Glow-worm F24 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F24 fault code mean?
The F24 code appears when your Glow-Worm boiler detects that the water temperature in the heating circuit is climbing far more rapidly than it should. Under normal operation, the water temperature increases gradually as the boiler fires; when it surges upward too quickly, the flow pipe NTC thermistor picks this up and sends a signal to the PCB, which locks the boiler out to prevent overheating. On certain Glow-Worm models the fault is triggered specifically when the central heating return temperature exceeds 90°C, but the root cause is the same either way: heat generated inside the boiler is not being dispersed into the heating system quickly enough, usually because water is not circulating properly. This is a safety lockout and should not be ignored or repeatedly reset without investigating the underlying cause.
General guidance only — not a substitute for professional advice. 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
- Failing or seized circulation pump Common
The pump is responsible for moving heated water around your radiators and back to the boiler. If it is failing, running too slowly, or has seized — often after a long summer lay-off — water barely moves and the heat exchanger temperature spikes rapidly, triggering F24. A blocked or incorrectly speed-set pump has the same effect.
- Sludge or scale build-up restricting flow Common
Over time, magnetite sludge and limescale accumulate inside the heat exchanger, pipework, and radiators, narrowing the passages through which water flows. This restriction slows circulation dramatically and causes localised overheating. Systems without a magnetic filter or inhibitor are particularly vulnerable. A kettling or rumbling noise from the boiler is a strong indicator of this problem.
- Low system pressure Common
Glow-Worm boilers typically need the system pressure to sit around 1–1.5 bar (1.3 bar is ideal). Below this, the pump cannot push water around the circuit effectively, and the boiler overheats before water can carry the heat away. Check the pressure gauge on the boiler front — if it reads below 1 bar, the pressure needs topping up via the filling loop.
- Trapped air or airlocks in the system Sometimes
Air pockets in radiators or pipework interrupt the flow of water, creating sections of the circuit where circulation stalls. The boiler then heats a smaller volume of water very quickly, causing the temperature to spike. Gurgling sounds from radiators or cold spots at the top of panels are telltale signs.
- Closed or partially closed isolation valves Sometimes
The isolation valves fitted beneath or near the boiler control water flow to and from the unit. If one has been accidentally knocked to the closed position — perhaps during other maintenance work — circulation is severely restricted and F24 can follow.
- Faulty flow or return NTC thermistor Rare
Although less common, a defective temperature sensor can send inaccurate readings to the PCB, causing the boiler to believe temperatures are rising dangerously fast even when circulation is perfectly normal. This is a diagnosis-by-exclusion finding, typically reached after other causes have been ruled out.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar. If it is below 1 bar, top up the pressure using the filling loop (usually a braided flexible hose with one or two valves beneath the boiler). Open the valve(s) slowly until the gauge reads around 1.3 bar, then close them. If pressure frequently drops, there may be a leak that needs an engineer to investigate.
- Check that all isolation valves are fully open DIY safe
Locate the isolation valves on the pipework directly connected to the boiler. When the slot on the valve head runs parallel to the pipe, the valve is open; when it is perpendicular (across the pipe), it is closed. Ensure all relevant valves are fully open before attempting a reset.
- Bleed the radiators to remove trapped air DIY safe
Turn the heating off and let it cool for around 20 minutes. Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve (a small square fitting, usually at the top corner of the radiator). Hold a cloth underneath. When the hissing stops and a small amount of water dribbles out, close the valve. Work your way back towards the boiler. After bleeding all radiators, recheck the system pressure and top up if needed.
- Attempt a boiler reset DIY safe
On most Glow-Worm models, turn the central heating control knob to the reset position (marked with a power symbol), hold it for a few seconds, wait 30 seconds, then turn the knob to your desired temperature. If the boiler fires up and runs normally, monitor it over the next hour. If it locks out again immediately or within a short time, do not continue resetting — move on to calling an engineer, as repeated resets can stress the PCB and burner components.
- Have the circulation pump inspected and tested Gas Safe engineer
A Gas Safe registered engineer will check whether the pump is running, confirm it is operating at the correct speed setting for the size of your heating system, and determine whether it can be freed, cleaned, and refitted or whether it needs replacing. Pump replacement typically involves isolating the water supply, draining the relevant section of the system, and refitting — work that must be carried out by a competent professional.
- Arrange a powerflush if sludge is suspected Gas Safe engineer
If the engineer suspects magnetite sludge or scale is restricting flow — particularly if the boiler is kettling or if multiple radiators have cold spots — a professional powerflush will be required. This involves connecting specialist equipment to the system to force clean water and cleaning chemicals through at high velocity, dislodging and flushing out debris. Afterwards, the engineer should add a corrosion inhibitor and, ideally, fit a magnetic system filter on the return pipe to catch future deposits before they reach the boiler.
- Have the NTC thermistor(s) tested and replaced if necessary Gas Safe engineer
If circulation appears normal but F24 keeps returning, a Gas Safe engineer can test the resistance values of the flow and return NTC thermistors using a multimeter and compare them against the manufacturer's specification. A sensor reading outside tolerance will need replacing — a straightforward job for an engineer but not safe for a homeowner to attempt given it involves partially draining the system and working on sealed components.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If your own checks have not resolved the lockout, or if you are at all unsure, contact a Gas Safe registered engineer. They have the tools, training, and legal authority to carry out a full diagnostic, safely replace components, and ensure the boiler is operating within safe parameters. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- Central heating circulation pump (e.g. Grundfos UPS2 or equivalent) · from £80
- Flow or return NTC thermistor sensor · from £25
- Magnetic system filter (e.g. MagnaClean Pro or Fernox TF1) · from £65
- Corrosion inhibitor (e.g. Fernox F1 or Sentinel X100, 1 litre) · from £18
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–£450, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the Glow-Worm F24 to get my heating back on?
One or two resets are acceptable while you check obvious things like system pressure and bled radiators. However, if the boiler locks out again quickly after resetting, you should stop. The F24 is a safety lockout — it is protecting the boiler from overheating. Forcing repeated restarts without fixing the root cause can damage the PCB, the heat exchanger, or the burner, turning a relatively straightforward repair into a much more expensive one.
How much does it cost to fix a Glow-Worm F24 fault?
For most households the repair falls between £120 and £450. A straightforward engineer call-out to diagnose and fix a minor issue such as low pressure or a stuck pump costs roughly £120–£180. Replacing a circulation pump typically adds up to £300–£450 all in. If sludge is the culprit, a professional powerflush runs from around £350 to £600 depending on the size of the system. In rare cases where the heat exchanger is damaged, costs can reach £550–£750 or more — if your boiler is older, it may be worth getting a replacement quote alongside a repair quote.
Why does F24 come back every winter on my Glow-Worm?
A recurring F24 — especially at the start of the heating season — usually points to a pump that is on its way out (pumps can seize after sitting idle all summer) or a gradual sludge build-up that has not been fully cleared. Fitting a magnetic system filter and having the system chemically flushed will address sludge; if the pump keeps causing issues, replacing it before it fails completely is the most cost-effective approach. An annual boiler service by a Gas Safe engineer will also catch these problems early.
Does a Glow-Worm F24 fault mean my boiler needs replacing?
Not usually. F24 is most commonly caused by circulation problems — a pump, sludge, air, or low pressure — all of which are repairable. Replacement is worth considering if the boiler is over 12–15 years old and is suffering multiple faults, or if a major component such as the heat exchanger needs replacing and the repair cost approaches the price of a new boiler. A Gas Safe engineer can give you an honest assessment of whether repair or replacement makes more financial sense for your situation.