Glow-worm F.83 fault codewhat it means and how to fix it
The burner lit but neither sensor registered any warming — Glow-worm call it dry fire. Low pressure or a sensor off its pipe; the Energy7 guide allows two resets before a 45-minute lockout.
Take care
What was the pressure when it happened?
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.
Below 1 bar
Top up and reset once. If the pressure had dropped a long way, ask why.
1 to 1.5 bar, and two resets have not cleared it
Sensor position or circulation. The sensors are inside the casing.
Book an installer or Gas Safe registered engineer.
What F.83 means
Glow-worm's Energy7 and Ultimate3 manuals print F.83 as "Fault: Dry fire", and the definition is exact: "when the burner starts, the temperature change registered at the flow or return temperature sensor is non-existent or too small". Water that is there and moving warms up when a burner lights; water that is absent, or a sensor that is not touching the pipe, does not, and the board stops before the heat exchanger cooks.
The two causes Glow-worm give are "insufficient water in the product" and "the flow or return temperature sensor is not in the correct position on the pipe", and the Compact manual, which calls it "NTC temperature fluctuation", adds low system pressure and tells the engineer to fill the installation. The Ultracom2 and Betacom2 manuals describe their F83 as "no water in the installation: the temperature does not increase when the burner is lit", with a closed return valve, a disconnected pump and a leak as the causes.
What makes this code unusual is that the Energy7 operating instructions give the householder a specific procedure: "If fault F.83 is displayed, press the reset button. If the fault persists and you have pressed the reset button once more, a burner anti-cycling time of 45 minutes is automatically activated. The display shows S.54." So two resets, then the boiler enforces a wait, and the guide says to consult a competent person if it persists. The homeowner page routes F.83 to an installer in the first instance because it can be about the system rather than the boiler, and lists low pressure, a circulation fault and the sensors as causes — which is the order to check them in.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Too little water in the boiler | Glow-worm's first cause on every range. Pressure well below 1 bar, or a system that has just been filled and still has air at the top of the boiler, leaves the sensors reading a pipe with nothing in it. | Common |
| Sensor slipped off its pipe | The flow and return sensors are clip-on NTCs. One that has been knocked or refitted loosely reads the air in the casing rather than the water, and sees no change when the burner fires. | Common |
| Pump not running or a valve shut | The Ultracom2 and Betacom2 manual's causes: a pump disconnected or a return valve closed. Nothing moves past the sensors, so nothing warms. | Occasional |
What to try yourself
Check the pressure. If it is low, top up to the range in your guide as the F.22 page describes; Glow-worm list insufficient water first for this code on every range.
Press the reset button once, as the Energy7 guide says. If F.83 returns, you may press it once more.
If it still persists, the boiler will show S.54 and refuse to try again for 45 minutes. Do not switch it off at the wall to skip the wait — use the time to book an installer or engineer, and tell them the pressure was fine if it was.
Do not keep resetting after the S.54 wait has run. Each attempt fires a burner over a heat exchanger the boiler believes is dry.
Get a Gas Safe registered engineer to look at your Glow-worm F.83
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.
What it costs to fix
Typical repair cost
£0–£420
Time on site
20 minutes
Free if it was pressure. Sensor £80–£160. Pump £250–£420.
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.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
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.20Get it checkedThe safety thermal cut-out has tripped and stopped the boiler. Glow-worm's homeowner page lets you check pressure, radiator valves and airlocks first; on Ultracom2 boilers F20 trips at 97 °C.Related: overheat and circulation
- F.24Get it checkedThe temperature at the sensor climbed faster than water flowing properly would allow. Glow-worm list the pump, air, low pressure and a stuck non-return valve, and send it to an engineer.Related: overheat and circulation
- F.23Get it checkedThe flow and return temperatures are too far apart, which means water is barely moving through the boiler. Glow-worm point at the pump, air, or the two sensors swapped over.Related: overheat and circulation
- F.76Stop — call an engineerThe thermal fuse on the primary heat exchanger has operated or failed. It exists to catch a dry fire the sensors missed, and Glow-worm's test for it is not one an owner performs.Related: overheat and circulation
- F.86Get it checkedThe external safety contact — usually an underfloor heating limit thermostat wired to the boiler — is open, or its link is missing. The boiler will not fire until the contact closes.Related: overheat and circulation
- F.28Get it checkedThree ignition attempts have failed and the boiler has locked out. Glow-worm's guides let you check the gas supply and the condensate pipe and reset three times before calling anyone.Related: same brand
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.83
What is S.54?
A status code, not a fault: the 45-minute anti-cycling wait the Energy7 enforces after F.83 has been reset twice. It clears itself.
Is F.83 the same as F.75?
Cousins. F.75 is the pump starting without the pressure changing; F.83 is the burner starting without the temperature changing. Both say water is not moving, from different evidence.
Cost?
Free if it was pressure. A sensor refitted or replaced £80–£160. A pump £250–£420.
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.