Glow-worm F.72 fault codewhat it means and how to fix it
The 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.
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Get a Gas Safe registered engineer to look at your Glow-worm F.72
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What F.72 means
The board on a current Glow-worm expects the flow and return sensors to differ by a plausible amount while the pump runs. F.72 is raised when they do not. The Energy7 and Ultimate3 manuals put it as "Flow/return NTC temperature difference too great → flow and/or return temperature sensor defective", and the Compact manual titles the same row "Fault: Return temperature sensor" with two measures, replace the flow sensor or replace the return sensor.
Glow-worm's homeowner page is broader than its trade manuals here: "the temperature spread between the flow and return temperature sensor is invalid", caused by "flow and/or return temperature sensor defective" or a "circulation fault, pump, valve, blockage", and it names the installer "in the first instance, as it can relate to how the boiler is working with the heating system".
The two-digit Ultracom2, Betacom 24a/28a and Betacom2 list F72 as a "permanent temperature difference between flow and return heating sensors — mismatch", and their measures include "check that there are no leaks" as well as the sensor connections. F.72 sits with F.84 and F.85 as one of three plausibility checks on the sensor pair; an engineer will normally test all three possibilities in one visit — that both sensors read true, that each is on its own correct pipe, and that water is actually moving between them.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| One thermistor reading wrongly | Glow-worm's trade cause, flow or return "defective". A sensor that has drifted several degrees makes the spread implausible without failing an open or short-circuit test, so it is proven against a thermometer on the pipe. | Common |
| Circulation fault holding one pipe hot and the other cold | The homeowner page's second cause — "pump, valve, blockage". A pump that has slowed or a valve left shut leaves the flow climbing while the return stays at room temperature, and the spread runs away. | Occasional |
| Sensor partly off its pipe | A clip half on the pipe reads somewhere between the water and the casing air. It is the same fault as F.71 seen from the pair rather than the single sensor. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£200
Time on site
1 hour
One or both thermistors £80–£160 fitted. If the spread is real and the pump is at fault, expect the top of the range or above.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
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Glow-worm flow or return temperature sensor (clip-on NTC)
£12–£35Authorised 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.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.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
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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.