Glow-worm F78 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F78 fault code mean?
The F78 fault code on a Glow-Worm boiler points to one of two related problems, depending on your specific model and setup. On boilers connected to Glow-Worm's eBUS control system — such as a Climapro or MiSet thermostat, or a Smart Wiring Centre — F78 signals a compatibility or communication breakdown between the boiler and those controls. This could be an incompatible controller, a wiring fault, or a PCB that can no longer process the eBUS signal correctly. On combi boilers without eBUS controls attached, F78 more commonly indicates a fault with the Domestic Hot Water (DHW) outlet sensor — the component that monitors the temperature of hot water leaving the boiler. If that sensor fails or loses contact with the PCB, the boiler shuts down as a precaution. In both cases, the boiler will typically lock out and display F78 until the underlying fault is resolved. A Gas Safe registered engineer with Glow-Worm experience can confirm which of these two causes applies to your specific installation.
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 or loose eBUS wiring Common
The eBUS wiring connects your boiler to its controls — thermostats, timers, or remote devices. If any of these low-voltage cables have become loose at the boiler terminal or the controller, kinked, or damaged through general wear, the communication signal breaks down and the boiler logs F78. This is the most common starting point for this fault.
- Incompatible or faulty eBUS controller Common
If an eBUS-compatible controller (such as a Climapro2 or MiSet) has developed an internal fault, or if a replacement controller was fitted that is not fully compatible with your particular boiler model, the boiler will detect the mismatch and raise F78. Firmware mismatches between the controller and boiler can also cause this.
- DHW outlet NTC sensor failure Common
On combi boilers not running eBUS controls, F78 typically points to the hot water outlet NTC thermistor. This small sensor monitors the temperature of water leaving the heat exchanger. If it fails, drifts out of calibration, or loses its electrical connection to the PCB, the boiler cannot safely regulate hot water output and locks out. Incorrect link-box wiring or a missing bridge at an external controller can produce the same symptom.
- Main PCB fault Sometimes
If the circuit board responsible for processing both eBUS signals and sensor readings develops a fault — due to age, moisture ingress, or a component failure — it may be unable to communicate correctly with either the controls or the DHW sensor, triggering F78 even when wiring and sensors are physically intact.
How to fix it
- Reset the boiler once or twice DIY safe
Refer to your boiler manual for the correct reset procedure. On most Glow-Worm models, turn the CH control knob to the reset (power symbol) position, hold it for a few seconds, then release and set it to your desired temperature. Wait a couple of minutes to see if the boiler fires successfully. A one-off reset can clear a transient communication glitch. If F78 returns quickly, do not keep resetting — a persistent fault needs investigation rather than repeated resets.
- Check that your gas supply is on DIY safe
Confirm other gas appliances in your home (hob, fire) are working normally. If there is no gas supply, contact your gas supplier rather than continuing to troubleshoot the boiler.
- Visually inspect the eBUS controller wiring DIY safe
If your boiler is connected to an eBUS thermostat or wiring centre, look at the cable running between the controller and the boiler. Check for any obvious signs of damage — fraying, kinking, or a loose plug. Do not open the boiler casing or touch internal terminals. If you can safely re-seat a connector at the external controller end (not inside the boiler), do so. If the wiring looks intact and undamaged, the fault lies inside the boiler or controller and requires a professional.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up using the filling loop following your boiler manual instructions. Low pressure alone is unlikely to cause F78, but it is worth ruling out before calling an engineer.
- Have a Gas Safe engineer test the eBUS controls and wiring internally Gas Safe engineer
An engineer will open the boiler casing and use diagnostic equipment to check the eBUS signal at the PCB terminals, verify the controller is correctly paired, and inspect all internal wiring connections. They can identify whether the problem is a damaged wire, a controller firmware issue, or an internal PCB communication fault. This step is not safe for homeowners to carry out.
- Have the DHW outlet NTC sensor tested and replaced if faulty Gas Safe engineer
The engineer will measure the electrical resistance of the DHW outlet thermistor against the expected values for the water temperature at the time of testing. A reading outside specification confirms sensor failure. Replacement involves draining the relevant section of the heat exchanger circuit and fitting a new manufacturer-approved sensor — work that must be carried out by a Gas Safe engineer.
- Have the PCB inspected and replaced if necessary Gas Safe engineer
If wiring, the controller, and the DHW sensor all test correctly, the fault lies with the main PCB itself. PCB replacement on a Glow-Worm boiler requires a Gas Safe registered engineer, ideally one familiar with Vaillant Group products. The engineer will source an approved replacement board and commission it correctly.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If resetting the boiler and checking the external wiring have not resolved F78, contact a Gas Safe registered engineer. Ask for one with experience of Glow-Worm or Vaillant Group boilers, as the eBUS diagnostic process benefits from familiarity with the platform. You can find registered engineers at gassaferegister.co.uk.
Parts you may need
- eBUS NTC DHW outlet sensor · from £25
- eBUS wiring harness / connector set · from £35
- Glow-Worm Climapro2 RF controller · from £120
- Main PCB (model-dependent) · from £220
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Is the Glow-Worm F78 the same as the Vaillant F78?
Yes, in practice. Glow-Worm has been part of the Vaillant Group since 2001 and many of their boiler platforms share the same underlying technology and fault-code logic. The F78 code appears across both brands and relates to the same eBUS communication and DHW sensor faults. If you are searching for information on the Vaillant F78, the diagnostic steps and causes described on this page are directly relevant.
Can I fix the Glow-Worm F78 myself?
There is very little a homeowner can safely do beyond resetting the boiler and checking the external eBUS cable for obvious damage. All internal work — testing wiring at the PCB, replacing the DHW sensor, or swapping the PCB — must be carried out by a Gas Safe registered engineer. Attempting to open the boiler and interfere with internal components is unsafe and will likely void any remaining warranty.
How much does it cost to repair a Glow-Worm F78 fault?
The most common repairs — a wiring fix, a replacement DHW outlet sensor, or a new eBUS controller — typically fall in the range of £120 to £350 including parts and labour. If the main PCB turns out to be the cause, costs rise significantly; PCB replacement on Glow-Worm boilers generally runs between £450 and £700 depending on the model and your location. London and the South East tend to be at the top of that range. If your boiler is still under its parts warranty and has been serviced annually, some of these costs may be covered by Glow-Worm — it is worth checking before authorising any repair.
Why does my F78 keep coming back after a reset?
A fault code that clears on reset but returns within a short time suggests the underlying component — a deteriorating sensor, an intermittently failing eBUS connection, or a PCB with a developing fault — has not been fixed. Repeated resets do not repair hardware. Each time the boiler locks out on F78 it is telling you the same problem is still present. If the fault has returned more than twice, stop resetting and arrange for a Gas Safe engineer to carry out a proper diagnosis.