Glow-worm F.24 fault codewhat it means and how to fix it
The 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.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.24
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 F.24 means
Glow-worm define F.24 as "Safety shutdown: Temperature rise too fast" on the current ranges, and the Ultracom2 and Betacom2 manuals as "water circulation fault — malfunction of the pump (excessively rapid temperature increase)". The burner has just lit and the flow sensor has jumped several degrees in a few seconds, which can only happen if the water at the sensor is not being replaced. The board stops the burner before F.20 would have to.
Glow-worm's causes are "pump blocked, insufficient pump output, air in product, system pressure too low, non-return valve blocked/incorrectly installed"; the homeowner page adds the sensors and the expansion vessel, and the Ultracom2 manual's check is that the flow and return isolation valves under the boiler are open.
Two of those an owner can look at — pressure and the valves — and Glow-worm's homeowner page nonetheless routes F.24 on a system or combination boiler to a Glow-worm engineer, with only open-vented installations sent to an installer first, so this page carries no steps beyond that. The Flexicom list has a different F24, "central heating return temperature above 90 degrees", and the older cxi and CI-era ranges used 24 the same way, so on those boilers the code is an overheat on the return rather than a fast rise on the flow — the same family of fault seen from the other pipe.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump stalled or too slow | Glow-worm's first two causes and the Ultracom2's only one. The burner heats water the pump is not moving, and the flow sensor a few inches above the heat exchanger sees it first. | Common |
| Air or low pressure | Listed together because they behave the same way: a pocket of air at the top of the boiler, or too little water to fill it, and the sensor is measuring steam rather than flow. | Common |
| Isolation valves under the boiler not fully open | The Ultracom2 and Betacom2 manuals put this first. After a service or a filter change the flow or return valve can be left a quarter turn short, and the boiler behaves as if the pump had failed. | Occasional |
| Non-return valve stuck or fitted backwards | Glow-worm's last cause on the current ranges. A check valve in the bypass or the flow that has jammed shut turns the primary circuit into a dead end. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£420
Time on site
1 hour
Valve opened or purge £80–£130. Pump £250–£420. Non-return valve £90–£180.
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.
Expansion vessel (8 litre, sealed system)
EV-8L£35–£85Plumbing merchant
Price checked 28 September 2026
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
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.83Get it checkedThe 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.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.24
Should I check the pressure before calling?
Yes — Glow-worm list "system pressure too low" as a cause, and if it is under 1 bar, topping up as the F.22 page describes and resetting once is a sensible first move. If F.24 returns at good pressure, the fault is circulation and wants an engineer.
Why does Glow-worm treat an open-vented boiler differently?
Because on an open-vented system the pump and the pipework that decide circulation are outside the boiler, so the homeowner page sends those to an installer first. On a combi or system boiler the pump is inside the casing.
Is Flexicom F24 the same thing?
Related but not identical. Flexicom, cxi and CI-era boilers use F24 for a return temperature above 90 °C — an overheat measured on the return — where the current ranges use it for a flow that rose too quickly. Both mean the water is not moving.
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.