Saunier Duval F22 Fault Code: Low System Water Pressure
The F22 code appears when your Saunier Duval boiler detects that the water pressure in the heating circuit has dropped too low — typically below 0.8 bar. Without sufficient pressure, the boiler cannot circulate water safely around the system, so it shuts itself down as a precaution. The result is a loss of both heating and hot water until the issue is resolved. Note: on certain Saunier Duval Condens models, F22 can instead indicate excessively high pressure (above 2.7 bar). If topping up pressure makes no sense given what you see on the gauge, check your specific model's documentation or call an engineer.
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 Saunier Duval F22 fault code mean?
The F22 code appears when your Saunier Duval boiler detects that the water pressure in the heating circuit has dropped too low — typically below 0.8 bar. Without sufficient pressure, the boiler cannot circulate water safely around the system, so it shuts itself down as a precaution. The result is a loss of both heating and hot water until the issue is resolved. Note: on certain Saunier Duval Condens models, F22 can instead indicate excessively high pressure (above 2.7 bar). If topping up pressure makes no sense given what you see on the gauge, check your specific model's documentation or call an engineer.
Common causes
- Low system pressure (no underlying fault) Common
The most straightforward cause. Heating systems lose small amounts of water over time through normal operation, especially after bleeding radiators. If pressure has simply drifted below 0.8 bar without any leak or component failure, repressurising via the filling loop is all that is needed.
- Water leak somewhere in the system Common
A slow drip from a radiator valve, a pipe joint, or the boiler itself can gradually reduce system pressure until F22 triggers. Check visible pipework, radiator valves, and underneath the boiler for damp patches, staining, or corrosion. If the boiler needs topping up repeatedly over days or weeks, a leak is the most likely culprit.
- Faulty or under-pressurised expansion vessel Common
The expansion vessel contains an air-filled membrane that absorbs the expansion and contraction of water as the system heats and cools. If the membrane has split or the vessel has lost its charge, pressure in the system can swing wildly or drop below the minimum threshold, triggering F22 — particularly if the fault reappears shortly after repressurisation.
- Pressure relief valve weeping Sometimes
The pressure relief valve (PRV) is a safety device that releases water if pressure climbs too high. Over time, PRVs can develop a slight weep even at normal pressures, slowly draining the system. A telltale sign is a discharge pipe outside the property that is damp or dripping when the boiler is not in overpressure.
- Faulty pressure sensor (pressostat) Sometimes
The pressure sensor monitors system pressure and signals the boiler's PCB. If it develops a fault or gives inaccurate readings, the boiler may display F22 even when the actual pressure is within a normal range. This is worth considering if pressure looks correct on the gauge but the code persists.
- Filling loop valve failure Rare
The filling loop connects the mains cold water supply to the heating circuit for repressurisation. If the valve inside it begins to weep, it can either allow slow ingress (causing overpressure on some Condens variants) or fail to hold a seal properly, making it difficult to repressurise the system effectively.
How to fix it
- Check the pressure gauge on your boiler DIY safe
Most Saunier Duval boilers have a built-in pressure gauge — either a dial or a digital display. A healthy system should read between 1.0 and 1.5 bar when cold. If it reads below 1 bar, low pressure is almost certainly the cause of F22.
- Top up system pressure using the filling loop DIY safe
Switch the boiler off and let it cool for around 20 minutes. Locate the filling loop — usually a short flexible silver or braided hose beneath the boiler with a valve or tap at each end. Slowly open both valves to allow mains water into the system, watching the pressure gauge as you go. Stop when the needle reaches 1.2–1.5 bar, then close both valves firmly. Never overfill beyond 1.5 bar.
- Reset the boiler and observe DIY safe
Press the reset button (consult your boiler's quick-start guide if you are unsure of its location) and allow the boiler to restart. Check that the F22 code has cleared and that heating and hot water have resumed. Monitor the pressure gauge over the next day or two.
- Check for visible leaks around radiators and pipework DIY safe
Walk around the property and inspect radiator valves, pipe connections at the boiler, and any visible pipework for signs of moisture, staining, or corrosion. Check whether the discharge pipe (the plastic pipe that exits through an outside wall, often near the boiler flue) is dripping — this can indicate a weeping pressure relief valve.
- Do not attempt to repressurise more than 2–3 times without investigation DIY safe
If pressure drops again within a few days of topping up, there is an underlying fault — a leak, a failed expansion vessel, or a faulty PRV — that needs professional attention. Continuing to top up without addressing the root cause risks masking a problem that could become more serious or costly.
- Call a Gas Safe registered engineer Gas Safe engineer
If the F22 code returns repeatedly, if you cannot identify or reach the filling loop, if the pressure gauge reads normal but the fault persists, or if you suspect a leak, an engineer is required. A Gas Safe registered engineer can pressure-test the system for leaks, inspect and recharge or replace the expansion vessel, test the pressure sensor, examine the pressure relief valve, and carry out any repairs safely and to the correct standard.
Parts you may need
- Expansion vessel (replacement) · from £60
- Expansion vessel valve/Schrader valve · from £5
- Pressure sensor (pressostat) · from £25
- Pressure relief valve · from £20
- Filling loop assembly · from £18
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 £100–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Saunier Duval F22 myself?
In many cases, yes — if the sole cause is pressure that has simply dropped below the minimum threshold, you can repressurise the system yourself via the filling loop at no cost. This takes around five minutes and is a routine homeowner task. However, if the code keeps returning after topping up, the underlying cause (a leak, a failed expansion vessel, or a faulty pressure sensor) requires a Gas Safe registered engineer.
Why does my Saunier Duval boiler keep losing pressure and showing F22?
Repeated pressure loss almost always points to one of three things: a slow leak somewhere in the system (radiator valves, pipe joints, or inside the boiler itself), a weeping pressure relief valve, or a faulty expansion vessel whose membrane can no longer absorb water volume changes. An engineer can pressure-test the system to pinpoint the cause. Topping up repeatedly without fixing the root problem is not a long-term solution.
How much does it cost to fix F22 on a Saunier Duval boiler?
If you can repressurise the system yourself, the cost is nothing. If an engineer is needed, most F22 repairs fall in the range of £100–£350. A straightforward call-out to repressurise and inspect typically costs £80–£150. Expansion vessel recharging or replacement usually runs £120–£250 depending on parts and labour. Pressure sensor replacement is generally £80–£150 including the part. In rare cases where a complex leak requires draining the system, costs can exceed £350, and major component failures such as PCB replacement can cost significantly more — but these are uncommon causes of F22.
My Saunier Duval Condens is showing F22 but the pressure looks high — is that normal?
On certain Saunier Duval Condens models, F22 is defined differently and can indicate that pressure has risen above 2.7 bar rather than fallen below the minimum. If your gauge shows high pressure and you have a Condens model, do not attempt to repressurise — the system is already overfull. Switch the boiler off and call a Gas Safe registered engineer. This variant of F22 can point to a faulty expansion vessel, overfilling, or a stuck filling loop valve.