The Vaillant F22 error code means your boiler has detected no water, or too little water, in the heating system, so the water pressure is too low for the boiler to run safely. It is one of the most common Vaillant boiler faults and, in many cases, a homeowner can clear it by topping up the pressure.
Quick answer: F22 on a Vaillant boiler means low water pressure. Vaillant ecoTEC boilers switch off when pressure falls below about 0.5 bar. You can usually top up to between 1.0 and 2.0 bar (cold) using the filling loop, as your manual describes. If F22 returns within days, there is likely a leak or faulty part, so call a Gas Safe registered engineer.
What Does the Vaillant F22 Error Code Mean?
F22 is a low water pressure lockout. Vaillant defines it on its official F22 fault code page as no water or insufficient water in the product, or water pressure that is too low. On the boiler display it usually appears as F.22.
A sealed central heating system needs a certain amount of pressure to circulate water through the boiler and radiators. The ecoTEC plus operating instructions say you should top up the system as soon as the pressure falls below 0.8 bar, and that the boiler switches off and shows F.22 when the pressure falls below 0.5 bar. The installation manual gives the correct cold filling pressure as between 1.0 bar and 2.0 bar.
F22 is not the same as F75. Vaillant describes F75 as a pressure change that is too low when the heating pump starts up, which points more towards the pump, the pressure sensor or air in the system. F22 is about the amount of water and the pressure reading itself.
What Causes the F22 Error?
Low system pressure is the usual cause, but Vaillant lists several possible faults behind F22. The first two below are the ones a homeowner is most likely to come across; the rest need an engineer to diagnose.
- Not enough water in the system: pressure has dropped over time, often after radiators have been bled or because of a small leak.
- A leak in the heating system: a weeping radiator valve, a joint under the floor or a fault inside the boiler.
- Defective internal expansion vessel: Vaillant lists this as a cause. A vessel that has lost its charge can make pressure rise when hot and fall when cold.
- Defective water pressure sensor, or a fault in its electrical connection.
- Loose, disconnected or defective cables to the pump or the pressure sensor.
- Interrupted pump operation.
- Defective solenoid valve in the automatic filling device, on models that have one.
How Do You Fix the F22 Error Yourself?
The only fix a homeowner should attempt is checking the pressure gauge and topping up the system through the filling loop, which the Vaillant operating instructions allow. Filling loop designs differ between models, so follow the steps in your own manual. Our guide on how to repressurise a combi boiler explains the general process in more detail.
- Turn the heating off and let the boiler and radiators cool. Pressure readings should be taken with the system cold.
- Read the pressure gauge or the pressure shown on the display. If it is below 0.8 bar, the system needs topping up.
- Find the filling loop. Depending on the model, it may be built into the boiler or be a separate braided hose with two valves on the pipework nearby. Your manual shows where yours is.
- Open the filling valve or valves slowly and watch the gauge. Close them when the pressure reaches between 1.0 and 2.0 bar (cold). Do not go above 2.0 bar.
- Make sure the valves are fully closed. If you have a removable external loop, disconnect it only if your manual tells you to.
- Reset the boiler using the reset button, as described in your manual, and check that heating and hot water work.
- Check the gauge again over the next few days. Write down the reading each day so you can tell whether it is dropping.
Safety: Stop after topping up and resetting. Do not remove the boiler casing, adjust the expansion vessel, or touch the pressure sensor, pump, wiring or gas valve. Vaillant states that repairs must be carried out by a qualified technician. In the UK, any work inside a gas boiler must be done by a Gas Safe registered engineer. If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.
When Should You Call an Engineer?
Call a Gas Safe registered engineer if F22 comes back within a few days or weeks of topping up, because water is escaping from somewhere or a component has failed. You should also call one straight away in these situations:
- The pressure gauge reads normal (1.0 to 2.0 bar) but F22 still appears. This suggests a sensor or wiring fault.
