- Boilers
- Gas boiler
F75
Glow-worm F75 on a gas boiler: no pressure change at pump start
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On Glow-worm boilers, F.75 means the pressure change is too low when the heating pump starts. If the gauge shows under 1 bar, Glow-worm says a competent user can slowly top up to 1 to 1.5 bar with the filling loop; above 1 bar, call a Glow-worm engineer.
Safety first
Stop and check this first
- If you smell gas, Glow-worm says to leave the property immediately and call the National Gas Emergency Service on 0800 111 999. Do not operate switches on the way out and make the call from outside.
- Glow-worm says top ups are at your own risk and only if you are competent; with pressure above 1 bar, F.75 is a job for its engineer.
Safe checks you can perform
- Pressure gauge reading
- Slow top up with the filling loop
- One reset
- Pump, sensor and harness test
Leave these tasks to a qualified professional
- Do not attempt this professional task: Pressure sensor and harness check
- Do not attempt this professional task: Annual safety check
- Do not attempt this professional task: Venting and pump test
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Pressure sensor and harness check
What they doThe engineer tests the water pressure sensor and checks the cable harness for an open circuit.
They must show youthe sensor reading against the gauge.
In writingthe test results and repair.
Annual safety check
What they doDuring the visit, the engineer can service the boiler, checking the flue and combustion.
They must show youthe flue and combustion checks.
In writingthe service record.
Venting and pump test
What they doThe engineer vents the boiler and system and checks the internal heating pump runs and builds pressure.
They must show youthe pump running and the pressure change.
In writingwhat was found and the reference of any part replaced.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Pressure gauge reading
- Who:
- you.
- How:
- read the system pressure on the boiler gauge.
- Result that matters:
- whether it is below 1 bar.
- If it is below 1 bar:
- top up as in the next check.
- If it is above 1 bar:
- Glow-worm says this is a case for its engineer.
- Evidence to keep:
- a photo of the gauge and display.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Slow top up with the filling loop
- Who:
- you, if you feel competent, as Glow-worm states.
- How:
- locate the filling loop shown in your user manual and slowly raise the pressure to 1 to 1.5 bar.
- Result that matters:
- whether F.75 clears and pressure holds.
- If it clears and holds:
- low pressure was the cause.
- If it drops again:
- a leak needs finding; call an engineer.
- Evidence to keep:
- daily gauge readings for a week.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
One reset
- Who:
- you.
- How:
- reset the boiler as your user manual describes.
- Result that matters:
- whether the pump starts without F.75.
- If F.75 returns with pressure correct:
- call a Glow-worm engineer.
- If it runs:
- note it.
- Evidence to keep:
- a note of the reset and reading.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Pump, sensor and harness test
- Who:
- a Glow-worm or Gas Safe registered engineer.
- How:
- vents the boiler, tests the internal pump, the water pressure sensor and the cable harness.
- Result that matters:
- why no pressure change is seen.
- If air was trapped:
- venting clears it.
- If the pump, sensor or harness fails:
- it is repaired or replaced.
- Evidence to keep:
- a job sheet with findings.
Glow-worm advises against accessing the inside of the boiler; the pump and sensor are engineer work.
Evidence: HSE gas safety for homeowners · Glow-worm boiler fault codes: what they mean and how to fix them
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Glow-worm
- Product type
- Gas boiler
Known code variants
- 75
- caldera Saunier Duval F.75
- code erreur F75 chaudière Vaillant
- codice F75 Vaillant
- error F75 Saunier Duval
- Error F75 Vaillant
- errore F75 caldaia Vaillant
- F 75
- Fehlercode F75 Vaillant
- Glow worm boiler F75
- Glow worm F75
- Glow-worm F.75
- Glow-worm F75
- Glow-worm F75 pump pressure
- panne F75 Vaillant
- Saunier Duval F75
- Saunier Duval fault code F.75
- Semia Condens F75
- Thelia Condens F75
- Vaillant F.75
- Vaillant F75
- Vaillant F75 fault
- Vaillant Fehler F75
What it means
Glow-worm's fault code page gives F.75 as the pressure change being too low when the heating pump is started up. When the pump starts, the boiler expects to see a pressure change at its water pressure sensor; if it does not, it stops. Possible causes Glow-worm lists are system pressure below the normal 1 to 1.5 bar, air in the product or heating system, a defective water pressure sensor, a defective internal heating pump and an open circuit in the cable harness.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas, Glow-worm says to leave the property immediately and call the National Gas Emergency Service on 0800 111 999. Do not operate switches on the way out and make the call from outside.
Evidence: HSE gas safety for homeowners · Glow-worm boiler fault codes: what they mean and how to fix them
- Mediumsource verified
Glow-worm says top ups are at your own risk and only if you are competent; with pressure above 1 bar, F.75 is a job for its engineer.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Symptoms and causes
Probable causes · in order
- Low system pressureCommon
Fits when the gauge shows under 1 bar; topping up clears it if nothing leaks.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
- Air in the boiler or systemPossible
Fits after draining or radiator work: the pump spins in air and cannot build pressure.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
- Pump, pressure sensor or harnessPossible
Fits when pressure is above 1 bar and the system is vented; Glow-worm sends its engineer.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Virtual technician
Does F75 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Health and Safety Executive · official_doc
- 2
Glow-worm boiler fault codes: what they mean and how to fix them
Glow-worm · official_doc
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