What the F75 code means
Each time the pump starts, it pushes water round the system and the pressure inside the boiler rises very slightly. The boiler's water pressure sensor watches for that change. It is a quick way for the boiler to confirm that the pump is working and that there is water for it to move.
Glow-worm's installation manual for the Energy range lists F75 as a pressure sensor fault, with a defective pressure switch or sensor as the cause. Installer sources describe it in more practical terms: the sensor did not register a pressure increase after the pump started. That covers two main possibilities. Either the sensor has failed and cannot see the change, or the pump is not creating one. Air or low water in the boiler can also stop the pump building pressure. The same code on Vaillant boilers is explained on our Vaillant F75 page, which is worth reading because the two brands share a lot of parts.
F75 is most often reported on the Glow-worm Energy range. It may not appear on every older model, so check the fault list for your own boiler.
Common causes of F75
A faulty or blocked pressure sensor
The sensor is the part named in Glow-worm's manual. It can fail with age, or its small inlet can become clogged with sludge or scale so that it reacts too slowly to register the change.
A pump that is sticking or failing
If the pump is seized, slow or has an electrical fault, it will not produce the pressure change the boiler is looking for. A pump can also stick after the boiler has been off for a long spell, such as over the summer.
Air in the boiler or system
A pocket of air in the pump or heat exchanger stops the pump moving water properly. This often shows up after the system has been drained, refilled or had new radiators fitted.
Low system pressure
If there is not much water in the system, the pump has little to push against. Very low pressure normally brings up F22, but a borderline reading can contribute to F75.
| Cause | What you might notice | Who fixes it |
|---|---|---|
| Low pressure | Reading below 1 bar when cold | You, using the filling loop |
| Air in radiators | Cold tops to radiators, gurgling | You, by bleeding radiators |
| Air inside the boiler | Noisy pump, F75 after system work | Gas Safe engineer |
| Faulty pressure sensor | Pressure looks fine, F75 keeps returning | Gas Safe engineer |
| Pump fault | No hum from the pump, radiators stay cold | Gas Safe engineer |
Safe checks you can do yourself
- With the system cool, check the pressure shown on the display or gauge. If it is below about 1 bar, top it up to 1 to 1.5 bar using the filling loop. Our guide on how to repressurise a boiler explains how.
- Bleed any radiators that are cold at the top, with the heating off and cool.
- Check the pressure again after bleeding and top up if it has dropped.
- Reset the boiler as your manual describes. On the Energy7, you press the reset button for about a second.
- Let the heating run and listen for the pump. A faint hum when the heating starts is normal.
- If F75 comes back after two or three resets, leave the boiler off and call an engineer.
If F75 appears the first time you turn the heating on after the summer, a sticking pump is a common reason. Do not keep resetting it. Book an engineer, who can free or replace the pump safely.
When to call a Gas Safe engineer
Because the likely causes are internal parts, most F75 faults need a Gas Safe registered engineer. Call one if the pressure is correct and the radiators are bled but F75 keeps returning, if the pump is noisy or silent, or if you are topping up pressure regularly. The engineer will check the pump is running and moving water, vent any air from inside the boiler, and test the pressure sensor. If the system water is dirty, they may also suggest cleaning it, since sludge can block the sensor and wear out the pump.
Do not remove the boiler casing to reach the pump or sensor. These sit alongside gas and electrical parts and must only be worked on by a Gas Safe registered engineer.
Typical repair costs in the UK
If topping up and bleeding sorts it, F75 costs nothing. Otherwise these are typical ranges, including parts and labour.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Venting air from the boiler | Typically £80 to £150 |
| Replacing the water pressure sensor | Typically £120 to £220 |
| Replacing the pump | Typically £250 to £450 |
| Cleaning the heating system | Typically £400 to £800 |
How to stop F75 coming back
Keep the pressure steady and the system water clean. Check the reading once a month, top up after bleeding radiators, and look into any pressure loss early, as it usually means water is escaping somewhere in the system. A magnetic filter and the right amount of inhibitor help protect both the sensor and the pump from sludge. In summer, running the heating briefly every few weeks can help stop the pump sticking, if your boiler does not already exercise the pump automatically. An annual service lets an engineer spot a weak pump or a slow sensor before it causes a lockout, and our boiler service cost guide explains what is included.