Boiler
E15
The E15 error code on a Glow-worm boiler indicates a fan or air pressure fault — specifically a problem with the boiler’s flue gas exhaust or air supply system. This is a safety lockout, meaning your boiler has shut itself down and will not restart until the fault is resolved.
What does this error mean?
Your Glow-worm boiler constantly monitors air pressure inside the flue system using a component called the air pressure switch (APS). The E15 code is triggered when the boiler’s control board detects that the fan is not generating the correct air pressure before or during a heating cycle. In plain terms, the boiler believes combustion gases may not be safely venting to the outside, so it locks out to prevent a dangerous build-up of carbon monoxide inside your home.
Est. repair cost
£80–£350 depending on the fault; fan replacement is the most expensive single repair at £150–£350 installed
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame symbol) for 3–5 seconds. If the boiler fires and runs normally, monitor it closely for 24 hours. A one-off lockout can occur after a power cut or brief pressure fluctuation.
Step 2 — Check the external flue terminal: Go outside and visually inspect the flue outlet on your exterior wall or roof. Look for obvious blockages such as a bird nest, debris, or ice. Do not attempt to reach a roof-mounted flue yourself — call a professional instead.
Step 3 — Check the condensate pipe: Locate the white plastic condensate pipe (usually exiting through an outside wall or into a drain). If the weather is freezing, this pipe may be iced up. You can carefully pour warm (not boiling) water over the external section to thaw it, then reset the boiler.
Step 4 — Do not attempt further DIY: The remaining causes — a faulty fan, failed APS, split hose, or PCB issue — are all internal boiler components. Gas boilers are legally required to be serviced and repaired by a Gas Safe registered engineer. Do not open the boiler casing yourself.
Step 5 — Note the boiler’s behaviour for the engineer: Before calling, jot down how frequently the lockout occurs, whether the fan is audible when the boiler tries to start, and your boiler’s model number (found on the front casing). This will help the engineer diagnose the fault faster.
📞 When to call a professional
Call a Gas Safe registered engineer if the boiler does not reset, returns the E15 code after a successful reset, or if you suspect a blocked flue that you cannot safely reach. Tell the engineer: ‘My Glow-worm boiler is showing an E15 fault — I think it may be the fan, air pressure switch, or flue.’ If you smell gas at any point, do not attempt a reset — leave the property, call the National Gas Emergency Service on 0800 111 999 immediately.
⚖️ Repair or replace?
A fan replacement typically costs £150–£350 all-in with labour, while an APS or condensate repair is usually £80–£180. If your Glow-worm boiler is under 10 years old and otherwise reliable, repair is almost always the right call. For boilers over 10–12 years old with a history of faults, a new A-rated combi boiler (£1,800–£3,000 installed) is worth serious consideration — newer models are far more efficient and come with manufacturer warranties of up to 10 years.