Glossary · Faults, servicing and gas safety
What does the F75 fault code mean?
F75 is the fault code a Vaillant or Glow-worm boiler shows when the pressure change is too low as its pump starts. The makers list low system pressure, air in the system, a faulty pressure sensor, a faulty pump or a wiring break as causes. Under 1 bar you can top up. Otherwise call an engineer.
Key points
- Glow-worm describes F.75 as the pressure change being too low when the heating pump is started up. Vaillant's list uses almost the same wording.
- The causes listed are system pressure below 1 to 1.5 bar, air in the heating system, a defective water pressure sensor, a defective internal pump or an open circuit in the cable harness.
- If the gauge reads under 1 bar, the makers say you can top it up through the filling loop if you are competent. Above 1 bar, it is a case for an engineer.
- F.75 is easy to confuse with F.22 because both involve pressure. F.22 is low water pressure itself, while F.75 is about the pressure change when the pump starts.
- Do not open the boiler casing. Only a Gas Safe registered engineer should work on the boiler's internal parts.
What causes an F75 fault
When a boiler’s pump starts, the pressure in the heating circuit changes in a way the boiler expects. On a Glow-worm boiler, F.75 means the pressure change is too low when the heating pump is started up.[2]Glow-worm, Boiler fault codes Vaillant’s list gives the same entry, though it words it as the heat pump being started up.[1]Vaillant, Boiler fault codes Check the manual for your own model if the wording matters to you.
Both makers list the same possible causes.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes
| Possible cause | Where it sits |
|---|---|
| System pressure below the normal range of 1 to 1.5 bar | The heating system |
| Air in the product or heating system | The heating system |
| Water pressure sensor defective | Inside the boiler |
| Internal heating pump defective | Inside the boiler |
| Open circuit in the cable harness | Inside the boiler |
Only the first can be checked safely by a householder. The others involve the inside of the boiler.
F.75 is easy to confuse with F.22. Both involve pressure and both lists mention the same normal range. F.22 is raised when the water pressure in the system is low. F.75 is raised when the boiler does not see the expected pressure change as the pump starts. Low pressure is one of the causes of both.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes
What you can check yourself
Start with the pressure gauge or display. The makers say the following.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes
| Gauge reading when F.75 shows | What the makers say |
|---|---|
| Under 1 bar | Can be solved without an engineer if you are competent. Use the filling loop to slowly raise the pressure to between 1 and 1.5 bar |
| Over 1 bar | A case for an engineer. Do not try to solve it yourself, because opening the boiler casing can be dangerous |
Both say their guidance is used at your own risk. Vaillant’s repressurising guide says to start with your boiler manual, because every boiler is different.[3]Vaillant, How to repressurise a boiler It also says that if the pressure drops again straight after you close the valve, there is likely a significant leak or a fault with the pressure release valve, and you should call an installer rather than keep trying to repressurise.[3]Vaillant, How to repressurise a boiler Our entry on low water pressure covers the topic in more depth.
When to call an engineer
If the pressure is above 1 bar, or you have topped it up and the code returns, the lists point to causes beyond a simple top-up. Air in the system, a pressure sensor, the pump and the wiring are all on the lists, and testing them involves the boiler’s internal parts. Work on a gas appliance must be done by a Gas Safe registered engineer.[4]Health and Safety Executive, Domestic gas: frequently asked questions A boiler repair visit can test the sensor and pump and look for air or a leak.
If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.[1]Vaillant, Boiler fault codes
Example
Yasmin in Wakefield has a Glow-worm boiler that shows F.75 after the heating has been off for a few days. The gauge reads 0.8 bar. She follows the manual to raise the pressure to 1.2 bar using the filling loop. The boiler runs normally.
If the gauge had read 1.3 bar when the code appeared, she would not have topped it up. She would have called an engineer, as the makers advise.
What people get wrong
Many people assume F75 means a faulty pump. It might, but a low reading on the gauge is the first thing to rule out. The makers list low pressure at the top of the causes, and air in the system is another one.
The reverse mistake is topping up repeatedly when the gauge is already in range. The makers say a reading above 1 bar is a case for an engineer, not a top-up.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes
The bottom line
F75 means the boiler did not see the pressure change it expected as its pump started. Check the gauge. Under 1 bar, you may be able to raise it to between 1 and 1.5 bar using the filling loop and your manual. Anything else, including a code that returns, is for a Gas Safe registered engineer.
Common questions
What does F75 mean on a Vaillant boiler?
Vaillant's list describes it as the pressure change being too low when the pump is started up. Its possible causes are low system pressure, air in the system, a faulty water pressure sensor, a faulty internal pump or an open circuit in the cable harness.
Is F75 the same as F22?
No, but they are linked. F.22 is low water pressure in the system. F.75 is raised when the boiler does not see the expected pressure change as the pump starts. Low pressure is one cause of F.75 on the makers' lists.
Can I fix F75 myself?
Only if the gauge shows under 1 bar. Vaillant and Glow-worm say you can then raise the pressure to between 1 and 1.5 bar using the filling loop if you are competent. If the pressure is above 1 bar, it needs an engineer.
Can I keep resetting an F75 fault?
Vaillant and Glow-worm say a reset can clear a fault or get the boiler working again for a while. If F.75 returns after a reset, stop resetting and book a Gas Safe registered engineer, because the cause is still there.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Vaillant. Boiler fault codes. Checked 11 November 2026. ↩
- Glow-worm. Boiler fault codes. Checked 11 November 2026. ↩
- Vaillant. How to repressurise a boiler. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