On this page
- What does F22 mean on a Vaillant boiler?
- First check: what does the pressure gauge say?
- How to clear F22 by topping up the pressure
- Vaillant F22 after topping up: why it keeps coming back
- F22 with normal pressure: the component faults
- Is it safe to use the boiler with F22?
- Get F22 fixed properly
- Frequently asked questions
Key takeaways
- F22 (shown as F.22 on the display) is Vaillant’s low water pressure code. The boiler shuts down to avoid “dry firing” with too little water inside.
- Check the pressure first. Below 1 bar: Vaillant says a competent homeowner can slowly top up to 1 to 1.5 bar using the filling loop.
- F22 with the gauge at 1 bar or above points to a sensor, pump, wiring or component fault. Vaillant says that is a job for an engineer.
- If you have to top up more than once, water is leaving the system somewhere. Find out where before it causes damage.
The Vaillant F22 error means the boiler has detected too little water, or water pressure that is too low, in the heating system. Vaillant describes it as low water pressure, below the normal range of 1 to 1.5 bar. The boiler locks out and will not light, which protects the heat exchanger from firing dry. If the gauge reads below 1 bar you can usually top it up yourself; if F22 shows while the pressure looks normal, or keeps coming back, it needs a Gas Safe registered engineer.
What does F22 mean on a Vaillant boiler?
Vaillant ecoTEC boilers have a water pressure sensor that checks there is enough water in the sealed heating circuit before and during operation. When pressure falls below the threshold, the boiler shows F.22 and refuses to fire. You will normally lose both heating and hot water.
Vaillant’s own UK fault code page lists F.22 as low water pressure in the system, below the normal range of 1 to 1.5 bar. Its international help centre describes the same code on boilers such as the ecoTEC plus as no or insufficient water in the product, or water pressure that is too low.
Why the boiler calls it dry fire protection
Inside the boiler, the burner heats water flowing through a heat exchanger. If there is not enough water, the metal has nothing to carry the heat away. It can overheat in seconds, damaging the heat exchanger and seals. F22 stops that happening. It is a safety lockout, not the fault itself, so the real question is why the water is not there.
First check: what does the pressure gauge say?
Your next step depends entirely on the pressure reading. On most ecoTEC models it shows on the digital display or on a gauge underneath. Check it with the heating off and the system cold.
| Gauge reading with F22 showing | What it suggests | What to do |
|---|---|---|
| Below 1 bar | Genuine low pressure | Top up slowly to 1 to 1.5 bar with the filling loop, then reset |
| Close to 0 bar | Large loss of water or a long-term slow leak | Top up, then look for leaks before relying on the boiler |
| 1 bar or above | Pressure is fine; the boiler cannot read it or something else is wrong | Do not keep resetting. Call an engineer |
| Normal after top-up, then F22 returns within days | Water is escaping | Engineer to find and fix the loss |
This split comes straight from Vaillant’s advice: a competent homeowner may top up when pressure is below 1 bar, at their own risk, but F.22 above 1 bar is a case for an engineer, and you should never open the boiler casing.
How to clear F22 by topping up the pressure
If the gauge is below 1 bar, here is the general process. Your exact filling loop may differ, so check your Vaillant user manual first.
- Turn the heating off and let the system cool. Topping up a hot system gives a false reading.
- Find the filling loop. On many ecoTEC combis it is a braided hose or a built-in key under the boiler, connecting the cold mains to the heating circuit. Some newer models use a lever or a blue tap.
- Open the valves slowly and watch the gauge rise. You will hear water flowing.
- Stop at around 1.2 to 1.5 bar. Close both valves fully. Going too high can make the pressure relief valve discharge later.
- Reset the boiler using the reset button as your manual describes.
- Watch it fire and check the pressure again once the heating has run for a while. It will rise a little when hot, which is normal.
Our step-by-step guide to repressurising a combi boiler has more detail on the different filling loop types. If you are not comfortable doing it, or the valves will not move, leave it for an engineer.
Vaillant F22 after topping up: why it keeps coming back
A single F22 after a winter of bleeding radiators is not unusual. Bleeding releases air, and the pressure drops with it. But if you are topping up every week, or the code returns soon after you clear it, the system is losing water. Pressure does not disappear on its own.
