The short answer
F22 on a Vaillant ecoTEC means the boiler has detected too little water in the heating circuit, usually because system pressure has dropped below about 0.6 bar. Check the pressure gauge. If it reads under 1 bar, top the system up to between 1 and 1.5 bar using the filling loop or filling key, and the fault clears. If the pressure falls again within days, there is a leak, a passing pressure relief valve or a failed expansion vessel. If the gauge reads normally but F22 stays, the pressure sensor or the pump has failed. London repair costs run from £90 to £400.
What the law requires
- What does F22 mean on a Vaillant?
- How to tell what is going on in one minute
- How to top up a Vaillant and clear F22
- Why does the pressure keep dropping?
- When F22 is not about water at all
- What an engineer does on an F22 call
- How much does a Vaillant F22 repair cost in London?
- Is F22 dangerous?
- How to stop F22 coming back
- Vaillant F22, quick answers
What does F22 mean on a Vaillant?
Vaillant’s fault list describes F22 as water shortage, shown when the boiler detects too little water or too low a pressure in the primary heating circuit. It is a protective shutdown. Firing a boiler with no water through the heat exchanger would overheat it in seconds and could crack it, so the boiler refuses to light until it is satisfied there is enough water.
You will see F22 on the ecoTEC plus, ecoTEC pro, ecoTEC exclusive and ecoTEC sustain ranges, and on older ecoMAX and turboMAX boilers, on both combi and system models. The display may show F22, F.22 or F 22. They are all the same fault.
Most F22 faults are simple. The system has lost a little water and needs topping up. The ones that are not simple are the ones where the pressure keeps disappearing, or where the gauge reads fine and the fault stays. This guide works through both.
How to tell what is going on in one minute
Look at the pressure gauge on the front of the boiler, with the boiler cold.
- Gauge under 1 bar: the system has lost water. Go to the top-up steps below.
- Gauge between 1 and 2 bar but F22 is showing: the pressure sensor, the pump or the wiring is at fault. This needs an engineer.
- Gauge above 2.5 bar when the heating is on, then near zero when cold: the expansion vessel has lost its charge. Topping up will make this worse. Book an engineer.
How to top up a Vaillant and clear F22
This is safe for you to do, needs no tools, and the boiler case stays shut. Topping up is not gas work.
Switch the boiler off
at the fused spur or with the mode dial, and let it cool.
Find the filling point.
On most ecoTEC models there is a filling valve or a removable filling key on the underside. Some systems use a flexible silver hose with a tap at each end.
Open the filling point slowly.
Watch the gauge. You will hear water entering.
Stop between 1 and 1.5 bar.
Do not go above 1.5 bar cold. Water expands as it heats, and 2 bar cold becomes roughly 3 bar hot, which lifts the pressure relief valve.
Close the filling point firmly.
If you used a braided hose, disconnect it at one end so it cannot feed the system while you are not watching.
Switch the boiler back on.
F22 should clear within a minute or two. If it does not, press the reset button once.
Bleed any radiators that are cold at the top,
then check the gauge again, because bleeding releases water and drops the pressure. Top up once more if needed.
If the pressure holds for a month, that was ordinary loss. If it is back under 1 bar within a week, move to the next section.
Why does the pressure keep dropping?
- A leak in the system. Radiator valve tails, pipe joints under floors, a corroded radiator seam or a weep at the boiler. Pressure that falls from 1.5 bar to F22 in a week means water is going somewhere. A damp patch, a stained ceiling below a bathroom, or a warm spot on a floor are the usual signs.
- A passing pressure relief valve. The relief valve dumps water outside if pressure goes above 3 bar. After a few lifts, the seat wears and it dribbles continuously. Look for a drip from the small copper pipe that exits the wall near the boiler.
- A flat expansion vessel. The vessel absorbs water expansion. When its air charge leaks away, pressure spikes when hot, the relief valve lifts and dumps water, and the gauge falls to nothing when cool. Every top-up then makes the next dump worse.
