Glow-worm Boiler Engineer in Surrey
Installation, servicing and repairs on all Glow-worm boilers. Backed by the Vaillant Group and a trusted name in UK heating for over 80 years.
Glow-worm boiler installation
Glow-worm is part of the Vaillant Group, which means their boilers share a lot of the same engineering and quality standards as Vaillant — but at a more affordable price point. The current Glow-worm Energy combi is a solid, reliable boiler that offers great value for homeowners who want quality without paying top-tier prices.
As a Vaillant accredited installer, I have in-depth knowledge of the Vaillant Group's technology and components, many of which are shared across the Glow-worm range. This means I can install and maintain Glow-worm boilers with the same expertise I bring to Vaillant installations.
Glow-worm servicing & repairs
I service and repair all Glow-worm models — from the current Energy range back to older Ultracom, Flexicom, Easicom, and Betacom boilers. Many Glow-worm boilers share parts with Vaillant, which often means better parts availability and quicker repairs. If your Glow-worm is showing a fault code or just needs its annual service, give me a call.
Common Glow-worm fault codes
Glow-worm is part of the Vaillant Group, so newer Glow-worm boilers (Energy range) use the same "F." fault codes as Vaillant. Older models (Flexicom, Ultracom, Easicom, Betacom) use a different numbering system — I've covered both below.
Newer Glow-worm models (Energy range)
Low water pressure
What it means: System pressure has dropped below the minimum operating level. Identical to the Vaillant F.22 — same boiler platform, same code.
Common causes: Leak on the system, recently bled radiators, or a faulty pressure relief valve.
What you can try: Re-pressurise using the filling loop to around 1.2 bar. If it drops again within a few days, you have a leak.
When to call Mark: If pressure keeps dropping. As a Vaillant accredited installer, I know the Vaillant Group platform inside out — which includes every current Glow-worm model.
Ignition failure
What it means: The boiler has failed to ignite after multiple attempts. The most common Glow-worm fault code.
Common causes: Frozen condensate pipe, gas valve fault, worn ignition electrode, or gas supply issue.
What you can try: Check gas supply (is the hob working?), thaw the condensate pipe in cold weather with warm water, then try resetting.
When to call Mark: If the simple checks don't fix it, it's an internal fault. Same components as Vaillant — I carry the parts as standard.
Pressure sensor fault
What it means: No pressure change detected when the pump starts — the boiler can't confirm water is circulating.
Common causes: Seized pump, faulty water pressure sensor, or air lock in the pump.
What you can try: Not much you can safely do here.
When to call Mark: Common autumn fault when heating gets switched on after summer. The pump may just need freeing off, or the pressure sensor swapping out.
Overheat protection
What it means: The boiler has overheated and shut down for safety.
Common causes: Pump failure, sludge restricting water flow through the heat exchanger, air locks, or a faulty thermostat.
What you can try: Check radiators for cold spots (sign of sludge). Don't repeatedly reset — overheating damages the heat exchanger.
When to call Mark: I'll check the pump and system flow. A power flush is often needed to restore proper circulation.
Older Glow-worm models (Flexicom, Ultracom, Easicom, Betacom)
Ignition lockout
What it means: The boiler has failed to ignite. The older Glow-worm equivalent of F.28 on newer models.
Common causes: Gas supply issue, frozen condensate pipe, faulty electrode, or gas valve failure. On older Flexicom and Ultracom models, electrode erosion is very common.
What you can try: Same as newer models — check gas supply, thaw condensate pipe if frozen, try a reset.
When to call Mark: If it won't clear after a reset, it needs an engineer. I've worked on hundreds of Flexicoms and Ultracoms over the years.
Overheat
What it means: The boiler has overheated. On older Glow-worms, this is a simple overheat code rather than the more detailed F.20 on newer models.
Common causes: Pump failure, blocked heat exchanger, or a closed valve restricting water flow.
What you can try: Don't repeatedly reset. Check that all radiator valves are open.
When to call Mark: Needs diagnosing — the pump and heat exchanger need checking.
No ignition
What it means: Similar to F1 but indicates the boiler attempted to fire and the flame wasn't sustained.
Common causes: Gas valve not opening fully, low gas pressure, or a burner fault.
What you can try: Check your gas supply and try a reset.
When to call Mark: If it won't clear, the gas valve or burner needs inspecting.
PCB / control board fault
What it means: The boiler's circuit board has detected an internal error. On older Glow-worms, F9 can cover a range of PCB-related issues.
Common causes: PCB failure, often caused by age or moisture damage. On older Flexicom and Easicom models, PCB faults become more common after 10+ years.
What you can try: Try a single reset. If F9 comes straight back, the PCB likely needs replacing.
When to call Mark: I'll test the PCB and give you an honest assessment. If the boiler is old enough, I'll advise whether a new PCB or a full boiler replacement makes better financial sense.
Return thermistor fault
What it means: The return water temperature sensor has failed or is giving incorrect readings.
Common causes: Sensor failure due to age, loose connection, or water damage.
What you can try: Nothing to try yourself.
When to call Mark: This is a straightforward sensor replacement — a common part that I carry on the van, so it can usually be sorted on the first visit.
Overheat on primary heat exchanger
What it means: The heat exchanger itself has exceeded a safe temperature. More specific than F3 and indicates a more severe overheating condition.
Common causes: Heavy sludge buildup in the heat exchanger, pump failure, or a combination of both.
What you can try: Don't reset repeatedly — this can crack the heat exchanger.
When to call Mark: The heat exchanger may be salvageable with a thorough flush, or it may need replacing. I'll give you an honest assessment.
Glow-worm fault code not listed here? Call Mark on 07311 811574. As a Vaillant accredited installer, I have in-depth knowledge of the Vaillant Group platform that underpins every current Glow-worm boiler, and I've been servicing and repairing older Glow-worm models throughout my 21+ year career.
Glow-worm FAQs
What people say
I'm so glad we chose Mark to fit our new boiler as he did a fantastic job. He arrived early and worked till late, not stopping until all work was complete in less than 2 days.
We recently had a new boiler fitted by Mark and the service was absolutely first class from start to finish. The attention to detail was spot on.
Mark completed a gas boiler replacement for me. The work was carried out in one day to a high standard of workmanship. Communication was always very good.
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