Glow-worm F91 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F91 fault code mean?
The F91 code (sometimes shown as F.91) appears on Glow-worm boilers that are part of the Vaillant Group platform and connected to an actoSTOR hot water cylinder. It means the cylinder has failed to reach its target temperature within the expected charging period. This fault does not appear on standalone combi boilers — it is exclusive to systems using an actoSTOR indirect cylinder. If your display is showing F9 rather than F91, that is an entirely different fault relating to low system pressure, so check your display carefully. The F91 fault is almost always caused by either an airlock or failure in the actoSTOR load pump, or a problem with the NTC temperature sensor or its wiring that monitors the cylinder. Because this fault can also produce related sensor codes alongside it — most notably codes relating to the DHW outlet sensor or the cylinder NTC — an engineer will typically investigate all of these together rather than in isolation.
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
- Air trapped in the actoSTOR load pump Common
The shift or load pump that circulates water through the actoSTOR cylinder can develop an airlock, preventing adequate flow. When flow is insufficient, the cylinder cannot heat up within the allotted time and the boiler raises the F91 fault. This is the single most common trigger and is often resolved by a Gas Safe engineer purging the pump.
- Faulty or seized actoSTOR pump Common
If the pump motor has failed mechanically — either seized or burnt out — water will not circulate through the cylinder at all. The boiler detects that the storage temperature is not climbing and flags F91. A failed pump requires replacement by a Gas Safe engineer.
- Defective cylinder NTC sensor or wiring Sometimes
The NTC thermistor mounted on the actoSTOR cylinder monitors its temperature and reports back to the boiler's PCB. If the sensor itself has drifted out of calibration, its wiring harness has developed a break, or the plug connection on the actoSTOR electronics is loose or corroded, the boiler receives an inaccurate or absent temperature reading and concludes the cylinder has not charged correctly. Related sensor codes may appear alongside F91.
- Blockage or debris restricting flow through the cylinder circuit Sometimes
Sludge or scale accumulation within the cylinder circuit pipework can restrict water movement enough to slow the heating process beyond the permitted window. This is more likely in older systems that have not been power-flushed or chemically treated.
- PCB fault Rare
In rare cases the PCB itself misinterprets signals from the actoSTOR circuit or fails to control the load pump correctly, causing a false or persistent F91 fault even after other components have been checked and found to be sound.
How to fix it
- Check that your boiler and actoSTOR cylinder both have a power supply DIY safe
Confirm both the boiler and the cylinder unit are switched on at the mains and that no fuses or circuit breakers have tripped. An actoSTOR cylinder that has lost power cannot signal correctly to the boiler.
- Verify that your overall hot water system pressure is within range DIY safe
Check the pressure gauge on your boiler. It should read between 1.0 and 1.5 bar when cold. If it is below 1 bar, top up via the filling loop following your boiler's user guide. Low system pressure can compound circulation problems.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (or follow the reset procedure in your user manual) once. If the boiler fires and the fault clears, monitor it over the next day or two. If F91 returns promptly, do not keep resetting — further resets will not fix an underlying component fault and may mask it.
- Do not attempt to access the actoSTOR pump, NTC sensor, or any internal cylinder wiring yourself Gas Safe engineer
Diagnosing and repairing the load pump, thermistor, wiring harness, or PCB connections on the actoSTOR system requires specialist tools, component knowledge, and in some cases involves working on a gas-connected appliance. This work must only be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will check the actoSTOR pump for airlock and purge it if needed, test the NTC sensor resistance and wiring continuity, inspect plug connections on the actoSTOR electronics, assess the cylinder circuit for sludge or restriction, and replace whichever component is found to be at fault. They will also check whether any companion sensor codes are present alongside F91 and address those at the same time.
Parts you may need
- actoSTOR load / shift pump · from £85
- Cylinder NTC thermistor sensor · from £35
- NTC wiring harness / loom · from £45
- actoSTOR PCB / electronics module · 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can F91 appear on a Glow-worm combi boiler without a cylinder?
No. The F91 fault is specific to Glow-worm boilers connected to a Vaillant Group actoSTOR hot water cylinder. If your boiler is a standalone combi with no separate cylinder and you are seeing what looks like F91, double-check the display — F9 (without the 1) is a separate, far more common code that simply means low system pressure, which you can often fix yourself by repressurising via the filling loop.
Why does my boiler show additional sensor codes alongside F91?
The actoSTOR system uses NTC temperature sensors at both the DHW outlet and the cylinder itself. If one of those sensors or its wiring is faulty, the boiler will flag F91 because it cannot confirm the cylinder has charged, and it will simultaneously show a related sensor fault code. An engineer will read all active fault codes together and work through them systematically — fixing just one and ignoring the others is unlikely to clear the fault permanently.
How much does it typically cost to fix a Glow-worm F91 fault?
Most homeowners pay somewhere between £120 and £380 depending on what the engineer finds. Purging an airlock from the pump is at the lower end, while replacing the pump itself or the NTC sensor and harness sits in the middle of the range. If the actoSTOR PCB or electronics module turns out to be at fault, costs can climb to £400 or beyond. In rare cases where multiple components have failed on an ageing system, a new boiler and cylinder installation may become the more economical option — typically £1,500 to £2,800 fully installed.
Is it safe to keep using hot water while the F91 fault is showing?
It depends on whether the fault has caused the boiler to lock out completely or merely flag a warning. If the boiler is still operational, you may have partial hot water, but the actoSTOR cylinder is not charging to its normal temperature, which means the stored water may not reach a temperature adequate to prevent legionella bacteria growth. You should contact a Gas Safe engineer promptly rather than running the system indefinitely in a degraded state.