- A leaking boiler component. Pump seals, the heat exchanger and the pressure sensor housing can all weep. This is usually visible as water on the base of the boiler casing or staining under it.
- Recently bled radiators or recent system work. Bleeding several radiators can drop the pressure by half a bar or more. This is normal and not a fault.
When F22 is not about water at all
If the gauge reads a healthy 1 to 1.5 bar and F22 refuses to clear, the boiler is wrongly reporting a shortage.
- A failed pressure sensor. It reads low when it is not. A common, inexpensive fix.
- A pump that is not circulating. On ecoTEC models the boiler infers water presence from flow, so a seized pump can throw F22 even at a normal pressure. Signs are a boiler that fires briefly then stops, and radiators that stay cold.
- An airlock in the primary circuit. After a drain-down or repair, trapped air stops water reaching the sensor. Bleeding the highest radiator often clears it.
- A wiring or connector fault between the sensor and the control board.
An engineer proves which one by comparing the sensor reading with a test gauge, and by measuring pump current.
What an engineer does on an F22 call
First, the pressure is set correctly and the system is observed over the visit to see whether it holds. Second, the expansion vessel charge is checked at its valve with a tyre gauge, with the system pressure at zero. Third, the relief valve and visible pipework are checked for weeps. Fourth, if all of that is sound, the pressure sensor and pump are tested. A leak under floors is found with dye, a pressure test or thermal imaging rather than lifting boards at random.
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.
How much does a Vaillant F22 repair cost in London?
Fixed price quoted after diagnosis. Expect these ranges.
- Top-up, system check and radiator bleed: £90 to £130.
- Pressure sensor replacement: £120 to £210 fitted.
- Filling loop or filling key replacement: £80 to £150.
- Pressure relief valve replacement: £130 to £230 fitted.
- Expansion vessel recharge: £90 to £150. Temporary if the diaphragm has failed.
- Expansion vessel replacement: £190 to £330 fitted.
- Pump replacement: £220 to £420 fitted.
- Leak location and repair: £120 to £400, depending on where it is.
If your boiler is over twelve years old and the repair is a pump or heat exchanger, ask for a replacement quote alongside it. Our repair or replace guide sets out the half-cost rule engineers use.
Is F22 dangerous?
No. The boiler stopped itself before any harm could happen. The danger is in how people react. Do not run the boiler repeatedly to try to force it, do not top up above 1.5 bar cold, and never open the boiler casing. If water is leaking near electrics, switch the boiler off at the fused spur and call an engineer. If you ever smell gas, leave the property and call the National Gas Emergency line on 0800 111 999.
How to stop F22 coming back
Look at the gauge once a month through winter. It takes two seconds and catches a slow leak early.
Ask for the expansion vessel to be checked at every annual service. It is the most commonly missed item.
If the pressure relief valve has been replaced, ask the engineer to check the vessel in the same visit. A valve that fails twice in three years is almost always a flat vessel nobody tested.
Fit a magnetic system filter if you do not have one, and keep inhibitor topped up. Sludge corrodes radiators from the inside, and that is where many slow leaks begin.
Vaillant F22, quick answers
My Vaillant shows F22, what do I do?
Check the gauge. If it is under 1 bar, top up to between 1 and 1.5 bar and the fault clears. If it does not, or it returns within a week, book an engineer.
What pressure should a Vaillant have?
Between 1 and 1.5 bar cold, rising to about 2 bar when hot.
Why does F22 only appear in the morning?
A small leak or a flat expansion vessel lets the pressure fall overnight as the system cools. Mention that pattern to the engineer.
The gauge reads 1.5 bar but F22 will not clear. Why?
The pressure sensor or the pump has failed. It is an engineer job.
Is F22 the same as F75?
No. F22 is a water shortage. F75 is a failure to detect a pressure change when the pump starts, which points at the pump or the sensor. Our F75 guide covers that one.
Is F22 covered by warranty?
A failed sensor or pump inside the boiler usually is, on a boiler within warranty with a service history. Leaks in the system, relief valve wear and filling loops usually are not.