The short answer: the F22 Vaillant fault code means the boiler has detected low water pressure and shut itself down to avoid running dry. Repressurise the system to around 1 to 1.5 bar using the filling loop, then reset. If F22 keeps returning, your heating system is losing water somewhere, often through a hidden leak.
Few things are more annoying than a boiler that refuses to fire on a chilly Devon morning. If your Vaillant is showing F22 on the display, take a breath. It is one of the most common fault codes the brand produces, and in most cases the boiler itself is fine. It has detected that the water pressure inside your heating system has dropped too low, so it has shut itself down rather than risk running dry.
This guide explains what the F22 Vaillant fault code means, how to read the pressure gauge, how to top the system back up safely, and the question that matters far more than the reset: why the pressure dropped in the first place. If F22 keeps coming back, something is letting water out of your system, and a reset will not cure that.
What the F22 Vaillant fault code means
F22 is Vaillant’s low water pressure fault. The manuals describe it as a dry fire protection shutdown: the boiler senses there is not enough water in the circuit and locks out rather than fire with too little water passing through the heat exchanger. Running dry causes rapid overheating and expensive damage, so the lockout is a safety feature doing its job, not a breakdown.
On many Vaillant models, including the popular ecoTEC range, the fault appears when system pressure falls to around 0.5 bar, and some models trip closer to 0.3 bar. The exact threshold varies, so check your boiler’s manual for the figure that applies to your model.
One thing worth knowing: F22 is occasionally triggered by a faulty water pressure sensor or a loose connection rather than genuinely low pressure. If your gauge shows a healthy reading but the fault will not clear, that is a job for a Gas Safe registered engineer. Never remove the boiler casing yourself.
How to read the pressure gauge
Every sealed heating system has a pressure gauge. On older Vaillant boilers it is a small dial on the underside of the appliance or on the pipework nearby, while newer models show the pressure digitally on the front display. The reading tells you how much water is in the system, and it only takes a glance to interpret.
What the gauge is telling you:
- Below about 0.5 bar: too low. This is F22 territory and the boiler has probably locked out.
- Around 1 to 1.5 bar when cold: healthy for most homes, though your manual gives the exact range for your model.
- A small rise while the heating runs: completely normal, because water expands as it heats.
- Approaching 3 bar: too high. Let the system cool and ask a heating engineer to take a look.
How to repressurise a Vaillant boiler safely
Vaillant designs its boilers so a householder can top up the pressure without tools and without opening anything up. The whole job happens on the pipework beneath the appliance, and the casing stays firmly on.
Most systems use an external filling loop, a short braided hose with a small valve at each end, linking the cold mains to the heating circuit. Some Vaillant models have a built-in filling key or lever instead. Have a quick look at your manual first, because arrangements vary between models and the manual will show exactly what you are working with.
Repressurising in general terms:
- Turn the boiler off and let the system cool. Pressure readings are only meaningful when cold.
- Find the filling loop beneath the boiler and check both ends are attached securely.
- Open the valves slowly and watch the gauge climb. Aim for around 1 to 1.5 bar, or whatever your manual specifies.
- Close both valves firmly once you reach the target. Leaving them open will over-pressurise the system.
- Restart the boiler. The F22 code normally clears on its own once the sensor reads normal pressure.
If the gauge will not rise, a valve is seized, or you are unsure at any point, stop and call a Gas Safe registered engineer. Forcing a stiff filling loop is a quick way to turn a small problem into a wet one.
Bleed the radiators afterwards
Fresh water carries dissolved air, so a few days after topping up you may find radiators that are warm at the bottom but cool at the top. That trapped air collects at the high points of the system.
Bleeding is straightforward. With the heating off, use a radiator key to open the bleed valve at the top corner of each affected radiator until you hear air hissing out, and close it the moment water appears. Keep a cloth handy. Bleeding releases a little pressure as well as air, so check the gauge afterwards and top up again if it has dipped.
The real question: why did the pressure drop?
Here is the part many guides skim over. A sealed heating system is exactly that, sealed. In a healthy system the water has nowhere to go, and the gauge holds steady for months at a time. Pressure does not vanish on its own.
