Glow-worm F.77 fault codewhat it means and how to fix it
F.77 on current ranges is the flue non-return flap or condensate pump not answering. On a Betacom2 or Ultracom2 it is the gas valve motor; on an Ultracom or Flexicom the optional condensate pump.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.77
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.
Which display are you looking at?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
F.77 with a stop
Flue non-return flap or condensate pump not responding.
Book a Gas Safe registered engineer. Tell them whether a condensate pump is fitted.
F77 on an Ultracom2, Betacom 24a/28a or Betacom2
Glow-worm's "fault in gas valve motor", or the optional condensate pump.
Book a Gas Safe registered engineer. This is a gas valve check; do not reset repeatedly.
F77 on an Ultracom hxi or a Flexicom
"Condensate pump error (optional)" — the pump, its float or its wiring.
If you can see the pump is full or its pipe is kinked, say so when you book the engineer.
What F.77 means
Three generations give F77 three meanings. On the Energy, Ultimate and Easicom it is "Fault: condensate or smoke — no response, flue non-return flap defective": where a boiler shares a flue with others, a non-return flap stops flue gas from another appliance being pushed back, and the board expects the flap to report its position. The Compact manual widens the title to "flue non-return flap/condensate pump" and adds "condensate pump defective — replace the condensate pump", because a condensate pump's alarm contact can be wired to the same input.
On the two-digit Ultracom2, Betacom 24a/28a and Betacom2, F77 is "fault in gas valve motor — defective gas valve, defective condensate pump (option)", and the measures are to check the gas valve connections and operation and, if fitted, the condensate pump. On the older Ultracom hxi and Flexicom the trade table has F77 as "condensate pump error (optional)", full stop. So a homeowner with a Betacom2 who reads the current manual's explanation will be told about a flue flap their boiler does not have.
In every version the common thread is an external device the board is waiting to hear from, and in every version Glow-worm give the householder nothing to do. The one useful observation an owner can make is whether a condensate pump is fitted at all — a small box under or beside the boiler with a pipe running uphill from it — because if there is one, its float or its wiring is the first place the engineer will look.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Condensate pump alarm — any range with a pump fitted | A condensate pump has a float switch that signals when it cannot keep up. Glow-worm name it on the Compact, the two-digit boards and the older Ultracom and Flexicom. A pump full of sludge or with a kinked discharge pipe trips the alarm and the boiler stops. | Common |
| Flue non-return flap not answering — Energy, Ultimate, Easicom, Compact | "No response, flue non-return flap defective." On a shared flue installation the flap's feedback contact has failed or its wiring has broken. On a boiler with its own flue and no flap, a board configured to expect one gives the same code. | Occasional |
| Gas valve motor — Ultracom2, Betacom 24a/28a, Betacom2 | Glow-worm's first cause on the two-digit boards is the gas valve, not the flue. The measures are to check its connections and its operation. A gas valve is a gas-carrying part and no owner should touch it. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£320
Time on site
1 hour
A condensate pump cleared or its float freed £80–£130; replaced £150–£250. A flue flap or its wiring £100–£200. A gas valve on a two-digit board £250–£320.
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.
Glow-worm gas valve assembly
£90–£200Authorised Glow-worm parts distributor
Price checked 28 September 2026
Glow-worm main PCB
£150–£320Authorised Glow-worm parts distributor
Price checked 28 September 2026
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Related Glow-worm fault codes
- F.32Get it checkedF.32 is a fan fault — the fan speed is outside what the board will accept, or the fan cannot be seen at all. Older ranges show F2 or F3. An engineer's code throughout.Related: fan and flue
- F.25Get it checkedOn current ranges F.25 is the flue gas safety cut-out — a break in its plug or harness. On an Ultracom, Flexicom or cxi, F25 is a circulation fault. Different generations, different fault.Related: fan and flue
- F.33Get it checkedF.33 is an air supply fault on the two ranges that carry it: a Compact's flue losing too much pressure, or a Betacom3's air pressure switch not proving. Not in the Energy or Ultimate tables.Related: fan and flue
- 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
- F.22Get it checkedGlow-worm's low-water-pressure shutdown, and the one code every Glow-worm user guide lets you clear yourself by topping the system up. The threshold differs by range.Related: same brand
- F.75Get it checkedGlow-worm's own documents give this code three different cause lists — pressure switch only, or sensor, pump or lack of water — and the fullest one is the Compact's eight-way diagnostic.Related: same brand
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Related guides
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.