Key takeaways
An F22 or F75 on your Vaillant boiler means it has locked out and stopped working. Both codes look the same on the display (a flashing error, no heating, no hot water) but they have different causes, different fixes, and a different answer to the question of whether you need an engineer.
F22 is usually a pressure problem you can resolve yourself. F75 points to the pump or the pressure sensor and, unless low pressure is behind it, needs a Gas Safe registered engineer to diagnose and repair, so it often leads to a boiler repair rather than a simple reset.
If you are not sure which code applies to you, this table covers the key differences before you work through each one in detail.
| F22 | F75 | |
|---|---|---|
| What it means | System pressure too low | Pump or pressure sensor fault |
| Can you fix it yourself? | Often, with the filling loop | Rarely: pressure check and reset only |
| Common cause | Pressure loss, slow leak | Low pressure, air, faulty sensor or pump |
| Reset resolves it? | Yes, if pressure holds | Sometimes: monitor for 24 hours |
| Engineer needed? | If pressure drops again | If it returns after a top-up and reset |
| Target repair | System leak, expansion vessel | Sensor replacement, pump replacement |
F22 is Vaillant's fault code for low water pressure. It appears when the system pressure drops below approximately 0.5 bar, at which point the boiler locks out to prevent it from running without adequate water circulation. Vaillant names low pressure as one of the most common reasons behind boiler problems.
The three most common causes are natural pressure loss over time, a slow leak at a radiator valve or pipe joint, and air released when bleeding radiators. In the last case, a slight pressure drop after bleeding is normal and expected. If F22 appears repeatedly after repressurising, water is escaping somewhere in the system and needs finding before the problem escalates.
Many F22 faults resolve with a pressure top-up and a reset. The table below separates what is safe to do yourself from the signs that the fault needs professional attention.
| What you can do yourself | When you need an engineer |
|---|---|
| Check the pressure gauge | Pressure drops again soon after topping up |
| Repressurise using the filling loop to between 1 and 1.5 bar | Visible damp, rust, or limescale around the boiler or pipework |
| Press the reset button to restart the boiler | Relief valve dripping or discharging water outside |
| Monitor pressure for 24 hours after topping up | F22 returns repeatedly over days or weeks |
Check the pressure gauge on the front of the boiler. If it reads below 1 bar, repressurise using the filling loop, press the reset button, and monitor for 24 hours. If the pressure holds, the fault is resolved. If the gauge already reads above 1 bar and F22 is still showing, do not add water: that points to a fault such as the pressure sensor or its wiring, which is a job for an engineer.
If it drops again soon after, stop topping up. The system has a fault (a leak, a faulty pressure relief valve, or a failed expansion vessel) that will not resolve itself. Each top-up only masks the problem. At that point, boiler repair by a Gas Safe registered engineer is the right next step.
F22 usually comes down to one of three things. Identifying which applies to your system determines whether a top-up resolves it or whether something needs investigating.
The most common cause. Pressure drops gradually over time in any sealed central heating system. Bleeding radiators, small leaks, or natural pressure loss over months can bring the system below the threshold the boiler needs to operate.
If F22 appears repeatedly after repressurising, water is escaping somewhere. Leaks at radiator valves, pipe joints, or internal boiler components cause the pressure to drop back down within hours or days of being topped up.
Releasing air from radiators also releases a small amount of water, which lowers pressure. An F22 appearing shortly after bleeding is normal and expected. Check and repressurise after bleeding any radiators.
F75 is Vaillant's fault code for a pressure sensor or pump fault. It appears when the boiler's pump starts but the pressure sensor does not detect the expected change in water pressure. The boiler interprets this as either the pump not running correctly or the pressure sensor failing to register its output.
Unlike F22, F75 is not mainly a pressure problem, although low system pressure is one of the causes Vaillant names, so check the gauge before anything else.
Apart from low pressure, F75 has four common causes, all of which involve the pump failing to move water correctly or the sensor failing to confirm that it has. None of them is resolved by topping up the system.
If the sensor is defective, stuck, or fouled by sludge, it cannot register the pressure change accurately and triggers F75 even when the pump is working correctly.
If the pump has seized, is running too slowly, or has failed internally, it cannot generate the pressure change the sensor is looking for. The boiler detects no movement in the system and locks out.
After a system has been drained and refilled, air trapped in the pump prevents it from circulating water correctly. The pump spins but moves no water, and the pressure sensor registers no change.
In older systems, sludge from corroding pipes and radiators can build up and restrict the pump. It runs but cannot generate adequate flow, which can trigger F75 alongside wider circulation problems.
