F22 on a Vaillant boiler means the water pressure in your central heating system has dropped too low - typically below 0.5 bar - and the boiler has locked out to protect itself. In most cases, repressurising the system using the filling loop will clear the fault and restore normal operation. If the pressure drops again within 24-48 hours, there is an underlying problem - most commonly a hidden leak, a failing expansion vessel, or a faulty pressure relief valve - which requires a Gas Safe engineer to diagnose and repair.
What Does F22 Mean on a Vaillant Boiler?
The F22 fault code is a safety lockout triggered by Vaillant boilers across the ecoTEC, ecoTEC Plus, ecoTEC Pro and ecoFIT Pure ranges. It appears when the boiler's pressure sensor detects that the water pressure in the sealed heating system has fallen below the minimum threshold required for safe operation.
Sealed central heating systems - those without a header tank in the loft - rely on the correct water pressure to push hot water through pipework, radiators and internal boiler components. When the pressure drops too low, there is insufficient water to circulate effectively, and the boiler cannot operate without risking damage to the heat exchanger and pump. The F22 safety lockout prevents this from happening.
On most Vaillant models, F22 triggers when the system pressure falls to around 0.3-0.5 bar. At this point, heating and hot water both stop working until the pressure is restored and the fault is cleared.
Vaillant Boiler Pressure Guide
Too low. Below 0.5 bar triggers F22 lockout. Repressurise immediately.
Ideal when cold. Most Vaillant boilers operate best in this range.
Too high. PRV may activate to release excess pressure - do not ignore.
Five Causes of the Vaillant F22 Fault Code
Pressure does not drop without a reason. The key distinction is whether the drop is a one-off occurrence or a recurring pattern. A one-off drop after months of normal operation is usually nothing to worry about. Repeated pressure loss - particularly within hours or days of topping up - points to one of the following underlying issues.
1. A Leak in the Heating System
The most common cause of recurring F22 faults. Leaks are not always visible - a pinhole in a buried pipe, a weeping radiator valve or a slow seep from a joint beneath the floor can drain the system gradually over hours or days. If the pressure drops again within 48 hours of repressurising, a leak is the most likely cause. Leaks in concealed pipework require specialist detection equipment to locate without lifting floors.
Gas Safe engineer required2. Natural Pressure Loss Over Time
All sealed heating systems lose small amounts of pressure gradually through normal operation - microscopic amounts of dissolved gas leave the water and the system contracts and expands with temperature cycles. A modest drop every few months is entirely normal and does not indicate a fault. Topping up once or twice a year via the filling loop is routine maintenance.
DIY - repressurise yourself3. Failing Expansion Vessel
The expansion vessel is a sealed chamber containing a rubber membrane that accommodates the expansion and contraction of water as the heating system heats and cools. If the membrane fails or the vessel loses its internal air charge, the system cannot buffer pressure changes properly. Pressure rises too quickly when hot and drops sharply when cold, triggering repeated F22 faults. An engineer can recharge or replace the vessel.
Gas Safe engineer required4. Pressure Relief Valve Passing or Weeping
The pressure relief valve (PRV) is a safety device that opens to release excess water if the system pressure rises too high. If the PRV has previously opened - due to overfilling or a fault - the internal seal can fail to reseat fully, allowing a slow, continuous trickle of water to escape through the discharge pipe (usually terminating outside the property). This causes gradual, persistent pressure loss and a recurring F22 fault.
Gas Safe engineer required5. Faulty Water Pressure Sensor
If F22 appears but the pressure gauge on the boiler reads within the normal range (1.0-1.5 bar), the pressure sensor itself may be faulty. The sensor is sending an incorrect low-pressure signal to the boiler's control board, causing an unnecessary lockout. This is less common than an actual pressure issue but can occur as boiler components age. Always check the physical pressure gauge first before assuming the sensor is at fault.
Gas Safe engineer requiredHow to Fix the F22 Fault: Repressurising Your Vaillant Boiler
If the pressure gauge shows a reading below 1.0 bar, repressurising the system is the correct first step and something most homeowners can do safely. You will need to locate the filling loop - typically a small flexible braided hose with a valve at each end, usually located beneath the boiler or in an airing cupboard.
Switch Off the Boiler and Allow It to Cool
Turn the boiler off at the controls and wait at least 30 minutes. The system pressure will read slightly differently when hot - always repressurise a cold system for an accurate result.
Check the Pressure Gauge
Locate the pressure gauge on the front of your boiler (either a dial gauge or a digital display). Confirm the reading is below 1.0 bar. If it shows 1.0 bar or above but F22 is displayed, the pressure sensor may be at fault - call an engineer rather than attempting to repressurise further.
Locate and Attach the Filling Loop
Find the filling loop - a short flexible braided hose with a valve handle at each end, usually located beneath the boiler. On some Vaillant models it is an integral internal connection. Ensure both valves are closed before proceeding. Remove end caps if fitted.
