Glow-worm F57 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F57 fault code mean?
The F57 code on Glow-Worm boilers signals a problem with the domestic hot water (DHW) circuit, but what exactly it means depends on which model you own. On the Flexicom range, F57 points to a fault with the hot water temperature sensor — the component that monitors the temperature of the DHW flow and feeds that data back to the boiler's controls. On the Betacom range, F57 instead indicates a domestic hot water pressure fault, meaning the boiler has detected an issue on the DHW side related to water pressure rather than temperature sensing. In both cases the boiler will typically lock out or restrict hot water delivery until the underlying problem is resolved. Check your boiler model first so you can focus on the correct cause.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty DHW temperature sensor (Flexicom) Common
The hot water temperature sensor has failed, drifted out of calibration, or developed an internal short or open circuit. When the boiler's PCB receives readings outside the expected range it triggers F57 to prevent unsafe operation. Sensor failure is the single most frequent cause on Flexicom models and often follows gradual corrosion or heat-related degradation.
- Loose or damaged wiring to the DHW sensor (Flexicom) Sometimes
The wiring loom connecting the hot water sensor to the PCB can suffer from loose terminals, chafed insulation, or corrosion at the plug. Even a partial break in the circuit produces erratic or absent sensor signals, which the boiler interprets as a fault. This is often found alongside sensor replacement during an engineer's visit.
- DHW pressure sensor fault or low incoming water pressure (Betacom) Sometimes
On Betacom models the F57 relates to the DHW pressure circuit. This can stem from a faulty pressure sensor giving a false reading, a blockage in the DHW pipework or strainer restricting flow, or genuinely low mains cold water pressure reaching the boiler. Any of these can cause the boiler to conclude that the DHW pressure is outside acceptable limits.
- PCB fault misreading sensor signals Rare
In a small number of cases the sensor and wiring are both intact but the PCB itself is misinterpreting or failing to process the signals correctly, producing a spurious F57. This is relatively uncommon and is usually diagnosed only after the sensor and wiring have been confirmed as healthy.
How to fix it
- Check which Glow-Worm model you have DIY safe
Look at the label on the front or inside the boiler casing. Confirming whether you have a Flexicom or Betacom model before anything else means you (and any engineer you call) can focus on the correct root cause — temperature sensor on the Flexicom, DHW pressure circuit on the Betacom.
- Attempt a single boiler reset DIY safe
Locate the reset button on the boiler fascia (usually marked with a flame or reset symbol) and hold it for the number of seconds stated in your user manual — typically 3 to 5 seconds. A one-off transient glitch can occasionally be cleared this way. If F57 returns immediately or reappears after a short time, do not keep resetting; repeated resets without a fix can mask a worsening fault.
- Check your cold mains water supply (Betacom owners) DIY safe
Turn on a cold tap elsewhere in the property to confirm you have a normal flow of mains water. If pressure is unusually low across the whole house, the issue may be with your mains supply rather than the boiler — contact your water supplier. This is a quick check that can save an unnecessary call-out.
- Arrange for a Gas Safe registered engineer to inspect the DHW sensor and wiring (Flexicom) Gas Safe engineer
An engineer will use a multimeter to measure the sensor's resistance at various temperatures and compare readings against the manufacturer's specification. They will also inspect the wiring harness for chafed insulation, loose terminals, or corrosion at the connector. A faulty sensor or damaged wiring will be replaced at this stage.
- Have the engineer examine the DHW pressure sensor and circuit (Betacom) Gas Safe engineer
The engineer will test the DHW pressure sensor electrically and assess the associated pipework and any in-line strainers for blockages or scaling. If the pressure sensor is sending incorrect readings it will be replaced. Any debris blocking the DHW circuit will be cleared.
- Request a PCB assessment if sensor and wiring tests pass Gas Safe engineer
If the engineer confirms the sensor, wiring, and pressure components are all functioning correctly but F57 persists, the PCB should be considered. PCB diagnosis and replacement is specialist work requiring appropriate components and programming — it must not be attempted by anyone other than a qualified engineer.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All of the above repair work — sensor replacement, wiring repair, pressure component servicing, and PCB work — must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at the Gas Safe Register website (gassaferegister.co.uk) before booking.
Parts you may need
- DHW temperature sensor (Flexicom compatible) · from £25
- Sensor wiring harness / connector · from £20
- DHW pressure sensor (Betacom compatible) · from £35
- PCB (model-specific) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£280, depending on the underlying cause.
Frequently asked questions
Will F57 affect my central heating or just hot water?
In most cases F57 specifically affects the domestic hot water circuit. Your central heating may continue to work normally depending on how the boiler handles the fault, but some Glow-Worm models will lock out all functions until the code is cleared. Check your boiler's behaviour — if heating has also stopped, a full lockout has occurred and a reset followed by an engineer visit is the right course of action.
Can I replace the hot water sensor myself to save money?
No. Although a DHW temperature sensor is not a gas component in itself, accessing it on a Glow-Worm boiler requires opening the casing and working on internal components. This work falls under boiler servicing regulations and should only be carried out by a Gas Safe registered engineer. Attempting it yourself could void any warranty and may create safety risks if wiring is disturbed incorrectly.
How much does it cost to fix F57 on a Glow-Worm boiler?
For the most common repair — replacing a faulty DHW temperature sensor on a Flexicom, or a DHW pressure sensor on a Betacom — most homeowners pay somewhere in the region of £150 to £280 including labour and parts. A straightforward wiring repair at the lower end of that range, or a sensor replacement with a slightly longer job, will sit comfortably within it. If the PCB turns out to be the culprit the cost rises considerably, typically to £350–£500 or more, but this is an uncommon outcome and worth discussing with your engineer before authorising the work.
My boiler is a Betacom and I have good mains pressure — why is F57 still showing?
If your mains cold water supply is confirmed as normal, the most likely explanation is that the DHW pressure sensor inside the boiler has developed a fault and is reporting an incorrect reading rather than there being a genuine pressure problem. An engineer can test the sensor electrically within a few minutes and replace it if it is out of specification. Scaling or a blockage in the DHW strainer is another possibility worth checking at the same visit.