Why Does Vaillant F.22 Keep Coming Back?

Why Does the Vaillant F.22 Fault Keep Coming Back?

The Vaillant F.22 fault code means the heat pump has detected that system pressure has fallen below the minimum level it needs to operate safely. When this happens, the unit shuts itself down as a protective measure. It will not restart until the pressure is restored and the fault is cleared.

Many homeowners top the system back up using the filling loop, the heat pump restarts, and they assume the problem is solved. A day later, or a week later, or a month later, the F.22 appears again. The cycle repeats. Each time the fix feels like a success, but each time the fault returns.

The reason for this is straightforward. Topping up the system replaces the water that has been lost, but it does nothing to stop the water from being lost in the first place. If the F.22 keeps coming back, the pressure has gone somewhere, and until that is identified and resolved, the fault will continue to return.

What the F.22 Fault Actually Means

A sealed heating system is designed to hold water under pressure. That pressure stays stable over time in a healthy system, rising slightly when the water heats up and falling back when it cools, but remaining within a normal operating band. If the system is losing pressure rather than simply fluctuating within the normal range, it means water is physically leaving the circuit somehow, somewhere.

The F.22 is a symptom, not a diagnosis. Vaillant use this code to tell you that pressure has dropped below the acceptable threshold, but the code itself does not tell you why. That distinction matters because without understanding the cause, the only option is to keep adding water while the underlying problem continues unchecked.

Why Topping Up Is Not Enough

A healthy sealed heating system should not normally require regular topping up. Occasional repressurisation after maintenance work or radiator bleeding is normal. But if you find yourself adding water to the system every few days, weeks, or even months in response to a recurring F.22, there is almost certainly an underlying issue that is causing the pressure to fall.

The danger with repeated topping up is that it can mask a problem for a long time. Water continues to leave the system, you continue to replace it, and nothing is ever properly investigated. Meanwhile, the cause of the pressure loss may be worsening, and in some cases it can lead to more significant damage if left unaddressed.

Common Reasons Pressure Keeps Dropping

There are several reasons a sealed system can lose pressure, and it is worth working through them methodically rather than assuming the worst or dismissing the issue as trivial.

A small leak somewhere on the heating pipework or at a joint, radiator valve, or fitting is one of the most common causes. These leaks do not always present as obvious drips or wet patches. Water can evaporate quickly from a weeping joint, particularly in warm areas of the property, meaning the source of the loss is never visible to the naked eye. A heating engineer can carry out a pressure test to identify whether there is an active leak and locate it.

A faulty expansion vessel is another frequent culprit, particularly in systems that have been in use for a number of years. The expansion vessel is a component that accommodates the natural expansion and contraction of water as the system heats and cools. When the vessel loses its internal charge, it can no longer perform this function correctly. Pressure may rise sharply when the system is hot and then fall significantly when it cools, eventually triggering F.22. If you notice that the system pressure climbs quickly during operation and then drops back after the heat pump shuts off, this is worth investigating. Our article on why your heat pump shows a flow error explains how system faults linked to circulation and pressure loss are interconnected, and why these issues are often related.

An automatic air vent that is releasing water rather than just air can also cause gradual pressure loss. These vents are designed to expel trapped air from the system, but if the valve inside is worn or contaminated, it can release small amounts of water alongside the air. The losses are often slow enough that they go unnoticed for some time.

The pressure relief valve is another component to consider. If the PRV has previously opened due to over-pressure and has not reseated cleanly, it may continue to weep water through the discharge pipe. The discharge outlet is usually directed externally, which means the evidence of this is easy to miss unless you specifically look for it.

Recent maintenance work is also worth considering. If the system was drained, flushed, bled, or interfered with in any way around the time the F.22 started recurring, the cause may be directly related. Components disturbed during maintenance do not always reseat correctly, and air introduced into the system during the work can cause pressure instability until it has been fully purged.

The Anti-Freeze Valve Problem That Many Engineers Miss

On many heat pump systems installed in the UK, an anti-freeze protection valve is fitted externally to protect the outdoor pipework during freezing conditions. These valves work by opening automatically when the water temperature in the external pipework drops close to freezing, releasing a small amount of water to prevent ice from forming in the circuit.

If one of these valves begins to fail, develops a fault, or starts passing water at temperatures well above the threshold it should be responding to, it can cause a slow but steady loss of system pressure. The water is discharged externally, which means there is often no visible evidence inside the property. Because the discharge happens outside and intermittently, it is easy to miss entirely on a routine inspection.

We see this pattern regularly on systems that experience recurring F.22 faults during or after colder weather. The valve may have been functioning correctly for some time and then begins to weep as it ages or after a period of sustained cold. If the heat pump has repeatedly lost pressure over winter months without any obvious internal leak being found, the anti-freeze valve should be one of the first things to check externally.

This is also one of the reasons that a recurring F.22 should not be attributed to a single cause without a proper inspection. The fault may appear to be a simple pressure issue, but the location of the water loss can require a more methodical investigation than a brief visual check of the indoor pipework.

What You Should Do If F.22 Keeps Returning

If you have topped up the system once and the F.22 has returned, it is time to investigate rather than simply represssurise again. The question to answer is not how to clear the code, but where the pressure is going and why.

Start by noting how quickly the pressure drops. A system that falls from normal pressure to F.22 within a day or two is losing water significantly faster than one that takes several weeks. The rate of loss gives a useful indication of the severity and sometimes the type of the underlying fault. A rapid overnight pressure drop, for example, often points to the expansion vessel or the PRV rather than a small weep at a joint.

Check the external discharge pipe for the pressure relief valve. If there is any sign of water around the outlet, or if the pipe shows staining or mineral deposits that suggest water has recently passed through it, the PRV should be examined by an engineer.

If there has been a period of cold weather around the time the faults started or intensified, ask your engineer to inspect the anti-freeze protection valve on the external pipework. This is a step that can be missed on a standard call-out, particularly if the engineer does not know it is installed or focuses only on the indoor components.

If no obvious leak is found during a visual inspection, a proper pressure test can be carried out to confirm whether the system is holding pressure or continuing to lose it. This will help determine whether a slow leak is present somewhere that cannot be seen.

A dirty system filter can also contribute to pressure-related problems in ways that are not always obvious. Debris accumulating in the strainer can cause flow restrictions that affect system behaviour and increase stress on seals and joints. Our article on how dirty filters can cause heat pump problems covers this in detail and explains why filter maintenance matters more than many homeowners realise.

If TRVs have been closed on a number of radiators around the property in an effort to save money, this can also affect how the system behaves under pressure. Our article on what happens if too many TRVs are turned off explains how restricting flow through the heating circuit can cause fault codes and performance problems that are sometimes mistakenly attributed to pressure loss.

If you have been through this process and are still unable to identify the cause, or if your installer has attended and not found the source of the pressure loss, an independent technical review may help. We work with homeowners across the UK who are experiencing exactly this kind of recurring fault, and in many cases we are able to identify the root cause from system documentation, photographs, and a structured review of the installation without requiring an engineer visit. You can find out more about how that process works.