A one-off F22 after bleeding radiators, or following work on the system, is nothing to lose sleep over. Top up and move on. But if you are repressurising every few weeks, then every few days, then watching the needle sag within hours, your system is losing water somewhere and the fault will keep returning until the escape route is found.
Sometimes the route is visible. A weeping radiator valve, a damp patch beneath the boiler, or a pressure relief pipe outside that drips after the heating has been on are all worth checking. A tired expansion vessel or a passing pressure relief valve can also bleed pressure away, and both are jobs for a heating engineer.
Very often, though, there is nothing to see at all. In our experience across Devon, the most common cause of a stubborn, recurring F22 is a small leak on the heating pipework itself, hidden somewhere you would never think to look.
Where hidden heating leaks tend to hide:
- Under solid floors, where pipes run through or beneath the screed
- Beneath timber floorboards and in underfloor voids
- Inside walls, where pipework drops down to radiators and towel rails
- Under kitchen units and built-in cupboards, at joints you cannot see
- On long runs to extensions and conservatories, where pipework flexes and ages
- At older soldered or compression joints that have corroded slowly over the years
A pinhole leak on a heating pipe can release a surprising amount of water over a week yet leave no visible trace, because the warmth of the pipe dries much of the moisture before it ever reaches the surface. Often the only symptom is a boiler that keeps asking to be topped up.
How we pinpoint hidden heating leaks
Finding these leaks without wrecking the house is exactly what Devon Leak Detection does. Rather than lifting floors on guesswork, we survey the system with specialist equipment and narrow the leak down to a precise spot before anything is disturbed.
Depending on the property, that can involve thermal imaging to trace pipe runs and highlight unusual warm patches, acoustic listening equipment that picks up the sound of water escaping under pressure, and tracer gas testing, where a safe hydrogen and nitrogen mixture is introduced into the pipework and detected at the surface as it rises through the leak point. You can read more about how it all works on our central heating leak detection page.
We should be clear about what we are not. We are leak detection specialists, not boiler engineers, so if your F22 turns out to be a faulty sensor, a flat expansion vessel or anything inside the boiler itself, the right person is a Gas Safe registered engineer. But when the boiler checks out fine and the pressure still keeps falling, the answer is in the pipework, and that is precisely what we find. We work across the whole of Devon, from Exeter and Plymouth to the smallest coastal village, with same-week appointments often available. For more guides like this one, browse our articles library.
Frequently Asked Questions
No. F22 is a protective shutdown rather than a warning of danger. The boiler has turned itself off precisely so nothing harmful can happen, and in most cases it restarts happily once the system is back up to pressure. Just never remove the casing. Anything beyond the filling loop and the external controls is work for a Gas Safe registered engineer.
Most Vaillant systems sit comfortably at around 1 to 1.5 bar when the system is cold, rising slightly as the heating warms up. Your boiler’s manual gives the exact recommended range for your model, and it is worth checking because models do vary.
Rarely. A healthy sealed system might want a small top-up once or twice a year, typically after radiators have been bled. If you are topping up every few weeks, or the F22 fault returns within days, the system is losing water and the cause needs finding. It will not fix itself.
Repeated F22 faults mean water is escaping somewhere. Check the visible places first: radiator valves, the pipework under the boiler, and the pressure relief discharge pipe outside. Ask a heating engineer to rule out the expansion vessel and pressure relief valve. If everything visible is dry, the leak is likely hidden under floors or in walls, which is when professional leak detection earns its keep.
Many household policies include trace and access cover, which can contribute towards locating a hidden leak and reinstating anything disturbed in the process. Policies differ widely, so check your documents or speak to your insurer before committing to work. We can provide a clear written report to support a claim.
Yes, that is the point of what we do. Thermal imaging, acoustic listening and tracer gas testing all work from the surface, so the leak is pinpointed before a single tile or floorboard is lifted. Any access needed for the repair is then kept to one small, accurate spot.
Boiler Keeps Losing Pressure? We Find the Leak
If the F22 fault keeps returning, the water is going somewhere. Devon Leak Detection pinpoints hidden heating leaks non-invasively across the county, with same-week appointments often available.