F22 on a Vaillant boiler means system water pressure has fallen too low and the boiler has shut down to protect itself. Top the system back up through the filling loop to typically around 1 to 1.5 bar cold. If it holds, you are done. If it drops again within days, water is escaping from the circuit.
A Vaillant boiler that stops mid-cycle and puts F22 on the display has not broken. It has protected itself. The Vaillant F22 fault is the low water pressure lockout, usually cleared with a filling loop and a gauge. What matters is what happens next: whether the pressure holds, or quietly slides back down.
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What the Vaillant F22 fault code means
F22, written F.22 in Vaillant’s own literature, means water pressure inside the sealed heating circuit has fallen below the level the boiler needs to fire. Rather than run with too little water through the heat exchanger, it locks out and puts the code on the display. Heating and hot water stop until pressure is back in range. Vaillant documentation also describes the condition as a dry fire, with no or insufficient water in the appliance.
The exact pressure at which a Vaillant cuts out varies by model, so treat any single figure with caution. Vaillant’s operating instructions for current ecoTEC plus models recommend a filling pressure of at least 1 bar, and say to top up if it falls below 0.8 bar. The response is the same either way: read the gauge cold, and if it sits below the band your manual gives, top the system up slowly to typically around 1 to 1.5 bar.
F22 reports a symptom. It tells you the system is short of water, not why.
Start by reading the pressure gauge
The gauge sits on the front of the boiler, either as a small dial with a needle or a figure on the digital display. Read it cold, before the heating has run that day, because pressure rises as water heats and a hot reading flatters the system. Cold pressure is typically around 1 to 1.5 bar, though the band is manufacturer dependent and your manual has the final word. What it says splits the job in two.
| Cold gauge reading | What it points to | Where it goes next |
|---|---|---|
| Below 1 bar | The system is genuinely short of water | Top it up through the filling loop, then watch whether it holds |
| Normal band, yet F22 still showing | The boiler is misreading pressure, or something on the boiler side has failed | A Gas Safe registered heating engineer. The casing stays shut. |
| Normal after a top up, then low again in days | Water is leaving a sealed circuit | Find where, before topping up becomes a monthly ritual |
How to repressurise the system safely
Topping a sealed system back up is a normal homeowner task, and Vaillant publishes instructions for it. The filling loop is usually a braided silver hose under the boiler with a small valve at each end, though several Vaillant models use an integrated loop worked by a lever or key behind a flap underneath.
- Switch the boiler off at the front panel and let it cool.
- Find the filling loop and check both ends are properly connected.
- Open both valves slowly. You should hear water moving in.
- Watch the gauge, not the clock. Start closing the valves as the needle approaches the band your manual specifies, typically around 1 to 1.5 bar cold.
- Close both valves fully, and disconnect an external loop if your manual says to.
- Switch the boiler back on. Vaillant says F.22 clears on its own once pressure is back up, though some models want the reset button pressing.
Ease the valves shut early rather than late. Overshooting leaves cold pressure too high, and once the water heats and expands, the pressure relief valve can open and discharge outside, which puts you back where you started. Our step by step guide to increasing boiler pressure covers what to do if you overfill.
The test that matters: does the pressure hold?
This is the part most people skip, and it is the part that answers the question. Write down the date and the reading the moment you finish topping up, then read the gauge cold once a week. Three or four dated readings settle it.
| What the gauge does next | What it usually means | Where it goes from here |
|---|---|---|
| Holds steady for weeks or months | A one off | A radiator was bled, or the system was drained for a repair. Nothing further to do. |
| Drifts down slowly, one top up a year | Worth watching | Older systems can seep slowly. Mention it at the next boiler service. |
| Back to F22 within days or weeks, repeatedly | Water is escaping | A sealed system does not use water up. Repeated loss means it is going somewhere. |
That last row is the reason for the exercise. A sealed heating system is a closed loop: the same water circulates for years, and nothing is consumed or evaporates. If fresh water goes in every fortnight, the same volume is leaving every fortnight, whether or not you can see it. Our guide to a boiler that keeps losing pressure runs through the causes in order of likelihood.
Where the water goes when there is nothing to see
Start with what you can actually look at. Vaillant puts radiator valves and system components at the top of its own list of leak sources. A weeping valve can leave a rusty tear stain down the valve body, or a crusty white deposit where drips have dried. Joints near the boiler can seep slowly enough that water evaporates before it falls. A discharging pressure relief valve can leave a limescale trail under the copper pipe outside, and it often drips only while the heating runs, so a dry check at midday proves nothing. Water at the boiler itself is covered in our guide to why a boiler leaks water.
