Vaillant F22 Fault Code: Low Water Pressure, Causes & How to Fix It

Fix & Repair Engineers diagnoses and repairs Vaillant F22 faults across London and the Home Counties. Our engineers investigate leaks, the pressure sensor, wiring, pump and expansion vessel properly before recommending a repair.

The table below summarises the most-asked questions about F22 before the full detail further down the page.

The simplest current definition is that F22 means the boiler has detected low or insufficient water pressure and has carried out a safety switch-off. Vaillant's current ecoTEC installation documentation describes F22 as "Safety switch-off: Low water pressure" and identifies the underlying condition as no or insufficient water in the appliance, or water pressure that is too low. Current homeowner guidance also identifies additional possible fault areas including water pressure below the normal range, a problem with the water-pressure sensor, loose, disconnected or defective wiring for the pump or pressure sensor, a defective water-pressure sensor, pump problems on open-vented boilers, and a defective internal expansion vessel. F22 is not always solved by simply adding water. First establish what pressure the boiler is actually showing.

Why Do Some Vaillant Manuals Call F22 "Dry Fire"?

If you search online for F22, you may also see the term "dry fire" or "dry firing". That wording comes from older Vaillant documentation. An earlier ecoTEC installation and servicing manual defines F22 as "Dry fire" and lists too little water in the appliance, a defective water-pressure sensor, faulty cable to the pump or water sensor, a blocked or defective pump and insufficient pump output as possible causes. Newer Vaillant documentation instead uses the clearer description "Safety switch-off: Low water pressure". The terminology has changed between boiler generations, but both relate to the boiler protecting itself when satisfactory water/pressure conditions cannot be confirmed. Whether you have seen "Vaillant F22 dry fire" or "Vaillant F22 low pressure", the troubleshooting principle is the same: check the pressure first and do not assume which internal component is responsible.

What Pressure Should a Vaillant Boiler Be?

Vaillant's current homeowner guidance generally recommends approximately 1-1.5 bar when the central heating is cold or switched off. Pressure normally rises somewhat as the heating operates because heating water expands. The correct value can vary by appliance and installation, so your own Vaillant operating manual remains the final reference for your particular model. The precise warning and lockout thresholds also vary between models: one current ecoTEC plus installation manual states that below 0.8 bar the displayed pressure starts flashing, below 0.5 bar that particular appliance switches off and displays F22, and after refilling the display continues flashing until at least 1.1 bar is reached. That does not mean every Vaillant boiler uses exactly the same threshold. For homeowners, the simpler rule is: below 1 bar, repressurisation may be appropriate; above 1 bar with F22 remaining, professional diagnosis is required.

Vaillant identifies several possible causes rather than a single failed component. Correct diagnosis means checking each area rather than assuming the boiler simply needs water.

A system leak is one possible reason why pressure becomes low, but Vaillant also identifies pressure-sensor faults, wiring problems, pump-related problems and expansion-vessel faults, and separately confirms that bleeding radiators can lower system pressure enough for F22 to occur. F22 means low pressure or a problem associated with detecting or managing that condition. It does not mean your boiler definitely has a leak. The first practical step is to look at the boiler's pressure gauge or digital pressure display and use the reading to decide what happens next.

Repressurisation is one of the relatively few boiler troubleshooting tasks Vaillant permits a competent homeowner to carry out. However, different Vaillant boilers use different filling arrangements, some with an integrated filling mechanism and others using an external filling loop. Check the manual for your particular boiler before beginning.

An F22 fault that repeatedly returns after repressurising deserves investigation. It tells us either that the system is repeatedly losing pressure, or that the boiler is still having difficulty correctly detecting or managing the pressure condition. Potential areas include water loss somewhere in the heating system, a pressure-sensor problem, pump or sensor wiring, pump operation depending on the boiler/system, and the internal expansion vessel.

Make a note of how quickly pressure falls, for example: drops again within hours, drops over several days, falls only after the heating cools, rises significantly when hot and then becomes very low when cold, or pressure appears normal but F22 remains. Those patterns can provide useful diagnostic information to the engineer.

