Protherm F.22 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.22 fault code mean?
The F.22 fault code on Protherm boilers signals that the water pressure in your sealed heating system has fallen too low — typically below 0.3 bar — for the boiler to operate safely. When this happens, the boiler locks out and shuts down to protect itself from running dry, which means you'll lose both central heating and hot water until the fault is cleared. You may also notice the pressure reading flashing on the display alongside the code. Protherm shares this fault code convention with other Vaillant Group boilers, so it carries the same meaning across that family of products. The fix is often straightforward — a simple top-up via the filling loop — but if pressure keeps dropping, there's usually an underlying problem that needs professional attention.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- System pressure too low — gradual loss over time Common
The most common trigger for F.22. Sealed heating systems naturally lose a small amount of pressure over months of use as microscopic amounts of water vapour escape. If the pressure has crept below 0.5–0.8 bar, the boiler may lock out. Topping up once or twice a year is perfectly normal, but more frequent drops point to a fault elsewhere.
- Air trapped in the heating system Common
Air pockets in radiators or pipework displace water volume, which can pull the system pressure down enough to trigger F.22 — particularly after a recent radiator bleed where the bleed screws were left slightly open, or after maintenance work that introduced air into the circuit.
- Water leak somewhere in the system Common
A dripping radiator valve, a weeping pipe joint, or a small leak at the boiler itself will cause pressure to fall steadily. Check for damp patches around radiators, under the boiler, and along visible pipework. Also inspect the pressure relief valve discharge pipe outside — if water is trickling out of it, the PRV may be releasing due to over-pressure cycling or the valve itself may be faulty.
- Faulty or waterlogged expansion vessel Sometimes
The expansion vessel absorbs pressure changes as water heats and cools. If its internal diaphragm has failed or the pre-charge pressure has dropped, the system pressure can become erratic — rising too high when hot and then falling below the F.22 threshold when the boiler cools. This is a common fault on boilers over five years old.
- Faulty water pressure sensor Sometimes
The pressure sensor (sometimes called a pressostat or pressure transducer) sends a reading to the PCB. If the sensor has drifted out of calibration or has developed an internal fault, it can report low pressure even when the actual system pressure is adequate. This is a component-level fault requiring an engineer to diagnose and replace.
- Circulation pump fault Sometimes
A seized or sluggish pump can cause localised pressure anomalies that the sensor interprets as low water. While a pump fault more commonly triggers its own dedicated code, it can occasionally accompany or contribute to an F.22 condition.
- Faulty PCB Rare
In rare cases where all other components check out, the PCB itself may be misreading or misprocessing the pressure sensor signal and generating a false F.22. This is a last-resort diagnosis made by an engineer after eliminating everything else.
How to fix it
- Check the pressure gauge reading DIY safe
Look at the pressure gauge on the front of the boiler (or on the pipework nearby). For a cold system — one that hasn't been running — the correct pressure is typically 1.0 to 1.5 bar. If the needle is below 0.5 bar, or the digital display is showing a very low figure, low pressure is almost certainly the cause of the F.22.
- Top up pressure using the filling loop DIY safe
Locate the filling loop — usually a braided silver hose with one or two valves beneath the boiler. Slowly open the valve(s) and watch the gauge rise. Aim for 1.2 to 1.5 bar, then close the valve(s) firmly. Never exceed 2.0 bar. Make sure both valves are fully closed before you walk away; a valve left even slightly open will allow the system to over-pressurise.
- Bleed your radiators if the system has been recently worked on or has cold spots DIY safe
If air has entered the system, use a radiator bleed key to open the bleed valve on each radiator (starting on the ground floor, working upward) until water dribbles out with no air hissing. Re-check and re-adjust the system pressure afterwards, as bleeding will cause a small pressure drop.
- Inspect for visible leaks around radiators, valves, and pipework DIY safe
Walk around your home and look for damp patches, rust staining, or wet spots beneath radiators and along accessible pipework. Check under the boiler casing for signs of dripping. Also look outside at the pressure relief valve discharge pipe — a steady drip from this pipe indicates either a PRV fault or that the system pressure is cycling too high. If you find an active leak, do not top up the system repeatedly; call an engineer.
- Reset the boiler DIY safe
Once pressure is restored to 1.0–1.5 bar, press and hold the reset button (usually marked with a flame or reset symbol) for a few seconds. The boiler should attempt to restart. If it fires up and runs normally, monitor the pressure over the next few days. If the F.22 returns within a short time, do not keep topping up — move to the next step.
- Call a Gas Safe registered engineer if the fault returns or a leak is suspected Gas Safe engineer
Recurring pressure loss, an unexplained drop that happened overnight, or any visible leak all require a professional investigation. A Gas Safe engineer will pressure-test the system to find hidden leaks, inspect and re-charge or replace the expansion vessel, test the pressure sensor, and check the pump and PCB. Repeatedly topping up a leaking system risks corrosion and long-term damage to heat exchanger components.
Parts you may need
- Water pressure sensor / pressostat · from £25
- Expansion vessel (internal replacement) · from £55
- Expansion vessel diaphragm / re-charge service · from £15
- Pressure relief valve (PRV) · from £20
- Filling loop assembly · from £18
- Circulator pump (Grundfos or equivalent) · from £85
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £80–£350, depending on the underlying cause.
Frequently asked questions
Can I just keep topping up the pressure to clear the Protherm F.22?
Topping up once or twice a year is normal for any sealed system, and doing it yourself is perfectly safe. However, if pressure keeps dropping — say, every few days or weeks — you should stop topping up and call an engineer instead. Repeated top-ups mask an underlying leak or expansion vessel fault, and continually introducing fresh oxygenated water accelerates internal corrosion. An engineer can find the root cause quickly and prevent more expensive damage later.
How much does it cost to fix a Protherm F.22 fault in the UK?
If you can top up the pressure yourself using the filling loop, there's no cost at all. If an engineer is needed — for example to trace and repair a leak, replace the expansion vessel, or fit a new pressure sensor — most people pay between £80 and £350 depending on the work involved. A straightforward call-out to repressurise and check the system typically runs £80–£130. Replacing an expansion vessel or pressure sensor usually falls in the £150–£300 range including labour. In rare cases where the PCB is faulty, costs can be higher, and at that point an engineer will advise whether repair or boiler replacement makes better financial sense.
Why does my Protherm boiler keep losing pressure and showing F.22?
Recurring pressure loss almost always means one of three things: a slow leak somewhere in the system (which may be tiny and hard to spot without a pressure test), a failed expansion vessel diaphragm that can no longer buffer pressure changes properly, or a faulty pressure relief valve that is weeping water to drain. A Gas Safe engineer can run a pressure-retention test to identify which of these is the culprit. Genuine gradual loss — needing a top-up every six months or so — can be normal, but anything more frequent than that warrants investigation.
Is the Protherm F.22 dangerous? Do I need to act immediately?
F.22 itself is not a gas or carbon monoxide safety hazard — it simply means the water pressure is too low. The boiler locks out as a precaution to protect its components from running dry. You can safely attempt a repressurise and reset as described above. That said, if you find an active water leak, address it promptly to prevent property damage and corrosion. If you are in any doubt, or the boiler won't restart after a top-up and reset, call a Gas Safe engineer rather than attempting further investigation yourself.