Glow-worm boiler repair across London
We repair current and older Glow-worm boilers. Ring us, day or night.
- Glow-worm Energy and Ultimate3 serviced
- All our engineers are Gas Safe registered
Makes we repair
How does booking a Glow-worm repair work, and what does it cost?
Booking: ring us any time, day or night, or send your details and we ring you back. We'll ask what the boiler is doing, and its model and any code on the display if you can see them. You don't need to know what's wrong. Finding the fault is the engineer's job. Then we tell you when an engineer can come, anywhere in Greater London.
What it costs: a repair's price depends on the part it needs, how long the job takes and when it's done. Our rates are higher in the evening, overnight and at weekends. Once the engineer has found the fault, the rate for the repair is agreed in writing before any work starts.
What happens on a Glow-worm repair visit?
A repair visit has three stages.
1. Before anything is touched
The engineer covers the floor around the boiler and asks what you have seen and heard since you rang.
2. Finding the fault
The engineer tells you which part has failed, whether they have it with them or must order it, and how long the work should take. No work is done until you say yes.
3. The repair
The engineer fits the part and checks the boiler runs as it should before leaving. The repair is guaranteed for 12 months, covering the parts we fit, the labour and any return visit.
What goes wrong on Glow-worm boilers most often?
Glow-worm's own help pages come back to the same problems:
- Low pressure. F.22, the code Glow-worm calls the easiest to put right at home, by topping up. If it keeps dropping, water is escaping and it needs an engineer.
- A frozen condensate pipe. In a frost the boiler gurgles and stops with F.28 or F.29.
- Not lighting. Glow-worm names five causes: the fan or the air flow, no gas, low pressure, the ignition parts, and a frozen condensate pipe.
- Turning itself off. Low pressure, a thermostat or sensor fault, an airlock or blockage, or overheating from limescale.
- No hot water, or hot water but no heating. The pressure, the diverter valve, the heating settings, the radiator valves or the thermostat.
- Noises. Gurgling, whistling, banging, vibrating or humming.
- Leaks. From corrosion, high pressure, a loose joint or a worn seal, underneath the boiler, or from the pressure relief pipe outside.
- Radiators. Cold at the bottom, or not warming at all.
Whichever it is, we'll ask what the boiler is doing when you ring.
Sources: Glow-worm's fault-code article and its advice pages on repressurising, frozen condensate pipes, not igniting, turning off, no hot water, hot water but no heating, noises, leaks and bleeding radiators, read on 6 October 2026.
Glow-worm fault codes and what they mean
A current Glow-worm shows a fault as F. and two figures, such as F.28. Glow-worm's own fault-code page writes F.0 and F.1 for the manuals' F.00 and F.01.
The user guides allow three resets for an ignition fault, or five on a Compact or MicraCom. A code that keeps coming back needs an engineer.
F.20Glow-worm F.20: the overheat cut-out has shut the boiler off
What it means: The water reached its top temperature, so the safety cut-out stopped the boiler.
Possible causes, as the manual lists them:
- a poor earth between the wiring and the boiler
- a flow or return sensor faulty or loose
- a stray spark from the ignition lead, plug or electrode
Glow-worm's fault-code page adds low pressure and a circulation fault: the pump, a valve or a blockage.
What you can do: Glow-worm's page suggests three steps:
- Check the pressure, and top up if it is low.
- See that the radiator valves are open.
- Reset the boiler.
Glow-worm sends F.20 to an installer first, because it can come from the heating system. If F.20 comes back, ring us.
On an older Flexicom or Ultracom, overheat is F5.
F.23Glow-worm F.23: the flow and return are too far apart
What it means: The water leaving the boiler is much hotter than the water coming back, so water isn't moving through it as it should.
Possible causes, as the manual lists them:
- a blocked pump
- a pump too weak
- air in the boiler
- the flow and return sensors connected the wrong way round
Glow-worm's page adds the expansion vessel inside the boiler.
What you can do: a reset may clear it. Glow-worm sends F.23 to an installer first, as it can come from the heating system. If F.23 comes back, ring us.
F.24Glow-worm F.24: the temperature rises too fast
What it means: The water heats up faster than it should, a sign it isn't moving round well.