F75 has far less user scope than F22. A pressure check and a reset are the only safe interventions. Everything beyond that requires a Gas Safe registered engineer with the right diagnostic equipment.
| What you can do yourself | When you need an engineer |
|---|---|
| Check the pressure gauge; repressurise if below 1 bar | F75 returns after a reset |
| Press the reset button and monitor for 24 hours | Boiler locks out immediately on restart |
| Note any unusual pump noise before calling | Pump making unusual noise or not running |
| Check when the system was last serviced or flushed | System not flushed in several years |
Check the pressure gauge first, as Vaillant lists low system pressure as one cause of F75. If it reads below 1 bar, repressurise to between 1 and 1.5 bar and attempt a reset. If F75 clears and does not return, low pressure was a contributing factor. If it returns immediately, the fault is most likely in the pump or sensor.
If the reset does not clear the fault or the code returns within a short period, a Gas Safe engineer needs to diagnose whether the fault is the pressure sensor, the pump, or system sludge.
A pressure sensor on a Vaillant ecoTEC is an inexpensive part. A pump replacement costs more. A system powerflush addresses the root cause rather than just the failed component, and is worth considering on any older system where sludge is a likely contributor.
Beyond a pressure check and reset, F75 is not a DIY repair. The pump and pressure sensor sit inside the boiler casing, and opening that casing or repairing any part of a gas boiler, not just its gas supply, is a job for a Gas Safe registered engineer.
For landlords with residential properties, boiler maintenance is a legal obligation. The Gas Safety (Installation and Use) Regulations 1998 require annual gas safety checks by a Gas Safe registered engineer. A boiler displaying recurring fault codes that has not been properly investigated and repaired is a compliance issue as well as a practical one.
Tenants who report a boiler fault are reporting a loss of heating or hot water, which falls under the landlord's repair obligations. A boiler showing F75 that is left unrepaired, particularly in winter, carries both legal and operational consequences.
For commercial properties and portfolios, recurring boiler faults across multiple sites are most effectively managed through a planned maintenance programme that includes annual servicing and pressure monitoring.
F22 is a fault many Vaillant boiler owners can resolve themselves: check the pressure, repressurise, reset. If the pressure holds, the job is done. If it drops again, there is a fault in the system that needs professional investigation.
F75 is a different problem. A reset is worth trying, but if the code returns, the boiler needs an engineer. The pump or pressure sensor has most likely failed, and neither is a DIY repair.
Still showing F22 or F75? Get it diagnosed
If repressurising has not cleared the F22, or F75 keeps coming back, the fault needs a Gas Safe engineer on site. FS Group's heating engineers cover London and the South East.
F22 means the system water pressure has dropped below the boiler's safe operating threshold, typically below 0.5 bar. The boiler locks out to prevent running without adequate water circulation. Often it resolves with a repressurise and reset.
F75 means the boiler's pressure sensor has not detected the expected change in water pressure when the pump starts. Vaillant lists low system pressure, air in the boiler or heating system, a faulty pressure sensor and a faulty pump as causes, and system sludge restricting flow can also play a part.
Yes, if the cause is low system pressure. Check the pressure gauge, repressurise to between 1 and 1.5 bar using the filling loop, and press the reset button. If the F22 clears and the pressure holds for 24 hours, the fault is resolved. If the pressure drops again, there is an underlying leak or component fault that needs a Gas Safe engineer.
You can attempt a reset, and in some cases it will clear a transient fault caused by air in the pump or a momentary sensor glitch. If F75 returns with the pressure between 1 and 1.5 bar, the fault is most likely in the pump or pressure sensor and needs professional diagnosis.
Recurring F22 faults after repressurising mean water is leaving the system somewhere. The most common causes are a slow leak at a radiator valve or pipe joint, a faulty pressure relief valve releasing water, or a failed expansion vessel.
Turn the boiler off. Locate the filling loop, usually beneath the boiler; some models have a filling key or a built-in filling device instead, so check your boiler's manual. Open the valve slowly and watch the gauge rise to between 1 and 1.5 bar. Close the valve, then press the reset button to restart the boiler.
It depends on the diagnosis. A pressure sensor is an inexpensive part to replace. A pump costs more. If the cause is system sludge, a powerflush addresses the root problem but is a more involved job. A Gas Safe engineer can confirm the cause and provide a fixed quote before starting work.
Boiler losing pressure and you are in London or the South East?
FS Group's Gas Safe registered engineers carry out boiler pressure diagnostics and repairs across London and the South East, and aim to reach emergencies within two hours, subject to availability.
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