Glow-worm F.11 fault codewhat it means and how to fix it
On Energy, Ultimate, Compact and the two-digit Ultracom2 and Betacom, F.11 is a shorted return sensor. On an Ultracom hxi or cxi, F11 is a main board connection fault. Engineer either way.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.11
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Which display are you looking at?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
F.11 with a stop, F11 on an Ultracom2, Betacom 24a/28a or Betacom2, or F11 on a Flexicom
Return temperature sensor shorted, or its harness.
Book a Gas Safe registered engineer for a return thermistor.
F11 on an Ultracom hxi, cxi or sxi, an Xtramax, or 11 on a CI display
Glow-worm's table says "main board connection fault" — the link between the display and the main PCB.
Book a Gas Safe registered engineer and say which range; this is an electrical check, not a sensor.
What F.11 means
The return sensor sits on the pipe bringing water back into the heat exchanger, and the board uses it against the flow sensor to judge whether water is moving. On an Energy, Ultimate, Easicom or Compact, F.11 is "Short circuit: Return temperature sensor", with the same causes Glow-worm give for F.10 — "NTC sensor defective, short circuit in the cable harness, cable/housing".
A shorted return sensor does more than lose one reading: every spread check on the boiler (F.23, F.24, F.72, F.84) depends on it. The two-digit Ultracom2, Betacom 24a/28a and Betacom2 list F11 as "return heating temperature sensor fault — return heating temperature sensor shorted", and the Flexicom trade table has "flow and or return thermistor fault".
The Ultracom hxi, cxi and sxi and the Xtramax diverge: their F11 is "main board connection fault", and the 24CI/18SI manual gives 11 as "no reception by the main board (MMI lead)" — the ribbon between the display and the main PCB, nothing to do with any sensor. The homeowner page carries F.11 with "this is a case for a Glow-worm engineer", and no Glow-worm user guide offers a householder step for the board version either.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Return thermistor shorted | Glow-worm's first cause on the F.xx and two-digit ranges. The bead fails low, the board reads an impossible return temperature and stops. Metered against the resistance table and replaced. | Common |
| Display-to-main-board link — Ultracom hxi, cxi, sxi, Xtramax and CI | On those ranges F11 is a connection fault between the user interface and the main PCB. The lead and its plugs are checked and reseated before either board is condemned, because the boards cost many times what the lead does. | Common |
| Harness short between the return sensor and the board | "Short circuit in the cable harness, cable/housing". The return sensor lead runs low in the casing where a leak from above lands on it, and a wet plug shorts as convincingly as a failed sensor. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
45 minutes
Return thermistor fitted £80–£160. On an Ultracom, a display lead reseated is a call-out fee; a main board replaced runs to £300.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Glow-worm flow or return temperature sensor (clip-on NTC)
£12–£35Authorised Glow-worm parts distributor
Price checked 28 September 2026
Glow-worm main PCB
£150–£320Authorised Glow-worm parts distributor
Price checked 28 September 2026
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Related Glow-worm fault codes
- F.00Get it checkedThe flow temperature sensor circuit is open — plug off, wire broken or thermistor failed — so the boiler has no flow temperature and will not run. Engineer's code on every range.Related: temperature sensors
- F.10Get it checkedF.10 on an Energy, Ultimate, Compact or two-digit Ultracom2 means the flow sensor has shorted. F10 on an Ultracom, Flexicom or Ultrapower names the return thermistor instead.Related: temperature sensors
- F.72Get it checkedThe flow and return sensors disagree by an amount the board will not accept — one sensor is wrong, or on Glow-worm's own account water is not circulating. An installer's call in the first instance.Related: temperature sensors
- F.71Get it checkedThe flow sensor is still connected but its reading has stopped changing while the burner runs — usually because it has come off the pipe. An engineer reseats or replaces it.Related: temperature sensors
- F.13Get it checkedOn a system or regular boiler with a cylinder sensor, F.13 means that sensor has shorted. F13 on an Ultracom, Flexicom or CI-range boiler points at the main board instead. Engineer's code.Related: temperature sensors
- F.84Get it checkedThe return is reading hotter than the flow — the two sensors are on each other's pipes, or the pipework is crossed. Glow-worm send this one to the installer, because it is nearly always workmanship.Related: temperature sensors
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.