Open the Filling Loop Valves Slowly
Open both valves gradually - you will hear water flowing into the system. Watch the pressure gauge rise. Increase the pressure slowly; do not rush. You are aiming for a reading of 1.2 bar when cold. It is easier to add more pressure than to release excess.
Stop at 1.2 Bar and Close the Valves
Once the gauge reaches 1.2 bar, close both valves fully. Do not overshoot above 1.5 bar. If air enters the system during filling, bleed any radiators that gurgle or feel cool at the top, then check and top up pressure again if needed.
Restart the Boiler and Reset the F22 Code
Remove the filling loop and replace any end caps. Switch the boiler back on. Most Vaillant models will clear the F22 code automatically once pressure is restored. If the fault remains on screen, press and hold the reset button (typically marked with a flame and cross symbol) for 3-5 seconds until the display resets.
Monitor the Pressure Over the Next 24-48 Hours
This is the critical step. Check the pressure gauge the following morning and again 48 hours later. If it has held steady, the repressurisation has resolved the issue. If the pressure has dropped again - even by a small amount - there is an underlying fault that needs professional diagnosis.
Pressure dropped again after topping up? A recurring F22 fault is the most common sign of a hidden heating leak. We connect you with specialist engineers across London.
Get HelpRecurring F22: What It Means When the Pressure Keeps Dropping
Topping up a boiler once every few months is normal. Topping it up every few days, every week, or finding that the pressure has halved again overnight is not. Recurring pressure loss is a diagnostic signal, not an inconvenience to manage by repeated repressurisation.
The most important point is this: repeatedly topping up a leaking system masks the underlying problem and allows the leak to continue causing damage. Water escaping from a central heating system does not simply disappear - it has to go somewhere. It may be saturating a timber subfloor, soaking into a wall cavity, corroding reinforced concrete or causing mould growth inside a partition wall, all while the system pressure gauge continues to be topped back up.
Call a Gas Safe Engineer If You Notice Any of the Following
- Pressure drops again within 24-48 hours of repressurising
- You need to top up the boiler more than once a month
- F22 returns immediately or very soon after resetting
- You can hear gurgling or trickling sounds in the pipework after topping up
- Radiators fill with air again quickly after bleeding
- Pressure rises unusually high when the heating is running
- Water is dripping from the boiler's external overflow pipe
- Damp patches, blistering paint or a musty smell near radiators or skirting boards
- F22 appears but the pressure gauge reads normal (1.0+ bar)
Hidden Heating Leaks and the F22 Fault: What to Do Next
If your Vaillant boiler keeps showing F22 and no leak is visible around the boiler, radiators or pipework, the leak is most likely concealed - within a buried pipe run under the floor, inside a wall cavity, beneath a screed bed or under a solid concrete slab. These leaks cannot be found by visual inspection alone.
Professional leak detection specialists use a combination of non-invasive methods to locate concealed heating leaks without lifting floors. Thermal imaging reveals the heat footprint of leaking water beneath floor surfaces. Tracer gas - an inert, safe mixture injected into the pipe under pressure - rises through concrete, screed and floor coverings and is detected at the surface with a sensitive probe. Acoustic detection listens for the sound signature of pressurised water escaping from a fault.
WaterLeakFinder connects London property owners with vetted specialist engineers who carry this equipment and have the experience to locate central heating leaks accurately - producing a written report suitable for Trace and Access insurance claims.
Preventing F22 Faults: How to Keep Your Vaillant Boiler Healthy
Most F22 faults are predictable and avoidable with straightforward maintenance habits. The following steps significantly reduce the likelihood of a pressure-related lockout.
Check Your Pressure Gauge Monthly
Build a quick pressure check into your monthly routine - it takes under 30 seconds. If the system is consistently holding between 1.0 and 1.5 bar without being topped up, the system is healthy. A slow downward trend over several months may indicate gradual pressure loss and is worth monitoring.
Have the Boiler Serviced Annually
An annual service by a Gas Safe registered engineer includes a full pressure test, expansion vessel charge check, pressure relief valve inspection and system flush assessment. Most F22-related component faults - including failing expansion vessels and weeping PRVs - are identified at a service before they cause a breakdown.
Do Not Ignore Small Leaks
A slow drip from a radiator valve, a weeping joint or a faint damp patch near the skirting board is not a minor cosmetic issue - it is a heating system leak. Small leaks grow. They cause structural damage to floors and walls, encourage mould growth and eventually trigger repeated F22 faults. Fix small leaks promptly before they become expensive problems.
Bleed Radiators Correctly
Bleeding radiators releases trapped air from the system, which is normal maintenance. However, bleeding always causes a small pressure drop. After bleeding radiators, always check the pressure gauge and top up to 1.2 bar if it has fallen below 1.0 bar. Failing to do so after bleeding is a common cause of an unnecessary F22 fault.
Do Not Overfill the System
Topping up past 1.5 bar when cold pushes the pressure too high when the system heats up. This causes the pressure relief valve to open and release water, and if the valve does not reseat cleanly, it will continue to drip - leading to gradual pressure loss and a recurring F22 fault. Aim for 1.2 bar when cold and stop there.