Heatline F22 Fault Code: Low Water Pressure
The F22 code appears when your Heatline boiler detects that the water pressure inside the sealed heating system has fallen too low for safe operation — typically below 0.5 bar. Without sufficient pressure, the boiler cannot circulate water properly through the heat exchanger and radiators, so it locks out to protect itself from damage. While locked out, you will have no central heating and no hot water until the underlying cause is resolved and the boiler is reset.
General guidance only — not a substitute for professional advice. 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.
What does the Heatline F22 fault code mean?
The F22 code appears when your Heatline boiler detects that the water pressure inside the sealed heating system has fallen too low for safe operation — typically below 0.5 bar. Without sufficient pressure, the boiler cannot circulate water properly through the heat exchanger and radiators, so it locks out to protect itself from damage. While locked out, you will have no central heating and no hot water until the underlying cause is resolved and the boiler is reset.
Common causes
- System pressure has dropped gradually over time Common
All sealed central heating systems lose a small amount of pressure naturally as the years pass. If the boiler has never been repressurised, the pressure can eventually fall below the 0.5 bar threshold that triggers F22. This is by far the most common reason for this fault code and is straightforward to resolve by topping up via the filling loop.
- Water leak from pipework, radiators, or valves Common
Even a slow drip from a radiator valve, a push-fit joint, or a section of buried pipework can drain enough water to cause repeated F22 lockouts. Signs to look for include damp patches on floors or ceilings, corrosion around radiator valves, and pressure that keeps dropping shortly after you top the system back up.
- Radiators bled without the system being refilled Common
Bleeding a radiator releases trapped air, but it also reduces the volume of water in the system and therefore lowers the pressure. If you recently bled your radiators and did not re-pressurise the boiler afterwards, this is the most likely cause of your F22 code.
- Faulty or weeping pressure relief valve (PRV) Sometimes
The PRV is a safety device that opens and releases water if the system pressure climbs dangerously high. If the valve seat has deteriorated, it may weep water continuously even at normal pressure levels, causing a steady pressure drop. You may notice a small amount of water discharging from the copper overflow pipe outside the property.
- Failed expansion vessel or low pre-charge pressure Sometimes
The expansion vessel absorbs the rise in water volume when the heating warms up. If the vessel's internal diaphragm has split or its air charge has been lost, pressure can swing dramatically between cold and hot operation, eventually causing the PRV to open or the boiler to display F22. This fault tends to produce pressure readings that look fine when cold but spike or drop when the boiler fires.
- Faulty pressure sensor Rare
The pressure sensor reports system pressure to the boiler's control board. If the sensor drifts out of calibration or develops a fault, the boiler may show F22 even though the actual pressure in the system is within the correct range. An engineer can verify this by cross-checking the sensor reading against an independent gauge.
- Faulty circulating pump or wiring Rare
A pump that has seized or a damaged pump wiring harness can, in some cases, contribute to abnormal pressure behaviour. This is an uncommon cause of F22 but may be flagged during an engineer's diagnostic if the more obvious causes have been ruled out.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Look at the pressure gauge on the front of the boiler (or a remote display if fitted). On a cold system, a reading comfortably below 1 bar — particularly anything approaching 0.5 bar or lower — confirms that low pressure is the reason for the F22 lockout. Make a note of the reading before you do anything else.
- Top up the system pressure using the filling loop DIY safe
Locate the filling loop — usually a short flexible braided hose with two inline valves, found beneath the boiler. With both valves initially closed, slowly open each one in turn to allow mains water into the system. Watch the pressure gauge closely and stop filling when it reaches approximately 1.2 to 1.5 bar. Close both valves firmly and make sure neither is left partially open, as this can cause over-pressurisation once the system heats up.
- Bleed any air-locked radiators if necessary DIY safe
If you suspect air is trapped in the radiators (they feel cold at the top but warm at the bottom, or you hear gurgling), use a radiator bleed key to release it. Hold a cloth under the bleed valve, open it slowly, and close it again as soon as water — rather than air — starts to emerge. Check the pressure gauge again afterwards and top up slightly if the pressure has dropped.
- Reset the boiler to clear the F22 code DIY safe
Once the pressure is back in the 1.2–1.5 bar range, reset the boiler by pressing and holding the reset button (consult your boiler's manual for the exact location on your model). Do not reset the boiler more than two or three times in quick succession. If it locks out again immediately after firing, stop resetting and call an engineer.
- Monitor the pressure over the following days DIY safe
Keep an eye on the pressure gauge over the next week or so. A system that holds pressure consistently is likely to have simply needed a top-up. If the pressure falls again within a few days, there is almost certainly a leak or a faulty component somewhere in the system that needs professional investigation.
- Arrange a visit from a Gas Safe registered engineer if the fault returns Gas Safe engineer
If the F22 code reappears after repressurising, or if you cannot find an obvious reason for the pressure loss, contact a Gas Safe registered engineer. They can pressure-test the system to locate hidden leaks, inspect the PRV and expansion vessel, check and replace the pressure sensor if required, and carry out any repairs that are beyond the scope of a homeowner. Do not attempt to dismantle gas components, valves, or internal boiler parts yourself.
Parts you may need
- System pressure sensor · from £35
- Pressure relief valve (PRV) · from £25
- Expansion vessel (replacement) · from £55
- Filling loop assembly · from £20
- Circulating pump · 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 fix an F22 fault myself, or do I need a Gas Safe engineer?
If the boiler has simply lost pressure gradually over time, you can often resolve the F22 fault yourself by topping up the system via the filling loop and resetting the boiler — no engineer needed. However, if the pressure drops again within a few days, or if you cannot identify how the pressure was lost, a Gas Safe registered engineer should investigate. Repairs involving the pressure relief valve, expansion vessel, pressure sensor, or any pipework joint will need a professional.
How often should I need to top up my Heatline boiler's pressure?
A well-maintained sealed system should hold pressure for many months between top-ups. If you find yourself repressurising more than two or three times a year, that is a sign the system is losing water somewhere and the root cause should be investigated. Frequent top-ups mask an underlying leak and can gradually introduce corrosion into the system if oxygen-rich mains water keeps being added.
Could the F22 code appear even if there is no actual leak?
Yes. A faulty pressure sensor can report low pressure to the boiler's control board even when the system is correctly pressurised. Similarly, a failed expansion vessel diaphragm can cause pressure to spike and then dump water through the PRV, leading to a drop that looks like a leak but is actually a component failure. An engineer can distinguish between these scenarios using a pressure gauge and a physical inspection of the expansion vessel.
How much does it typically cost to fix an F22 fault in the UK?
If you can resolve it yourself by topping up the pressure, the cost is nothing. An engineer call-out to repressurise and reset the boiler usually costs £60–£100. Repairing a minor accessible leak is typically £80–£200, replacing a pressure relief valve or expansion vessel usually falls in the £120–£300 range including parts and labour, and more extensive leak detection or pipework repairs can reach £300–£350. In rare cases where an internal component such as the pump or PCB is at fault, costs can be higher — your engineer will advise once the diagnosis is confirmed.