Boiler
E10
The E10 error code on a Glow-worm boiler indicates a problem with the central heating water pressure — specifically that the system pressure has dropped too low for the boiler to operate safely. It’s one of the most common boiler faults UK homeowners encounter, and in many cases it can be resolved without calling an engineer. In this guide, we’ll explain exactly what’s causing it, walk you through the safe steps you can take yourself, and tell you when it’s time to pick up the phone to a Gas Safe registered engineer.
What does this error mean?
The E10 fault code on Glow-worm boilers (including popular models in the Flexicom, Ultracom, and Energy range) is a low water pressure lockout. Your boiler has a built-in pressure sensor that continuously monitors the water pressure inside the sealed central heating circuit. The safe operating range is typically between 1.0 and 1.5 bar when the system is cold. When the pressure drops below roughly 0.5 bar, the boiler detects it cannot circulate water safely and shuts itself down, displaying E10 to let you know. This is a safety feature — the boiler is protecting itself and your home from running dry, which could cause serious internal damage. The error does not mean your boiler is broken; it means it needs the pressure restoring before it will restart.
Est. repair cost
£0 for a DIY re-pressurise; £80–£200 for a Gas Safe engineer call-out; £150–£300 for expansion vessel replacement; up to £400–£600 for a heat exchanger repair
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Check the pressure gauge: Look at the pressure gauge on the front of your Glow-worm boiler (it may be a dial gauge or a digital reading on the display). If it reads below 1.0 bar, low pressure is confirmed as the issue. If it reads 1.0–1.5 bar and you’re still getting E10, skip to Step 7.
Step 2 — Inspect for visible leaks before you do anything else: Walk around every radiator in your home and check the valves at both ends for drips or white limescale residue (a sign of dried water). Check under your boiler, along any visible pipework, and around the boiler’s discharge pipe outside — if this pipe is dripping, your PRV may be faulty. Do not re-pressurise the system if you find an active leak; call a Gas Safe engineer instead.
Step 3 — Locate the filling loop: The filling loop is a short braided metal hose (sometimes built into the boiler, sometimes a separate loop beneath it) with one or two valves. On most Glow-worm combi boilers, you’ll find it underneath the boiler casing. Consult your boiler’s manual or the label inside the front panel if you’re unsure — no tools are needed for this step.
Step 4 — Re-pressurise the system slowly: Open the filling loop valve(s) slowly (usually a quarter-turn on a slotted or lever valve) and watch the pressure gauge rise. Aim for 1.2–1.3 bar — do not exceed 1.5 bar. Close the valve(s) firmly once you reach this level. If you have a keyless filling link (a blue plastic device), follow the specific instructions in your Glow-worm manual for your model.
Step 5 — Reset the boiler: Once the pressure is back in range, press and hold the reset button on your Glow-worm boiler (typically marked with a flame symbol or labelled ‘Reset’) for 3–5 seconds until the display changes. The boiler should attempt to restart.
Step 6 — Test the boiler: Turn your heating or hot water on and observe the boiler for a few minutes. Check that it fires up, that the pressure remains stable, and that the E10 code does not return. If everything looks normal, monitor the pressure gauge over the next few days.
Step 7 — If the pressure gauge reads normal but E10 persists: This suggests a faulty pressure sensor or a PCB issue inside the boiler. Do not attempt to open the boiler casing or touch any internal gas or electrical components. This is the point to call a Gas Safe registered engineer — it is not a DIY repair.
Step 8 — If E10 returns within days of re-pressurising: A system that needs topping up more than once every few months has an underlying leak or a failed expansion vessel. Re-pressurising is only masking the problem. Stop re-pressurising repeatedly (it dilutes the system’s inhibitor and can accelerate corrosion) and call a Gas Safe engineer to diagnose the root cause.
📞 When to call a professional
Because this is a gas boiler, you should call a Gas Safe registered engineer in any of the following situations — and in truth, if you have any doubt at all, calling an engineer is always the right choice. Contact one immediately if: you find any visible water leak on the system or boiler; the E10 error returns within a few days of re-pressurising; the pressure gauge reads normal but the fault persists; your boiler is over 10 years old and pressure loss is recurring; or you are not confident locating or operating the filling loop. When you call, tell the engineer: ‘My Glow-worm boiler is showing an E10 error code, which I understand is a low pressure fault. I have checked for visible leaks [and/or re-pressurised the system], but the fault has returned.’ This saves diagnostic time and helps them arrive prepared. Always verify your engineer is Gas Safe registered at gassaferegister.co.uk before booking.
⚖️ Repair or replace?
For most homeowners, a single E10 fault resolved by re-pressurising costs nothing at all. If a Gas Safe engineer is needed, a call-out to diagnose and fix a minor issue such as a faulty PRV or pressure sensor typically costs £80–£200 depending on your region and the engineer’s rates. A failed expansion vessel replacement (a common root cause of recurring E10 faults) costs around £150–£300 including labour. A leaking heat exchanger is a more expensive repair — parts and labour can reach £400–£600 — and on a boiler over 10 years old, replacement is likely the more sensible option. As a rule of thumb: if your Glow-worm boiler is under 8 years old and within warranty, always repair. Between 8–12 years old, weigh repair cost against a new boiler (typically £1,800–£3,000 installed for a quality combi). Over 12 years old with a major component failure, replacement almost always makes better financial sense, particularly given the improved efficiency of modern A-rated boilers and potential savings on energy bills. Glow-worm is part of the Vaillant Group, so parts availability is generally good for models up to around 15 years old.