Possible causes, as the manual lists them:
- a blocked pump
- a pump too weak
- air in the boiler
- low system pressure
- a non-return valve blocked or fitted the wrong way
What you can do: a reset may get it going. On a combi or system boiler, Glow-worm calls F.24 an engineer's job. On a regular boiler, Glow-worm sends it to the installer first. If F.24 comes back, ring us.
On a Flexicom cx or cxi, F24 means something else, the return water over 90 degrees.
F.27Glow-worm F.27: a flame signal with the burner off
What it means: The boiler sees a flame while the burner should be off.
Possible causes, as the manual lists them:
- damp on the electronics
- the flame-monitoring electronics
- a gas valve letting gas through
What you can do: nothing at home. Glow-worm calls F.27 a case for an expert, as it can be dangerous, so ring us.
F.32Glow-worm F.32: fan fault
What it means: The fan's speed is outside its limits, and the boiler won't light without it.
Possible causes, as the manual lists them:
- the fan's plug loose
- the board's plug loose
- a break in the wiring
- a blocked fan
- the fan's speed sensor
- the electronics
Glow-worm's signs of a fan fault are no whirr before the boiler lights, and no air from the flue.
What you can do: nothing at home. Glow-worm calls F.32 an engineer's job.
On an older cxi, CI, Flexicom or Ultracom, a fan fault is F2 or F3.
F.28Glow-worm F.28: the boiler tried to light and failed
What it means: The boiler tried to light and failed.
Every cause of F.28, and what fixes it, is on the F.28 fault code page.
F.29Glow-worm F.29: the flame went out and wouldn't relight
What it means: The flame went out and wouldn't relight.
Every cause of F.29, and what fixes it, is on the F.29 fault code page.
F.22Glow-worm F.22: there isn't enough water in the boiler
What it means: There isn't enough water in the boiler.
Every cause of F.22, and what fixes it, is on the F.22 fault code page.
F.75Glow-worm F.75: a pressure or pump fault found as the pump starts
What it means: A pressure or pump fault found as the pump starts.
Every cause of F.75, and what fixes it, is on the F.75 fault code page.
F.62Glow-worm F.62: the flame went out late after the gas valve shut
What it means: The flame went out late after the gas valve shut.
Every cause of F.62, and what fixes it, is on the F.62 fault code page.
F.83Glow-worm F.83: the burner lit, but the water barely warmed
What it means: The burner lit, but the water barely warmed. F.83 on a Compact or MicraCom means the temperature swings.
Every cause of F.83, and what fixes it, is on the F.83 fault code page.
F1Glow-worm F1 on an older Flexicom, Ultracom or cxi: an ignition fault
What it means: An ignition fault.
Every cause of F1, and what fixes it, is on the F1 fault code page.
F3Glow-worm F3 on an older cxi, Flexicom or Ultracom: the fan couldn't reach its lighting speed
What it means: The fan couldn't reach its lighting speed. F3 on a CI Plus is an air fault.
Every cause of F3, and what fixes it, is on the F3 fault code page.
MoreEvery other Glow-worm code, in one list
Every other code in Glow-worm's manual for the Energy combi, its most popular, with the two the Compact and MicraCom add. The manual's table is shared across boilers, so some codes never show on a given model.
