the flame symbol on the control panel, held for around five seconds until the display responds. That much is straightforward. The discipline is in how often you use it.

• Only reset after you have changed something. Topping the pressure up, thawing a condensate pipe or turning the gas back on are changes. Pressing reset again five minutes later is not.
• Two or three attempts, maximum. Once a fault persists through a small number of resets, stop and get qualified help.
• A code that clears and returns the same day is a persistent fault. Book the visit rather than resetting through the evening.
• Never reset an overheat or safety shutdown repeatedly. If the boiler is banging, kettling or smells of scorching, turn it off at the switched fused spur and book the visit as an emergency.

The lockout is not the boiler being awkward. It is a safety mechanism that has decided a start sequence is not safe to repeat, and overriding it by habit is how a cheap sensor fault becomes an expensive one.

Where You Must Stop: The Gas Safe Line

This is the part that matters more than any code on this page. Under the Gas Safety (Installation and Use) Regulations 1998, work on a gas appliance in the UK may only be carried out by an engineer on the Gas Safe Register. Removing the casing of a sealed-system gas boiler counts as work on the appliance. It is a criminal offence for anyone else to do it, it invalidates the boiler warranty and often the home insurance, and it puts you alongside live mains electrics, a pressurised water circuit and a gas carrying component simultaneously. It also breaks the room-sealed integrity of the appliance, which is what keeps combustion products in the flue rather than in your home.

So the boundary is simple and it is physical: outside the casing is yours, inside the casing is not.

When an engineer does attend, ask to see their Gas Safe ID card and check the registration number at gassaferegister.co.uk. The card shows which appliance types they are qualified to work on, and the number on the card should match the one on your invoice.

What These Repairs Typically Cost in London (2026)

Indicative London pricing. The call-out covers travel and the first 45 minutes on site including diagnosis, and any parts are quoted before fitting rather than after.

Two things are worth knowing before you book. First, the call-out is not dead money: on most fault code jobs the diagnosis and a minor fix both happen inside that first window, so plenty of visits finish at the £60 to £120 mark. Second, if the diagnosis lands on a PCB or a heat exchanger on a boiler that is already 12 to 15 years old, pause and price a replacement before committing. Our emergency boiler repair guide for London sets out the full repair versus replace maths.

Landlords: A Fault Code Is a Repair Clock Starting

If you let the property, a locked-out boiler is a legal matter as well as a practical one. Under Section 11 of the Landlord and Tenant Act 1985 you must keep the installations for space heating and hot water in repair and proper working order. Under the Homes (Fitness for Human Habitation) Act 2018, a home left without heating or hot water, particularly in winter or where a child, an elderly or a vulnerable tenant lives, can be judged unfit for human habitation.

There is no single statutory deadline, but the accepted benchmark is to act within roughly 24 hours for a total loss of heating or hot water, and same day where a vulnerable person is affected. Two practical points: ask the tenant to photograph the display showing the code and to note the model name from the front panel, which speeds up the dispatch and gives you a dated record, and never coach a tenant into opening the boiler. Separately, the annual Gas Safety Record (CP12) from a Gas Safe engineer is required for every gas appliance in the property regardless of whether anything has broken.

Frequently Asked Questions

The Bottom Line

Of the codes on this page, F22 is the only reliable homeowner fix, and even then a repeating F22 is a leak or an expansion vessel telling you something. F1 on the ranges where it is an ignition fault gives you four checks worth doing (gas on, other appliances, meter credit, frozen condensate) and a decent chance of clearing it yourself in a London winter. F75 gives you one check worth doing (the pressure gauge) before the diagnosis moves inside the casing. And F4, along with the other low-numbered codes, is a reminder that on a Glow-worm the code alone is not enough: the model name is half the answer.

Note the code exactly as displayed along with your model name, do the outside-the-casing checks, reset no more than twice, and if you smell gas ring 0800 111 999 before anything else.

Emergency Repairs London provides same-day and 24/7 emergency boiler repair with Gas Safe registered engineers across all 32 London boroughs, including Glow-worm diagnostics and sensor, pump, fan and expansion vessel replacement. Fixed £60 daytime call-out, quoted before work starts. Lines are open 24/7 on 0207 046 1363.