Glow-worm F7 fault codewhat it means and how to fix it
F7 on an older Glow-worm combi is the domestic hot water thermistor — the sensor on the hot water outlet. It has no equivalent on the current F.xx ranges. An engineer's part.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F7
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What F7 means
F7 belongs to the combis of the F1-to-F26 generation. Glow-worm's trade tables for the cxi, sxi, SI, CI and CI Plus ranges and for the Flexicom cx and sx list F7 as "domestic hot water thermistor", the 24CI/18SI manual has 07 as "domestic hot water NTC fault", and the Xtrafast table has F07 as "domestic hot water thermistor/wiring fault". The sensor sits on the hot water outlet of the plate heat exchanger and tells the board how hot the tap water is leaving; without it the boiler cannot modulate for hot water and most of these ranges stop hot water while leaving heating alone.
The Ultracom hxi and Flexicom hx, which are system boilers with no hot water side of their own, do not carry F7 at all. Alongside it the same tables list F8 as "tank thermistor (N/A for this appliance)" — a row inherited from a storage variant that the ordinary combi never uses, so an F8 on one of these boilers is worth reading twice.
The Betacom 24c and 30c, with no F codes, flash their temperature scale at 40 for the same hot water thermistor. There is no F.07 in the Energy, Ultimate, Easicom, Betacom3/4 or Compact tables, and the two-digit Ultracom2 and Betacom boards have no 07 either; on the Compact the nearest thing is F.46, a cold water sensor short. The 24CI/18SI manual's fault-finding tree for "no hot water but the heating is working" starts with the flow rate and the hot water thermistor, and none of its steps is a householder's.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Hot water thermistor failed | Glow-worm's only named cause. The clip-on or immersion NTC on the hot water outlet reads open or short and the board stops hot water production. Metered against the resistance table and replaced. | Common |
| Wiring to the thermistor | The Xtrafast table says "thermistor/wiring fault" in one breath. The lead runs past the plate heat exchanger where a weep from a joint lands on it, and a wet plug reads as a failed sensor. | Occasional |
| Scale on the sensor pocket | Not a Glow-worm cause but a hard-water reality on a fifteen-year-old combi: a sensor coated in scale reads slowly and low, and the board may report it as faulty at the extremes. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£160
Time on site
45 minutes
A hot water thermistor fitted £80–£160 on the older combis, parts availability permitting.
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
Own this appliance and had this code? . Nothing is published until a person has read it.
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.11Get it checkedOn 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.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
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F7
I have heating but no hot water and the display shows F7. Is that consistent?
Yes. F7 is the hot water thermistor, and these boilers stop hot water while heating carries on. Book an engineer; it is a small part.
The display shows F8. Is that the same thing?
F8 is the tank thermistor and Glow-worm's tables mark it "N/A for this appliance" on the ordinary combis. If your boiler is not a storage model, read the display again and tell the engineer exactly what you saw.
Is there an F.07 on a newer Glow-worm?
No. The current ranges do not publish a hot water sensor code; the Compact's F.46 is a cold water sensor. If your display has a full stop after the F you are on the wrong page.
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 support portal
- Manufacturer technical and service documentation
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.