F.00The flow temperature sensor can't be read
Possible causes
A loose plug at the sensor or the board, a break in the wiring, or the sensor
F.01The return temperature sensor can't be read (not the old F1)
Possible causes
As F.00, for the return sensor
F.10The flow sensor has a short circuit
Possible causes
The sensor, or a short in its wiring
F.11The return sensor has a short circuit
Possible causes
The sensor, or a short in its wiring
F.12, F.91The cylinder temperature sensor has a short circuit (Energy)
Possible causes
The sensor, or a short in its wiring
F.13The hot water cylinder sensor has a short circuit
Possible causes
The sensor, or a short in its wiring
F.25A safety shut-down, as the flue gas is too hot
Possible causes
A broken plug on the optional flue gas cut-out, or its wiring
F.33 (Compact, MicraCom)Too much pressure is lost in the air and flue pipes
Possible causes
A blocked flue, the fan or the board, badly connected sensors, or wind blowing back
F.46 (Compact, MicraCom)The cold water sensor has a short circuit
Possible causes
The sensor or its wiring
F.49The voltage on the controls' wiring is too low
Possible causes
A short on the controls' wiring, or too much connected to it
F.61The gas valve can't be controlled
Possible causes
A short in the gas valve's wiring, its coils, or the electronics
F.63Memory chip fault
Possible causes
The electronics
F.64Electronics or sensor fault
Possible causes
A flow or return sensor shorted, or the electronics
F.65The electronics are too hot
Possible causes
Heat from around the boiler, or the electronics
F.67The flame signal makes no sense
Possible causes
The electronics
F.68Unstable flame
Possible causes
Air in the gas, low gas pressure, the gas and air mix, the injector, or the flame-sensing current
F.70The boiler's product code is missing or wrong
Possible causes
The display and the board both replaced without resetting it
F.71The flow sensor is stuck at one reading
Possible causes
The sensor badly placed on its pipe, or faulty
F.72The flow and return sensors disagree too much
Possible causes
A flow or return sensor
F.73The water pressure sensor isn't connected, or is shorted
Possible causes
A break or short in its wiring, or the sensor
F.74Electrical fault in the water pressure sensor
Possible causes
A short in its wiring, or the sensor
F.76The main heat exchanger's safety cut-out is faulty
Possible causes
Its signal doesn't match the gas valve's
F.77Condensate or flue fault
Possible causes
A condensate pump or a flue flap not responding
F.78A hot water sensor setting with an outside control
Possible causes
A link box fitted without the sensor bridged
F.84The flow and return readings aren't believable
Possible causes
Sensors swapped or fitted wrongly
F.85A temperature sensor is fitted wrongly
Possible causes
Both on the same pipe, or not connected properly
F.86Underfloor heating contact open, or an outside safety cut-out on a Compact
Possible causes
The contact or its sensor. On a Compact, a limit thermostat, the diverter valve or a condensate pump.
F.87Fault in the ignition electrode's connection
Possible causes
Not connected, connected wrongly, or a short
F.88Fault in the gas valve's connection
Possible causes
Not connected, connected wrongly, or a short
F.89Fault in the pump's connection
Possible causes
Not connected, the wrong pump, or a short
F.97The main board's self-test failed
Possible causes
The main board
ConnectionThe main board and the control panel can't talk to each other
Possible causes
The electronics
Flexicom, Ultracom, cxi and older Glow-worms: the codes that mean something else
From Glow-worm's manuals and fault-finding pages for its older ranges. The Betacom2, Betacom a and Ultracom2 use today's numbers without the dot, so the list above fits them. On the older ranges in this list, the numbers mean something else.
F2 (cxi, CI)Fan or air pressure switch fault
Possible causes
The fan or the air pressure switch
F4The flame lit, then went out
Possible causes
The ignition or the flame sensing
F5Overheat
Possible causes
The heating water got too hot. The user guide says let it cool, then reset.
F6Heating flow sensor fault
Possible causes
The sensor or its wiring
F7Hot water sensor fault
Possible causes
The sensor or its wiring
F9Water pressure sensor fault, a blocked flow filter or a pump fault
Possible causes
The pressure sensor, the filter or the pump
F10Return sensor fault
Possible causes
The sensor or its wiring
F11Main board connection on an Ultracom, or a flow or return sensor on a Flexicom cx
Possible causes
The board's connections, or the sensor
F13Board memory or sensing fault
Possible causes
The main board
F14Gas valve control fault, or the flow over 95 degrees on a cxi or CI
Possible causes
The gas valve's control, or overheating
F15Controls' wiring voltage fault, or the gas valve's motor on a cxi or CI
Possible causes
The controls' wiring, or the gas valve
F16, F17 (cxi, CI)F16 is a flame detection fault, and F17 the mains supply under 170 volts
Possible causes
The flame sensing for F16, and the electricity supply for F17
F22Low water pressure, or the temperature rising too slowly at lighting
Possible causes
Too little water in the system
F24 (Flexicom cx, cxi)The return water is over 90 degrees, not today's F.24
Possible causes
Too much heat coming back to the boiler
F25, F26Poor circulation
Possible causes
Water not moving round the system
F43A general error
Possible causes
One for an engineer's tests
F70The software doesn't match
Possible causes
The board's software
F73The pump wasn't seen to start
Possible causes
The pump
F77Condensate pump fault, where one is fitted
Possible causes
The condensate pump
Er, or a blank display (Ultracom)A loose connection
Possible causes
A connection inside the boiler
A flashing 30 to 90 (Betacom C)30 means the heating sensor, 40 the hot water sensor, 50 the outside sensor and 60 low pressure. 70 means the air pressure switch, 80 the overheat circuit and 90 a flame or gas failure.
