Glow-worm F9 fault code: what it means and how to fix it

What does the Glow-worm F9 fault code mean?

The Glow-worm F9 fault code means the boiler is sensing low system water pressure. Check the pressure gauge — if it reads below about 1 bar, top up to 1.0–1.5 bar using the filling loop and reset. If the pressure keeps dropping or F9 returns, you likely have a leak or a faulty pressure sensor.

In plain terms, your boiler needs a certain amount of water pressure in the heating circuit to run safely. When that pressure falls below the minimum it needs, the boiler shuts down and shows F9 rather than firing up with too little water in the system. It's a protective stop, not a breakdown in itself.

F9 shows up across the Glow-worm range — Energy, Betacom 4, Easicom, Ultracom and Flexicom models all use the same digital fault display. It's closely related to F22, Glow-worm's dedicated low-pressure shutdown code. Both point at the same thing (pressure that's too low) and both are usually cleared the same way, by repressurising the system.

F9 is one of the few fault codes a homeowner can often clear themselves, by topping the pressure back up — a normal maintenance job, as long as you stick to the pressure your boiler's manual specifies and don't over-fill. Do it once, though: if the pressure won't hold, that usually points to a leak, and repeated top-ups only hide it rather than fix it.

Key facts

The quick version, before we get into the detail.

• What it means: low system water pressure, or a pressure-sensor fault.
• DIY-fixable: yes — repressurise to 1.0–1.5 bar via the filling loop, then reset.
• Gas Safe job if: the pressure keeps dropping (a likely leak) or the fault persists at correct pressure (sensor).
• Related code: F22, Glow-worm's low-pressure shutdown — same cause, same fix.
• Smart Plan boiler cover: parts and labour up to £500 if your boiler is under 7 years old, or up to £200 if it's over 7 years.
• Call-outs run Mon–Fri 08:00–18:00, other than a genuine emergency; a £95 call-out fee applies in defined cases set out in the plan terms.
• Once you use a service, cover runs as a 12-month agreement (there's a 14-day cooling-off period before then), with a 75% charge if you leave early.

F9 and F22 at a glance

Why does the Glow-worm F9 code appear?

F9 appears when the system pressure falls below the level the boiler needs. A few things can cause that, and it's worth knowing which is which because they don't all have the same fix.

A small leak somewhere in the system is the most common reason pressure slowly disappears — a weeping radiator valve, a joint on the pipework, or the pressure-relief valve passing water out through its outside discharge pipe. If you've recently bled your radiators, that alone can drop the pressure enough to trigger F9, and a quick top-up sorts it.

Sometimes debris gets caught in the pressure sensor, so the boiler misreads the pressure and stops even when the system is fine. Glow-worm notes that this can also be a sign the system would benefit from a magnetic filter, and possibly a power flush, to clear out the circulating sludge. And occasionally the pressure sensor itself simply fails and needs replacing.

So the three headline causes are: genuinely low pressure (often after bleeding radiators), a small leak letting pressure escape over time, or a pressure sensor that's blocked or failing. The first is a simple top-up; the other two usually need an engineer to trace and fix.

How to fix the Glow-worm F9 code yourself

You can usually clear F9 by repressurising the system. It's a safe homeowner job and doesn't involve opening the boiler or touching anything to do with gas. Do it with the boiler cold, and take it slowly — the aim is to bring the gauge up gently, not to rush it.

1. Check the pressure gauge while the boiler is cold. If the needle is sitting below about 1 bar (often down in the red or near zero), low pressure is the cause of F9.

2. Find the filling loop — usually a silver braided hose underneath the boiler, with a valve or tap at each end. Some Glow-worm models have a built-in filling key instead; your manual shows exactly where yours is.

3. Open both valves slowly and watch the gauge rise. Close them again once it reads about 1.0–1.5 bar. Don't go past roughly 1.5 bar — over-filling can push the pressure-relief valve to discharge.

4. Close the filling-loop valves fully, then reset the boiler. It should now clear F9 and fire up normally.

If the gauge won't hold its pressure, if F9 comes straight back, or if you can see water leaking anywhere, stop there. Repressurising over and over won't fix the underlying cause, and it adds fresh oxygenated water that speeds up corrosion inside the system. That's the point to book a Gas Safe registered engineer.

One safety note that applies to any boiler work: F9 itself is a pressure fault, not a gas emergency. But if you ever smell gas or your carbon monoxide alarm sounds, leave the gas alone, open the windows, get everyone out, and call the National Gas Emergency Service on 0800 111 999.

When to call a Gas Safe registered engineer for F9

Call an engineer if the pressure keeps dropping after you've topped it up, if F9 returns soon after a reset, or if you can see water leaking from a radiator, a pipe or the boiler's outside discharge pipe. A pressure that won't hold almost always means there's a leak to find, and that's a job for someone who can trace it and seal it properly.

The other engineer case is a sensor stuck on F9 even though the gauge shows a healthy 1.0–1.5 bar. If the pressure is genuinely fine but the boiler still won't accept it, the pressure sensor is likely blocked with debris or has failed, and it needs replacing. Both of these are inside the boiler, so they're engineer-only work — you can check any engineer is qualified on the Gas Safe Register.

On cost, many F9 faults cost nothing at all when a simple top-up does the job; a leak repair or a replacement pressure sensor is a chargeable engineer visit. Ask your engineer for a quote before any work goes ahead.

If you'd rather not face a surprise repair bill, an ongoing Smart Plan boiler module covers parts and labour up to your cover limit, so a fault like a leaking valve or a failed sensor is sorted for you. Cover is modular — you pick only what you want, so you don't pay for what you don't use. Boiler cover runs up to £500 a year if your boiler is under 7 years old, or up to £200 if it's older, and a £95 call-out fee applies in defined cases set out in the plan terms. Worth being clear-eyed about the limit: on an older boiler that £200 a year can be less than a single bigger repair, so check the cover fits your boiler before you rely on it. Once you use a service the plan runs as a 12-month agreement (with a 14-day cooling-off period before then, and a 75% charge if you leave early). Smart Plan is a service plan, not insurance. We've looked after over 15,000 customers, and we're from UK Boiler Company Ltd, trading since 2014. Call-outs run Mon–Fri 08:00–18:00, other than a genuine emergency. To set cover up, build your plan online or call us on 0333 772 6247.

Glow-worm F9 fault code FAQs

Seeing F9? Book a repair or set up cover.

If a top-up won't hold or F9 keeps coming back, book a one-off repair and a Gas Safe registered engineer will trace the cause, or set up an ongoing Smart Plan boiler module for the next fault. A service plan, not insurance — parts and labour up to your cover limit (boiler £500 under 7 years, £200 over 7 years), with a £95 call-out fee in defined cases. Call-outs run Mon–Fri 08:00–18:00, other than a genuine emergency; once you use a service, cover runs as a 12-month agreement.

Helpful links

• Glow-worm F22 fault code
• Boiler pressure too low? How to fix it
• How to bleed a radiator
• Boiler & central heating cover
• No heating? Book a repair
• Glow-worm boiler fault codes (official)
• Check an engineer on the Gas Safe Register