- The pressure climbs close to 3 bar when the heating is hot, then falls low again when it cools. This pattern can point to the expansion vessel.
- Water is dripping from the boiler, or from the small pressure relief pipe that usually runs outside through the wall.
- You cannot find the filling loop, or the pressure will not rise when you open it.
- You see damp patches on ceilings or floors near heating pipes.
If your boiler is still under a Vaillant guarantee, contact Vaillant or your installer first. Vaillant’s UK boiler fault codes page lists its customer service number as 0344 736 0049.
Which Vaillant Codes Are Related to F22?
The table below uses Vaillant’s own descriptions from its official fault code list. Only F22 has a step that a homeowner can safely try.
| Code | Vaillant’s description | Who fixes it |
|---|---|---|
| F22 | No or insufficient water in the product, or water pressure too low | Homeowner can top up; engineer if it returns |
| F20 | Safety cut-out has interrupted the gas valve because a temperature limit was exceeded | Gas Safe engineer |
| F23 | Temperature spread between flow and return is too great | Gas Safe engineer |
| F25 | Flue gas temperature is too high | Gas Safe engineer |
| F28 | Flame signal not detected during the ignition phase | Reset, then Gas Safe engineer |
| F75 | Pressure change too low when the heating pump starts up | Gas Safe engineer |
How Can You Stop the F22 Error Coming Back?
The simplest habit is to glance at the pressure gauge once a month when the heating is cold, and top up before it drops below 0.8 bar. Beyond that:
- Check the pressure after bleeding radiators, because releasing air also lowers the pressure.
- Look regularly at radiator valves and visible pipework for drips, green or white crusting, or rust marks.
- Have the boiler serviced every year. Vaillant recommends annual servicing, and an engineer can check the expansion vessel charge and the pressure relief valve during the visit.
- Never overfill. Pressure above 2.0 bar when cold can make the relief valve open when the system heats up, which then lowers the pressure again.
If you are replacing radiators or planning heating upgrades while the engineer is there, our radiator size calculator helps you work out the output each room needs, and the gas bill calculator shows what your boiler costs to run from your meter readings.
Vaillant F22 Error Code FAQs
Can I reset a Vaillant boiler showing F22?
Yes, but only after topping up the pressure. Resetting without adding water will not clear F22, because the boiler is protecting itself from running with too little water. Top up to between 1.0 and 2.0 bar when cold, then press the reset button.
What pressure should a Vaillant boiler be?
Vaillant’s ecoTEC plus installation manual gives a cold filling pressure of between 1.0 and 2.0 bar. The operating instructions say to top up below 0.8 bar. The pressure rises a little when the heating is hot, which is normal.
Why does my Vaillant boiler keep losing pressure?
Repeated pressure loss usually means water is escaping through a leak, the pressure relief valve, or that the expansion vessel has lost its charge. A faulty pressure sensor can also cause false F22 readings. A Gas Safe registered engineer needs to find the cause.
Is it safe to use the boiler after an F22 error?
F22 is a protective shutdown, so the boiler stops itself before damage occurs. Once you have topped up to the correct pressure and it runs normally, it is generally fine to use. If the code returns or you see water leaking, stop and call an engineer.
Where is the filling loop on a Vaillant boiler?
It depends on the model. Some boilers have a filling device built in, while others use a separate braided hose with two valves on the pipework nearby. Your boiler’s operating instructions show the exact location and the steps to follow.
What is the difference between Vaillant F22 and F75?
Vaillant describes F22 as no or insufficient water, or water pressure that is too low. F75 means the pressure change is too low when the pump starts. F22 is often cleared by topping up; F75 usually needs an engineer to check the pump and sensor.
Checked October 2026. Sources: Vaillant: F22 fault code, Vaillant: Official boiler fault codes list, Vaillant UK: How to handle common fault codes, Vaillant: ecoTEC plus operating instructions, Vaillant: ecoTEC plus installation and maintenance instructions.