Repeated repressurisation should not become routine. Occasionally restoring pressure, particularly after radiator bleeding, can be normal, but if the boiler needs frequent topping up, the reason the system is losing or mismanaging pressure needs investigating. Repeatedly restoring the pressure can hide the symptom without correcting the underlying problem.

These are among Vaillant's best-known pressure-related faults, and they are frequently confused. Vaillant's current ecoTEC professional manual lists these as separate diagnostic faults.

F22 concerns insufficient pressure/water, while F75 concerns the pressure response associated with pump operation. This explains why a boiler can potentially display F75 even when its static pressure appears reasonable, whereas checking the actual pressure is the primary first step for F22. See our separate guide to the Vaillant F75 boiler fault for the pump-start pressure-change fault.

F73 and F74 are more specifically associated with the water-pressure sensor signal. Older Vaillant ecoTEC documentation distinguishes them as F73, a water-pressure sensor signal outside range on the low side, and F74, a water-pressure sensor signal outside range on the high side. F22, by contrast, describes the boiler's low-water/low-pressure safety condition. A pressure sensor can be involved in F22 without F22 itself simply meaning pressure sensor failure.

A reset may be appropriate after the low-pressure condition has been corrected, using the reset procedure for your particular Vaillant model. Vaillant notes that resetting can sometimes clear a fault or temporarily return the boiler to operation. However, resetting does not restore lost system water, and it does not repair a leak, defective pressure sensor, wiring fault, pump problem or expansion-vessel fault. Repeatedly resetting an F22 boiler without addressing the pressure problem is not useful troubleshooting. The boiler's F22 safety switch-off exists specifically to prevent normal operation when acceptable water/pressure conditions cannot be confirmed, so the correct response is to resolve the pressure condition rather than attempting to force the boiler to operate. Never bypass an F22 safety lockout or interfere with internal controls.

Is F22 Dangerous?

F22 itself is principally a low-water-pressure safety fault, not a gas-leak warning. The boiler has stopped normal operation because it cannot confirm appropriate water/pressure conditions. In most cases, that means the immediate practical problem is loss of heating and hot water rather than a gas emergency. However, do not open the boiler casing or carry out internal repairs.

The exact process depends on the Vaillant model and heating-system design, but a professional investigation can include several stages.

How Much Does It Cost to Fix a Vaillant F22 Fault?

There is no single meaningful F22 repair price because F22 is not a failed-part code. In some cases, there may be no repair charge beyond correctly repressurising the system. In others, diagnosis may identify a leak, pressure-sensor fault, electrical/wiring problem, pump issue, expansion-vessel fault or another heating-system problem causing repeated pressure loss. Those repairs vary substantially in labour, parts and complexity, so generic online "F22 repair prices" should not be treated as a quotation for your boiler. We diagnose recurring Vaillant pressure faults and provide a quotation based on the problem actually found.

Does F22 Mean I Need a New Boiler?

No. F22 is one of the last fault codes that should automatically lead someone to conclude that the boiler needs replacing. A simple low-pressure situation can potentially be resolved by correctly restoring system pressure. Even where an underlying component is responsible, pressure sensors, expansion-vessel issues, wiring and other system faults can often be repaired. Boiler replacement becomes worth considering only where the diagnosed repair is significant and the wider condition of the appliance makes further investment difficult to justify. Relevant factors include boiler age, overall condition, previous breakdown history, other components showing deterioration, parts availability and total repair cost. Diagnose first. Make the repair-versus-replacement decision afterwards.

Can Servicing Prevent F22?

Regular boiler servicing cannot guarantee that an F22 fault will never occur. For example, bleeding radiators can reduce system pressure even where the boiler itself is perfectly serviceable. However, routine maintenance can help identify problems affecting the boiler or heating system before they develop into repeated pressure faults. If your boiler frequently loses pressure between services, do not simply wait until the next annual visit. Have the underlying cause investigated.