Possible causes
The part each number names
02 to 20, with a phone symbol (Xtrafast)02 means air flow, 04 no ignition and 05 overheat. 06 and 07 mean the sensors, 09 water pressure, 12 the controls, 13 the main board, 14 the temperature limiter and 20 communication.
Possible causes
The part each number names
Sources, read on 6 October 2026: Glow-worm's installation and maintenance instructions for the Energy7 (0020300875_01) and the Compact and MicraCom AS/2 (8000043326_01), its user guides, its boiler fault codes page, and its trade fault-finding pages for the older ranges.
Which Glow-worm boiler do you have, and how does it show a fault?
Glow-worm, the Vaillant Group's British brand, has made boilers in Derbyshire since 1934. Most show a fault as F and a number, but the same number doesn't always mean the same thing on an older boiler. The model is on the data plate, and we'll ask for it and the code when you ring.
Not every code or sign is a fault:
- An S. code shows what the boiler is doing.
- The display shows rE while a reset is running.
- The display shows OFF on standby, and oF on a Compact when the heating is switched off.
- A flashing pressure figure means the pressure is out of range.
- A plume of steam from the flue in cold weather is normal.
We'll ask what the display shows.
How do you reset a Glow-worm boiler?
Each Glow-worm range resets in its own way:
- Energy, Ultimate3, Easicom3 and Betacom4: press the reset key, for a second for F.28. Try it three times at most.
- Compact and MicraCom: hold the on/off button for more than 3 seconds, and rE shows. Try it five times at most.
- Betacom2 and Ultracom2 cxi: press reset and give it 5 seconds. Try it three times in all.
- Betacom C: turn the control knob off for 15 seconds, then back on.
On an Energy, if F.83 shows again after a second reset, the boiler pauses for 45 minutes before it tries again. If a code keeps coming back, ring us rather than resetting again.
The pressure: with the heating and hot water off and the boiler cool, the gauge should read 1 to 1.5 bar, Glow-worm says. It shouldn't go above 2.5 bar, and bleeding a radiator brings it down. The user guide shows how to top up at the filling loop under the boiler, or with a filling key on some older combis. Afterwards the boiler purges itself for about 5 minutes, the gauge moves, and the heating and hot water pause. If the pressure keeps dropping, ring us.
Where a frozen condensate pipe is at ground level and easy to reach, thaw it with warm, never boiling, water, then reset the boiler. When a boiler won't fire up, and frozen condensate pipes.
Sources: Glow-worm's user guides and installation manuals for each range, and its advice pages on repressurising and frozen condensate pipes.
Is your Glow-worm boiler still under guarantee?
Glow-worm's guarantee depends on the range and how it was registered. As standard, the guarantee runs:
- 7 years on the Energy
- 5 years on the Ultimate
- 3 years on the Easicom
- 5 years on the Compact
- 2 years on the MicraCom
When the boiler is registered by an installer in Glow-worm's myREWARDS scheme, with a Glow-worm Power System Filter fitted, the guarantee runs longer:
- 10 years on the Energy and the Ultimate
- 5 years on the Easicom
A MicraCom registered by a myREWARDS installer reaches 5 years.
The boiler must be registered within 30 days of fitting, or the guarantee falls to 12 months. It must also be serviced every year by Glow-worm or any Gas Safe registered engineer, recorded in the Benchmark logbook.
While the guarantee runs, Glow-worm or an engineer it authorises carries out the repairs. Glow-worm doesn't pay other engineers' invoices unless it agreed in writing first. Its terms say parts that aren't Glow-worm's own void the guarantee. Topping up the pressure, bleeding the radiators and thawing a frozen condensate pipe aren't covered. A fault in the heating system rather than the boiler, such as sludge or limescale, is charged.
A boiler still under its maker's guarantee is usually repaired by the maker, at no charge, so ring Glow-worm first. Once the guarantee has ended, or for a fault it doesn't cover, ring us. We are an independent firm, with no connection with Glow-worm.
Sources: Glow-worm's guarantee registration page and its guarantee terms and conditions, and its Energy and Ultimate product pages, read on 6 October 2026.