Common places water escapes from:
- The pressure relief valve. It vents water outside through a small copper pipe, often when the pressure has been allowed to climb too high or the valve itself is worn. Look for drips or staining below the pipe outside. See our guide to a pressure relief valve dripping outside.
- A failed expansion vessel. When the vessel loses its air charge, pressure swings high when hot, the relief valve dumps water, then pressure crashes when cold. Vaillant lists a faulty internal expansion vessel as one cause of F22.
- Leaks inside the boiler from seals, the pump or the heat exchanger. Look for drips or corrosion under the boiler, but do not take the cover off.
- Leaks on radiators, valves or pipework, including hidden ones under floors.
If you have checked every radiator and pipe and can see nothing, our article on a boiler losing pressure with no visible leak walks through the less obvious culprits.
F22 with normal pressure: the component faults
When the gauge shows healthy pressure but the boiler still reports F22, the boiler cannot confirm that water is there or circulating. Vaillant’s F22 help centre page lists these possible causes:
- A faulty water pressure sensor, or a poor electrical connection to it
- Loose, disconnected or damaged wiring to the pump or pressure sensor
- Interrupted pump operation, such as a seized or failing pump
- A faulty solenoid valve on the automatic filling device, on models that have one
- A faulty internal expansion vessel
All of these are inside the casing. Diagnosing them means testing live electrical components and sometimes draining the boiler, so they are engineer repairs. Resetting over and over will not fix them and can mask a pump problem that later causes a different fault. If your boiler has also shown F75, which concerns the pump and pressure sensor, mention it to the engineer.
Is it safe to use the boiler with F22?
The F22 lockout is the boiler keeping itself safe, so there is no gas danger from the code itself. The risk comes from what people do next. Do not open the casing, bypass sensors, or keep topping up a system that is clearly leaking. Water from a leak can reach electrics and ceilings.
If you smell gas at any time, turn off the gas at the meter if you can do so safely, open windows, do not use electrical switches, leave the property and call the National Gas Emergency Service on 0800 111 999.
In a cold snap, while you wait for a repair, keep the house protected from freezing. Our guide on protecting pipes from freezing has practical steps.
Get F22 fixed properly
Below 1 bar, a careful top-up and reset is usually all F22 needs. Normal pressure with F22, or a code that keeps coming back, means a fault inside the boiler or a leak in the system that will not fix itself.
Gasmaster’s Gas Safe registered engineers repair Vaillant ecoTEC boilers across Stoke-on-Trent, Staffordshire, Cheshire and Derbyshire. Visit our boiler repair page or contact us to book an engineer. Regular servicing catches failing expansion vessels and pumps early; our Vaillant boiler service guide explains what is checked, and if you have also seen ignition codes, read our guide to Vaillant F28 and F29.
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Frequently asked questions
What pressure should my Vaillant boiler be at?
Vaillant gives a normal range of 1 to 1.5 bar for the heating system. Check it when the system is cold. It will rise when the heating is running, which is expected. If it climbs close to 3 bar when hot, or drops below 1 bar when cold, the system needs attention.
How do I reset a Vaillant boiler after F22?
First fix the cause, which usually means topping the pressure back up to between 1 and 1.5 bar. Then press the reset button on the control panel, as shown in your user manual. If F22 returns straight away with good pressure, stop resetting and call an engineer. Our guide on how to reset a boiler safely explains more.
Can a Vaillant F22 fault damage my boiler?
The code exists to prevent damage. The boiler stops firing before the heat exchanger can overheat without water. Damage is more likely if the underlying cause is ignored, such as a leak dripping onto electrics, a pump left failing, or a relief valve dumping water repeatedly because the expansion vessel has lost its charge.
Why does my Vaillant lose pressure after bleeding radiators?
Bleeding lets air out, and the water that replaces it comes from the system itself, so the pressure falls. One top-up afterwards is normal. If pressure keeps falling in the days after, the system is losing water and should be checked for leaks, a worn relief valve or an expansion vessel problem.
Is F22 covered by the Vaillant guarantee?
It depends on the cause and your guarantee terms. A failed internal part such as a pressure sensor or pump may be covered on a registered boiler with a full service history. Low pressure from a radiator leak or bleeding is a system issue rather than a product fault. Check your terms or contact Vaillant.
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
