On the Glow-worm ranges that use it, F1 is an ignition fault: the boiler tried to light five times, failed every time and locked out. It is not the low pressure code, which on Glow-worm is F22. Other ranges show the same failure as F.28. Either way, the repair is Gas Safe work.
No heating, no hot water, and an F1 on the display is a bad hour to be hunting through a boiler manual. F1 does not mean the same thing on every make, and on a Glow-worm it points at ignition, which changes who you should be ringing.
On this page
- What a Glow-worm F1 fault is telling you
- Why so much F1 advice talks about pressure
- What to check before you call anyone
- When you need a heating engineer, and not us
- If the gauge is low, repressurise and watch it
- When the pressure loss is a hidden leak
- Heating leaks in Cornwall and Devon homes
- Frequently asked questions
What a Glow-worm F1 fault is telling you
Every time a Glow-worm is asked for heat, the control board runs the same short sequence. It runs the fan, sparks the ignition electrode and opens the gas valve, then looks for a flame signal to confirm the burner has actually lit. If that signal does not arrive, the board shuts the gas valve and tries again.
F1 is what appears when that keeps failing. Glow-worm’s manuals define the code precisely: the boiler has attempted to light five times and failed on all occasions. It then locks out, and central heating and hot water both stop.
The code does not tell you which part failed
F1 describes the outcome rather than the cause. Glow-worm’s fault table lists a long run of possibilities against that one code: a closed gas tap, low gas inlet pressure, a broken or badly positioned electrode, a blocked air inlet or flue, a poor earth, or a blocked condensate pipe. Diagnosis needs an engineer with test equipment.
Why so much F1 advice talks about pressure
Search the code and much of what comes back tells you to top up your boiler pressure. That advice was written for a different badge. On Ideal boilers, for example, F1 is the low water pressure code, so cross-brand articles lean on filling loops. Glow-worm numbers things differently: its low water pressure code is F22, written F.22 on some ranges.
Which Glow-worm boilers show F1 at all
Glow-worm has used two different fault code sets. One uses short codes with no decimal point, F1, F3, F4, F9, F22, and the Flexicom cx and Ultracom cx combis are in that group. The other writes its codes with a decimal point and has no F.1 for ignition at all: on the Energy, Ultimate 3, Essential, Betacom and Ultracom2, a failed light-up is F.28, and F.01 is a return temperature sensor fault. So a plain F1 points at the first group.
| Code | Meaning in Glow-worm’s manuals | Where you see it |
|---|---|---|
| F1 | Boiler attempted to light five times and failed on all occasions | Ranges such as Flexicom cx and Ultracom cx |
| F9 | A water pressure sensor or pump fault, not simply a low gauge | Ranges such as Flexicom cx and Ultracom cx |
| F22 or F.22 | Low water pressure, or not enough water in the boiler | Both code sets |
| F.28 | Ignition unsuccessful when starting up, the equivalent of F1 | Energy, Ultimate 3, Essential, Betacom, Ultracom2 |
| F.01 | Return temperature sensor fault, nothing to do with ignition | Ranges using the decimal-point codes |
So if your gauge reads 1.2 bar and the display says F1, pressure is not the complaint. A low gauge is a second problem, not an explanation for the code.
What to check before you call anyone
A householder can safely make a short list of checks, all of them outside the boiler casing.
- Read the pressure gauge and write the figure down. Glow-worm’s manuals put the cold filling pressure between 1 and 1.5 bar; check your own. Not the cause of an F1, but you will want it shortly.
- Check gas is reaching the house. If the hob will not light or another gas appliance is dead, the supply is the problem, not the boiler. On a prepayment meter, check the credit.
- In cold weather, look at the condensate pipe, the plastic pipe running from the boiler to a drain. Glow-worm lists a blocked condensate as one cause of F1, and says the blockage is usually ice in the section outside the house. It is often reported alongside a gurgling noise.
- Look at the flue terminal outside. A bird’s nest or drifted snow across the outlet will stop a boiler firing.
Not on that list: the front panel. Everything behind the casing is gas side, and out of bounds.
Does a Glow-worm F1 clear with a reset?
Glow-worm boilers have a reset button on the control panel; your manual will show which one. Glow-worm’s instructions are to hold it down for more than three seconds to clear a fault code. Sometimes that is all it takes after a brief interruption to the gas supply.
One reset is reasonable. Glow-worm’s advice is that if the code will not clear, or comes back despite several attempts, you stop and call for service. Resetting will not repair an electrode, clear a flue or thaw a pipe.
When you need a heating engineer, and not us
Worth being blunt: DCI is a leak detection company. We find water escaping from pipework. We do not repair boilers, service them or fit parts.
An F1 lives on the gas side of the appliance. Under the Gas Safety (Installation and Use) Regulations 1998, work on a gas fitting is reserved for a Gas Safe registered engineer, and Glow-worm says the same in its manuals. That covers the gas valve, the electrodes, the fan and the flue. A boiler that will not fire, an ignition lockout, a failed sensor or electrode: nothing we could usefully do for you.
