The short answer
The Vaillant F22 fault code means low water pressure: the system has dropped below 0.5 bar and the boiler has shut down to protect itself. You can usually fix it yourself by repressurising to 1.5 bar using the filling loop. A recurring F22 means water is leaking somewhere.
What the law requires
- What does F22 mean on a Vaillant boiler?
- First, read the gauge
- How to fix a Vaillant F22 in five steps
- Why does my Vaillant keep showing F22?
- Why topping up forever makes things worse
- F22, F23, F24 and F75: reading Vaillant’s pressure family
- What the engineer will do for a recurring F22
- What does an F22 repair cost in London?
- F22 just after a new boiler was installed
- A landlord’s note on F22
- Vaillant F22, quick answers
What does F22 mean on a Vaillant boiler?
F22 is Vaillant’s low water pressure lockout, and it appears across the range: ecoTEC pro, plus and exclusive, ecoFIT pure and sustain, and most older models. A sealed central heating system runs at pressure, typically 1 to 1.5 bar when cold, and the boiler watches that number constantly through its pressure sensor. When the reading falls below roughly 0.5 bar, the boiler can no longer circulate water safely: run a pump against too little pressure and it cavitates, run a heat exchanger with too little water and it overheats. So the boiler stops, shows F22, and waits.
Two things make F22 the most approachable code in the Vaillant list. First, it is genuinely a user-fixable condition in most cases, one of the very few on a gas appliance. Second, it is honest: the code tells you exactly what is wrong right now (not enough water pressure) while leaving you the more interesting question this guide spends most of its time on, which is where the pressure went.
First, read the gauge
Before touching anything, look at the pressure display: a dial on older models, a digital bar reading on newer ecoTECs. Below 1 bar is low; at or below 0.5 bar is F22 territory. That reading is your baseline, because everything in diagnosing this fault comes down to how the number behaves over time: how far it fell, how fast it falls after you refill, and what it does when the heating runs. A photo of the gauge today, and another next week, tells an engineer more than most descriptions.
How to fix a Vaillant F22 in five steps
Turn the boiler off and let it cool for a few minutes. You are about to add cold water, and a cold system gives an accurate reading.
Find the filling loop. On most installations it is a silver braided hose beneath the boiler with a small valve at each end. Many ecoTEC models instead have built-in filling keys or levers under the casing edge; your manual shows which your model uses. If the loop is missing entirely (some installers remove them after filling), an engineer can refit one in minutes.
Open both valves slowly. You will hear water entering the system. Watch the gauge climb.
Close both valves at 1.5 bar. Do not chase a higher number: much above 2 bar cold and the pressure relief valve will start dumping the excess through the discharge pipe outside.
Restart the boiler. F22 should clear on its own or after one press of the reset button, and heating resumes.
Afterwards, bleed any radiators that feel cold at the top, since refilling can move air around, and recheck the gauge once more when finished. If the display still shows F22 with the gauge sitting at 1.5 bar, the pressure sensor itself is the suspect, and that is an engineer’s job.
Why does my Vaillant keep showing F22?
This is the question that matters, because F22 is only ever the symptom. A sealed system holds its water; if you are refilling every few weeks, water is leaving, and it is going somewhere. The suspects, in order of frequency across thousands of London callouts:
A leak in the system. The commonest and the best hidden. It rarely announces itself with a puddle: a weeping radiator valve, a pinhole in pipework under a floor, a joint behind a kitchen unit. A heating system holds a surprisingly small volume of water, so a leak too slow to see can still cost half a bar in a fortnight. Look for crusty white or green deposits at radiator valves, staining on ceilings below pipe runs, and lifted or darkened flooring near pipework.
A weeping pressure relief valve. Every sealed system has a safety valve that dumps water outside through a small copper pipe when pressure climbs too high. Once a PRV has lifted, it often never reseals perfectly, and it drips your pressure away invisibly. The check costs nothing: find that pipe on your outside wall and look for dripping, staining or a green tail below it.
A failed expansion vessel. The vessel is an air cushion that absorbs the water’s expansion as it heats. When its charge leaks away or its diaphragm fails, pressure swings hard: climbing high when the heating runs, forcing the relief valve to vent, then reading low once cool. The signature is exactly that pattern, high hot and low cold, on repeat. A recharge is quick; a replacement is routine.
The boiler’s own internals. Less often, the leak is inside the case: a weeping pump seal or heat exchanger joint. Any dampness at or under the boiler casing moves this suspect up the list, and it is strictly engineer territory.
The rule of thumb we give every caller: repressurising once or twice a year is normal life. Once a month is a leak. Once a week is a leak with a deadline, and the honest move is a leak trace, which usually finds the source within the hour.
