Vaillant F22 Fault Code: Low Water Pressure — 5 Common Causes & How to Fix It

What does the Vaillant F22 fault code mean?

The Vaillant F22 fault code indicates that your boiler's water pressure has dropped below the safe operating threshold — typically below 0.5 bar. When the pressure sensor detects insufficient water pressure, the boiler locks out as a safety measure to prevent damage to the pump and heat exchanger. You'll lose both heating and hot water until the fault is resolved.

F22 is one of the most common Vaillant fault codes we see across London, particularly on the ecoTEC range installed in thousands of properties from Marylebone to Hampstead. While it can be alarming when your boiler suddenly stops working, F22 is often straightforward to resolve — though the underlying cause needs proper diagnosis to prevent it recurring.

The 5 most common causes of the Vaillant F22 fault

1. Natural pressure loss over time

The most frequent cause of F22 is gradual pressure loss in your central heating system. Just as a car tyre slowly loses air over months of use, heating systems naturally lose a small amount of pressure through microscopic leaks, temperature-driven expansion and contraction, and trapped air pockets. This is entirely normal and doesn't necessarily indicate a fault — but if left unchecked, the pressure will eventually drop low enough to trigger F22.

If your boiler pressure has been slowly declining over weeks or months and this is the first time F22 has appeared, simple re-pressurisation via the filling loop will likely resolve it. However, if the pressure drops again within days or weeks, there's an underlying issue that needs investigation.

2. A leak in the heating system

A more rapid loss of pressure usually indicates a leak somewhere in your central heating system. Leaks can occur at radiator valves, pipe joints, under floorboards, or — less commonly — inside the boiler itself. Even a tiny weeping joint can cause significant pressure loss over time, and leaks in hidden areas (under floors, behind walls, in ceiling voids) are notoriously difficult to spot without specialist equipment.

Signs of a system leak include:

• Pressure dropping repeatedly after re-pressurising
• Damp patches on walls, ceilings or floors
• Visible water staining near radiator valves or pipework
• A smell of damp or mould in areas where pipes run
• Higher-than-usual water bills if the filling loop is letting mains water into the system continuously

If you suspect a leak, our non-invasive leak detection service uses thermal imaging and acoustic equipment to pinpoint the source without damaging your property.

3. Faulty pressure relief valve (PRV)

The pressure relief valve is a critical safety component that releases excess pressure if it builds to dangerous levels. However, if the PRV becomes faulty — through wear, scale build-up or debris — it can release pressure unnecessarily, causing a continuous drop that triggers F22.

Signs of a faulty PRV include:

• Water dripping from the overflow pipe outside your home
• Needing to re-pressurise your system every few days
• Visible corrosion or mineral build-up on the valve
• Pressure rising above 3 bar when the heating is on, then dropping rapidly when it cools

PRV replacement is a straightforward job for a Gas Safe registered engineer and typically costs £120–£200 including parts and labour.

4. Expansion vessel failure

The expansion vessel accommodates the expansion of water as it heats up, preventing excessive pressure build-up. If the vessel loses its air charge, becomes waterlogged, or ruptures, the system pressure will fluctuate dramatically — rising too high when the heating is on and dropping too low when it cools, eventually triggering F22.

Common expansion vessel problems include:

• Loss of air pressure within the vessel (recharge needed)
• Perished or ruptured diaphragm (replacement needed)
• Waterlogging — water has entered the air space

Our engineers test the expansion vessel during every boiler service and can recharge or replace it before it causes an F22 fault.

5. Faulty pressure sensor

In some cases, the F22 fault code is triggered not by actual low pressure but by a faulty pressure sensor. The sensor monitors water pressure and sends signals to the boiler's control board — if it fails, it may report low pressure even when the system is correctly pressurised. This is less common than the other causes but does occur, particularly on older ecoTEC models or systems with sludge contamination that has damaged the sensor membrane.

Signs of a faulty pressure sensor include:

• F22 appearing even when the pressure gauge reads between 1.0 and 1.5 bar
• Inconsistent pressure readings between the gauge and the digital display
• F22 recurring shortly after sensor replacement if sludge is the root cause

How to fix the Vaillant F22 fault code

Step 1: Check the pressure gauge

Look at the pressure gauge on the front of your Vaillant boiler. If it reads below 0.5 bar, F22 is being triggered by genuine low pressure. If it reads between 1.0 and 1.5 bar but F22 is still showing, the pressure sensor may be faulty — call an engineer.

Step 2: Re-pressurise the system

If the pressure is genuinely low, you can re-pressurise using the filling loop:

1. Turn off the boiler and allow it to cool for 20 minutes
2. Locate the filling loop — a flexible silver hose beneath or beside the boiler
3. Open both isolation valves slowly to allow mains water into the system
4. Watch the pressure gauge — stop when it reaches between 1.0 and 1.5 bar
5. Close both isolation valves and disconnect the filling loop if it's removable
6. Reset the boiler — the F22 code should clear

Step 3: Monitor for recurrence

If the pressure holds for weeks or months, natural pressure loss was the cause and no further action is needed. If F22 returns within days, you have a leak, a faulty PRV, or an expansion vessel issue that requires professional diagnosis.

Do not repeatedly re-pressurise without investigating the cause — this wastes water and masks an underlying problem that will worsen over time.

When to call a Gas Safe engineer

Call RARE Plumbing immediately if:

• F22 returns within days of re-pressurising — you have a leak or component fault
• The filling loop won't hold pressure or is damaged
• The pressure gauge reads normal but F22 persists — sensor fault
• You see water dripping from the outside overflow pipe — PRV fault
• The pressure rises above 3 bar when heating is on — expansion vessel fault

Our Vaillant specialists carry pressure sensors, PRVs and expansion vessels on every van, so most F22 faults are fixed on the first visit.

How to prevent F22 in future

• Annual boiler servicing — we check the expansion vessel, PRV and pressure sensor every year
• Check your pressure gauge monthly — catch gradual loss before it triggers F22
• Bleed radiators regularly to remove trapped air that affects pressure
• Install a magnetic filter to catch sludge that can damage the pressure sensor
• If your boiler is over 12 years old and F22 is recurring, consider a boiler replacement — modern Vaillant ecoTEC models have improved pressure management

Related Vaillant fault codes

F22 is often confused with two other common Vaillant pressure-related fault codes:

• F20 — Also low water pressure, but triggered by a different sensor threshold. The diagnosis and fixes are similar.
• F75 — No pressure change detected on pump start. Related to the pump rather than static pressure, but can be caused by the same sludge and leak issues.

For a full list of Vaillant and other brand fault codes, use our Boiler Error Codes lookup tool.

Book a Vaillant engineer

If you're unable to re-pressurise your boiler, or if pressure keeps dropping, RARE Plumbing's Gas Safe registered engineers are available 24/7 across London. We'll diagnose the root cause — not just top up the pressure — and fix it the same day in most cases. Call 020 7458 4949 or book a Vaillant specialist online.