Glow-worm F.86 fault codewhat it means and how to fix it
The 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.
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Get a Gas Safe registered engineer to look at your Glow-worm F.86
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What F.86 means
On the Energy7, Ultimate3, Easicom3, Betacom3 and Betacom4, Glow-worm print F.86 as "Fault: Underfloor heating contact — underfloor heating contact open, sensor disconnected or defective". The Compact manual generalises it to "external safety shutdown" and lists what might be behind it: limit thermostat settings incorrect, the flow sensor reading deviating values, a blocked three-port diverter valve, a defective condensate pump. The idea is the same on every range. The board has a pair of terminals for an external safety device — in a UK house almost always the high-limit stat on an underfloor heating manifold, sometimes a condensate pump's alarm contact — and it will not run while that circuit is open.
On a boiler without any such device the terminals carry a factory link, and F.86 on one of those means the link has been removed or has fallen out, which is exactly what the Ultracom2, Betacom 24a and Betacom2 manuals say for their two-digit F86: "check that the sensor is connected to the main board and the shunt is removed" when a sensor is fitted, and by implication that the shunt is present when it is not.
So F.86 divides neatly. If you have underfloor heating, the floor got too hot or its stat tripped, and the boiler is protecting your floor; let it cool and see whether the code clears. If you have no underfloor heating and no condensate pump, a wire has come adrift inside the casing, and that is an engineer's five minutes. Glow-worm's user guides give the householder nothing for F.86 on any range.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Underfloor limit thermostat tripped | The stat on the manifold opens when the floor circuit gets too hot — a mixing valve failed, a pump on the manifold stopped, or the stat is set too low. Glow-worm's "underfloor heating contact open". | Common |
| Factory link removed or fallen out | On a boiler with no external safety device the terminals must be bridged. The two-digit manuals talk about the shunt; a link disturbed during installation or a controls change leaves the circuit open. | Common |
| Sensor or wiring defective | Glow-worm's "sensor disconnected or defective". A broken wire to the manifold, or a stat that has failed open, reads as a permanent trip. | Occasional |
| Condensate pump alarm — Compact | The Compact manual lists a defective condensate pump against F.86, because a pump's alarm contact can be wired to the same terminals. If your boiler drains through a pump, its float or its wiring is a candidate. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
45 minutes
Link or wire refitted £80–£130. Limit stat £60–£120. Manifold valve or pump £150–£300.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
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Glow-worm main PCB
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Price checked 28 September 2026
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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.24Get it checkedThe 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.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.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.86
I have no underfloor heating. Why am I seeing this?
Then the terminals should be bridged and are not — a link out, or a wire off. Glow-worm's two-digit manuals talk about the shunt for exactly this reason. It is a quick job, but it is inside the casing.
I do have underfloor heating. Can I wait for it to cool?
Yes. If the manifold's limit stat has tripped, the code clears when the floor circuit cools and the stat closes. If it trips again, the manifold mixing valve or pump wants attention from whoever fitted the floor.
Cost?
A link refitted or a wire reconnected, a call-out fee of £80–£130. A limit stat £60–£120. A manifold mixing valve or pump, £150–£300, from a heating installer rather than a boiler engineer.
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
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.