Why topping up forever makes things worse
Endless refilling feels harmless and is quietly corrosive, literally. Fresh mains water carries dissolved oxygen, and oxygen rusts a steel heating system from the inside. Every top-up feeds a little more of it to your radiators, accelerating the corrosion that becomes magnetite sludge: the black sediment behind cold radiator bottoms, kettling noise and, on Vaillants specifically, the F75 sensor faults described in our F75 guide. A system that holds its water stays clean for years longer than one being constantly refreshed. That, more than the inconvenience, is why a £150 leak repair is cheaper than it looks.
F22, F23, F24 and F75: reading Vaillant’s pressure family
Vaillant gives its circulation problems separate codes, and they point different directions. F22 is the static low-pressure lockout this guide covers. F23 and F24 report circulation faults, temperature differences and rises that suggest water is not moving properly, often pump or blockage related. F75 means the boiler could not detect the pressure pulse that proves the pump is running: pressure may read normal, and topping up often changes nothing. If your boiler alternates between F22 and F75 across a season, it is telling one combined story, a system losing pressure and carrying dirty water, and it deserves one combined diagnosis rather than two separate part swaps.
Repressurising more than once a month? That is a leak, not a habit. Our Vaillant specialists trace and fix pressure leaks across London, usually same day.
What the engineer will do for a recurring F22
A pressure-loss visit is detective work with instruments. The engineer confirms the fill and watches the gauge behaviour hot versus cold, which alone separates the expansion vessel from the leak. The relief valve and its discharge pipe are inspected, the vessel’s charge is tested and recharged if low, and the system is pressure-tested to expose leaks that only open under heat and pressure. Where the loss is in hidden pipework, tracing works from evidence (staining, flooring, thermal patterns) rather than exploratory destruction, and the fix is quoted fixed before anything is opened. If the sensor is suspect, it is tested directly. The visit ends with a named cause; “just keep topping it up” is not a diagnosis, and you should treat any firm offering it as guessing.
What does an F22 repair cost in London?
The self-fix costs five minutes. Engineer work runs: leak trace with repressurise £95 to £220 depending on how well the leak hides, pressure relief valve replacement £110 to £190, expansion vessel recharge £80 to £140 or replacement £120 to £380, pressure sensor replacement £120 to £200, and internal component leaks quoted on diagnosis. Everything is fixed-quoted before work begins, and catching a recurring F22 early keeps you at the cheap end of that list: the same leak, left through a winter, tends to add damaged flooring to the invoice.
F22 just after a new boiler was installed
A special case worth its own note, because it worries people unnecessarily and occasionally hides a real problem. In the first weeks after any installation or major system work, trapped air works its way to the automatic air vents and escapes, and the pressure drops a little as it does. One or two top-ups in the first month of a new boiler’s life is commissioning, not a fault. What is not normal is the drop continuing past that settling period, or needing a top-up weekly from day one: that suggests a joint disturbed during the works is weeping, and it is squarely the installer’s to fix under their workmanship guarantee, at no cost to you. Date-stamp your top-ups in that first month; if the pattern has not flattened by week five or six, call the installer back with the log rather than quietly refilling into the winter.
A landlord’s note on F22
F22 is one of the most common tenant-reported “boiler broken” calls in London, and the most commonly mis-handled. A tenant sees a fault code and a dead boiler; a contractor attends; the fix was a thirty-second repressurise, invoiced at a callout. Two habits prevent that cycle. First, phone triage: our team talks tenants through the gauge reading and, where appropriate, the filling loop on the phone, free, before any visit is booked, and a large share of F22 reports end right there. Second, pattern tracking across the tenancy: a property whose boiler needs repressurising every few weeks has a leak on the clock, and for a landlord that leak is also a slow-motion insurance claim working through a ceiling somewhere. Fold the pressure check into the annual service and CP12 visit, and recurring F22s get caught as maintenance instead of arriving as winter emergencies; the combined visit is covered on our landlord gas safety certificate page.
Vaillant F22, quick answers
Can I run the boiler with F22 showing?
No, and it will not let you; the lockout protects the pump and heat exchanger. Repressurise first.
What pressure should a Vaillant boiler be?
1 to 1.5 bar cold, rising a little when hot. Much above 2.5 bar cold is its own problem, usually the vessel or a passing filling valve.
Does F22 mean I need a new boiler?
Almost never; it is a symptom code, not a verdict, and every common cause is a repair.
My filling loop valve will not turn. What now?
Valves seize; do not force one, because a snapped valve turns a top-up into a callout. An engineer frees or replaces it quickly.
Is F.22 different from F22?
No; Vaillant displays the point, people type it both ways, and this guide covers both.
Why does F22 always seem to strike in winter?
Cold contracts the water slightly and drops standing pressure, so a system already losing water crosses the 0.5 bar line on the coldest mornings, exactly when you notice most.