What does a Glow-worm boiler service cover?
Glow-worm asks for a service every year by Glow-worm or any Gas Safe registered engineer, to keep the guarantee after its first year. Each service is recorded in the Benchmark logbook. The manual's yearly list covers:
- the fault history
- a combustion reading
- the flue
- the burner and heat exchanger seals
- the heating water and its inhibitor
- the condensate trap, cleaned and refilled
- a leak test
Glow-worm puts a service at 30 minutes to an hour.
We service every Glow-worm model, and give you the rate on the call. What a boiler service includes, check by check.
Sources: Glow-worm's annual boiler servicing page and its advice on servicing, its guarantee terms, and the yearly maintenance list in its Energy7 installation and maintenance instructions.
Do you cover my part of London?
Yes: we travel all over Greater London, day and night, to repair Glow-worm boilers in every borough, north and south of the Thames. Pick your side of the river to see the areas.
North of the river
Every borough north of the Thames: Barnet and Enfield at the top, Hillingdon in the west, Havering in the east, and central London in between.
South of the river
Every borough south of the Thames: Richmond and Kingston in the west, Bexley in the east, and down through Croydon to Sutton and Bromley.
The other makes we repair
Every main make, combi, system and regular. Pick yours for its common faults, its codes and how we repair it.
Questions about Glow-worm boilers
Do I need to know what's wrong with my Glow-worm boiler before I ring?
No. Ring and say what the boiler is doing, or what you can see or hear, and we'll ask the rest. Finding the fault is the engineer's job.
Do you charge more in the evening or at weekends?
Yes. Our rates are higher in the evening, overnight and at weekends.
Can you repair an older Glow-worm boiler?
Usually, yes. It depends on the boiler's condition and whether its parts are still made. We'll ask for the model when you ring.
Is my Glow-worm boiler worth repairing?
Often, yes. Age alone doesn't decide it. What decides it is the rate for the repair against a new boiler, and how often the boiler has failed. The engineer gives you the rate for the repair, and the choice is yours. When a new boiler makes more sense.
Will a repair affect my Glow-worm guarantee?
While the guarantee runs, ring Glow-worm first. The maker usually repairs a boiler under guarantee itself, and its terms say who may work on it. Once it has ended, any Gas Safe registered engineer can repair it.
Do you fit Glow-worm's own parts?
Yes, the maker's own parts wherever they are available. Each repair is guaranteed for 12 months, covering the parts we fit, the labour and any return visit.
How much does a Glow-worm boiler repair cost?
The cost of a repair depends on the part, how long the job takes and when it's done. The rate is agreed in writing before the repair starts. How our rates work.
Why does my Glow-worm boiler keep losing pressure?
A Glow-worm boiler that keeps losing pressure has water escaping somewhere. Glow-worm's guides treat frequent topping up as a sign of a leak, from a joint, a seal, corrosion, or the pressure relief pipe outside. Top up to 1 to 1.5 bar as the user guide shows. If the pressure keeps dropping, ring us.
What does F1 mean on a Glow-worm boiler?
F1 on an older Flexicom, Ultracom, cxi or CI is an ignition fault. The boiler tried to light five times and failed. On a current Glow-worm, F.1, or F.01 in the manuals, is the return sensor. We'll ask for the model when you ring. What F1 means on every make.
Why does my Glow-worm show F.28 or F.29 in cold weather?
Glow-worm links F.28 and F.29 in cold weather to a frozen condensate pipe, when the boiler gurgles and stops. Thaw a pipe at ground level with warm, never boiling, water, then reset the boiler. What F.28 means on every make.
How long do Glow-worm boilers last?
Glow-worm says its boilers last 10 to 15 years with a yearly service. Age alone doesn't decide whether to repair one.
Who makes Glow-worm boilers?
Vaillant Group UK makes Glow-worm boilers. Glow-worm is its brand, bought by the Vaillant Group in 2001. The brand has made boilers in Derbyshire since 1934, and its Energy, Ultimate and Easicom are made in Belper. How we repair Vaillant boilers.
Don't know what has failed? You don't have to.Ring and say what you can see or hear. Finding the fault is our job.
Calls answered 24 hours a day
A real person answers and gives you the rate on the call. No work starts until you say yes, and repairs are guaranteed for 12 months: parts, labour and any return visit.