Then there is the pipework nobody can look at. Heating pipes run under floorboards, buried in screed and chased into wall plaster, and a pinhole in one releases a small volume continuously. Under a solid floor that may show as a patch that feels slightly warm underfoot, or carpet that never quite dries. Often it shows as nothing at all, and the gauge is the only evidence.
Finding that pipe without taking up the floor is the whole discipline of central heating leak detection. Heated water gives a leak away thermally, so a FLIR thermal camera traces the warm plume through a floor or wall. Acoustic sensors pick up the sound of water escaping under pressure. Where a circuit can be isolated and charged, tracer gas, a safe mix of 5% hydrogen and 95% nitrogen, rises through the structure to a probe at the surface. The aim is to open one small area rather than a whole floor, and to leave a trace and access report for your insurer.
When F22 is not a leak at all
Not every F22 is about escaping water. Bleeding radiators or draining down for a repair takes water out of the circuit, and a drop afterwards is expected. Two other causes are worth knowing.
A faulty pressure sensor or its wiring
The boiler does not read the gauge on the front. It reads a sensor, and a failed sensor or loose connection can report low pressure that is not there. The giveaway is a gauge sitting comfortably in the normal band while the display insists on F22. That is a boiler side repair for a heating engineer.
An expansion vessel that has lost its charge
A tired vessel does not lose water by itself, but it lets pressure swing hard as the system heats and cools, and those swings can push the pressure relief valve open. The valve discharges outside and the system loses pressure for real. It looks like a leak because it is one, just not in a pipe.
What we do not do
DCI is a leak detection firm. We do not service boilers, repair them or replace parts, so if your F22 turns out to be a sensor or a vessel, a heating engineer is who you need. Our work starts where a system is provably losing water and nobody can find where it goes.
F22 in Cornwall and Devon homes
Two things about the housing stock down here change how this plays out. The first is construction: in granite and cob cottages, heating pipes are often chased into solid walls or run under solid floors rather than beneath liftable boards, and a buried leak there can stay invisible for longer than in a house with timber floors. The second is occupancy. A holiday let standing empty between bookings can lose pressure for weeks with nobody there to read the gauge, and a guest arriving to no hot water finds the fault.
Winter concentrates it. Systems that idled through the summer run hard for the first time in months, and we hear the same account every heating season: topped up in October, F22 again by Christmas, no sign of water anywhere. That pattern is what our heating circuit leak tracing across Cornwall and Devon exists for.
Frequently asked questions
What does F22 mean on a Vaillant boiler?
F22 means the water pressure in the sealed heating circuit has dropped below the level the boiler needs to run, so it has shut itself down. Heating and hot water stop until the pressure is restored. Vaillant literature describes it as a low water pressure or dry fire condition.
What pressure should a Vaillant boiler be set to?
Read the gauge when the system is cold. Typical cold system pressure is around 1 to 1.5 bar, and Vaillant guidance for a low reading is to top up slowly into that band. The figure is manufacturer dependent, so check the manual for your own model.
Will the F22 fault clear once I top the pressure up?
Usually, yes. Vaillant’s own operating instructions say the F.22 message goes out and the boiler works again automatically once pressure is back in range. Some models want the reset button pressing as well, so the manual for your boiler is the reliable answer.
Why does my Vaillant keep showing F22 after I repressurise it?
Because the water is going somewhere. A sealed heating system does not consume water, so pressure that falls back within days or weeks points to a leak in the circuit. The pressure sensor or its wiring can also be at fault, which a heating engineer can check on the boiler.
Can I fix a Vaillant F22 fault myself?
Topping the system back up through the filling loop is a normal homeowner task and Vaillant publishes instructions for it. Opening the boiler casing is not. If the gauge already reads inside the normal band and F22 still shows, the work belongs to a Gas Safe registered engineer.
Does F22 always mean I have a leak?
No. A single F22 after bleeding radiators, a repair or a system drain down is normal and one top up settles it. It becomes a leak question when pressure keeps falling back after repeated top ups with no puddle, damp patch or dripping valve to explain it.
How is a hidden heating leak found without lifting floors?
Thermal imaging picks up the warm trail that heated water leaves under a floor or behind plaster. Acoustic sensors listen for escaping water, and tracer gas, a safe mix of 5% hydrogen and 95% nitrogen, is detected at the surface where it rises through the structure.
Topped it up more than twice this month?
If F22 returns within days of every top up and there is nothing on the floor to explain it, water is leaving a sealed circuit, and finding where is the job we do across Cornwall and Devon. If the pressure held after one top up, you have no leak and no need of us.
Call Dickie on 07822 025 911