What does become our territory is a system that keeps losing water.
If the gauge is low, repressurise and watch it
Topping a sealed system back up through the filling loop is a normal householder task: no tools, no panels removed. Our walkthrough on how to increase boiler pressure covers the sequence.
Done properly, it becomes a test rather than a patch:
- Fill slowly to the cold figure in your manual. Glow-worm gives 1 to 1.5 bar. Do not overshoot.
- Write the reading and the date on paper taped near the boiler.
- Read it again after a week, then again after a month.
| What the gauge does after topping up | What it means | What to do |
|---|---|---|
| Holds steady for weeks or months | No leak. The system shed a little through bleeding radiators or a one-off | Nothing. You do not need us |
| Drops back within days or weeks, again and again, with nothing visible | Water is leaving a sealed system somewhere you cannot see | Rule out the visible suspects, then have the circuit traced |
Before assuming the worst, check where water escapes in plain sight. A weeping radiator valve can drag the gauge down over a fortnight while leaving little more than a stain on the carpet. A damp patch behind the boiler, or the relief valve discharge pipe dripping outside, points at a plumbing repair.
When the pressure loss is a hidden leak
Barring the expansion vessel and the relief valve, pressure in a sealed system only falls if water leaves the circuit. Once an engineer has checked those two and nothing is visible at the radiators or the boiler, what remains is pipework you cannot see: buried in a screed floor, chased into a wall, or under floorboards.
A buried heating leak has its own tells. A patch of floor that is warm when the heating runs, or a length of pipe that never warms up, is worth noting. Our guide to a boiler that keeps losing pressure works through the fuller list of causes.
Finding it means narrowing down before opening up. Thermal imaging shows where warm water is tracking beneath a floor, and acoustic sensors pick up water escaping under pressure. Where a circuit can be drained, tracer gas, a safe mix of 5% hydrogen and 95% nitrogen, is picked up at the surface with a sensitive probe. That is what central heating leak detection is for.
Heating leaks in Cornwall and Devon homes
Two things about the housing stock down here shape how this plays out. Granite and cob cottages tend to have heating pipe retrofitted through solid floors and thick walls long after the house was built, so very little of the circuit is on view. And holiday lets stand empty for weeks, so a gauge that creeps down goes unread until somebody arrives to a cold property.
Exposed coastal and moorland properties add the condensate problem on top. A pipe run along an outside wall on high ground can freeze on a night when a sheltered valley house is fine, and the boiler locks out with nothing wrong beyond a plug of ice. Where the cause turns out to be water rather than gas, the job is to trace the heating circuit and mark the spot before anything is lifted.
Frequently asked questions
What does F1 mean on a Glow-worm boiler?
On the Glow-worm ranges that use it, F1 means the boiler has attempted to light five times and failed on all occasions, so it has locked out. Heating and hot water both stop until an engineer clears the cause. On ranges using decimal-point codes, the same failure shows as F.28.
Does F1 on a Glow-worm mean low water pressure?
No. That is the meaning on some other makes, Ideal among them, which is why so much F1 advice online is about filling loops. Glow-worm reports low water pressure separately, as F22 or F.22, so read the pressure gauge on its own merits rather than reading it into an F1.
Can I reset a Glow-worm boiler showing F1?
Yes, once. Glow-worm’s instructions are to hold the reset button down for more than three seconds to clear a fault code. If it locks out again immediately, or the code comes back despite several attempts, stop resetting and book a Gas Safe registered engineer instead.
What pressure should a Glow-worm boiler run at?
Glow-worm’s manuals give a cold filling pressure of 1 to 1.5 bar, though you should check the figure for your own model. Read it when the heating has been off for a few hours, because the needle sits higher once the system is hot and that reading tells you less.
Can a frozen condensate pipe cause an F1 fault?
It can. Glow-worm lists a blocked condensate as one of the causes of F1, and its manuals note that in freezing weather the blockage is often ice forming in the section of pipe outside the house. Thawing that section with warm cloths or warm water, then resetting, usually clears it. Lagging helps prevent a repeat.
My Glow-worm keeps losing pressure after I top it up. Is that a leak?
Probably, if nothing is visible. Once the expansion vessel and the pressure relief valve have been ruled out, a sealed system loses pressure because water is leaving it, so repeated topping up with no drip at a radiator, a valve or the boiler itself points to buried pipework. Have those two checked first, then trace the circuit.
Do I need a Gas Safe engineer or a leak detection specialist?
A boiler that will not fire, an ignition lockout or a sensor fault is a Gas Safe registered engineer’s job, and no leak detection company can help with it. A system that keeps losing water with nothing visible is the opposite case, and that is when leak detection earns its place.
Topped the pressure up more than twice this month?
Repeat top-ups on a sealed system, with nothing visible on the floor, mean water is escaping somewhere, and tracing it across Cornwall and Devon is what we do. If the pressure held after one top-up, you do not have a leak and you do not need us.
Call Dickie on 07822 025 911