Saunier Duval F.20 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.20 fault code mean?
The F.20 code appears on most Saunier Duval boilers — including the Themaplus Condens, Thema Condens, Isomax, and ThemaClassic — when the water temperature inside the heat exchanger climbs past the manufacturer's safety limit of approximately 97°C. At that point, the boiler shuts itself down in a protective lockout to prevent component damage, dangerous pressure build-up, and any risk of scalding or explosion. The display will flash and alternate between two numbers (the second carrying a decimal point) to signal the fault type. Once the underlying cause has been addressed, pressing the Reset button restarts the boiler. Note: on the Condens range specifically, F.20 can occasionally point to a software incompatibility between components — often after an incorrect replacement part has been fitted — rather than a straightforward overheating event. Always cross-reference with your model's manual if the code reappears after a seemingly successful fix.
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
- Faulty or seized circulation pump Common
If the pump has seized, is blocked with debris, or has simply failed, hot water cannot circulate around the heating circuit. Heat builds up in the heat exchanger very quickly, pushing the temperature past the 97°C threshold and triggering the lockout. This is the single most common culprit behind a persistent F.20.
- Limescale or sludge blocking the heat exchanger Common
Years of hard water use can leave limescale deposits inside the heat exchanger, while system sludge can narrow pipework and radiator flow passages. Both reduce heat transfer efficiency and cause localised overheating even when pressure and pump operation appear normal. Condensing models such as the Themaplus Condens are particularly sensitive because the secondary heat exchanger runs at lower temperatures and furs up more readily.
- Low or unstable system pressure Common
When system pressure drops below around 0.8 bar — due to a slow leak, a waterlogged expansion vessel, or simply natural pressure loss — there is insufficient water volume circulating. The reduced flow allows the heat exchanger to overheat rapidly, triggering F.20.
- Faulty NTC temperature sensor Sometimes
The NTC (negative temperature coefficient) probe monitors water temperature and reports back to the control board. If the sensor has drifted out of calibration or developed an intermittent fault, it can feed inaccurate temperature readings to the PCB, causing the board to believe the boiler is overheating even when it is not. This often shows up as an F.20 that clears on reset but returns within a day or two.
- Air trapped in the system Sometimes
Air pockets in radiators or the pipework prevent water from circulating freely, creating localised hotspots. While bleeding radiators is a straightforward homeowner task, persistent air ingress usually points to a deeper problem such as a leak or a failed auto air vent.
- Faulty expansion vessel Sometimes
The expansion vessel absorbs the increase in water volume as the system heats up. If its internal diaphragm has split or the pre-charge pressure is wrong, system pressure can spike sharply during a heating cycle, contributing to overheating and F.20 lockouts.
- PCB communication fault (ThemaFast) Rare
On the ThemaFast specifically, F.20 can appear intermittently — the boiler resets successfully but the code returns days later with no obvious hydraulic cause. In these cases, the fault often originates from a communication breakdown between the two PCB boards rather than a genuine overheating event. Software incompatibility after fitting incorrect replacement parts can produce the same pattern on the Condens range.
- Restricted ventilation or blocked air ducts Rare
Poor ventilation around the boiler or a partial blockage in the air intake or flue can cause heat to accumulate inside the casing, raising internal temperatures and contributing to an F.20. This is less common but worth checking if the boiler is in a confined space.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when the system is cold. A reading below 0.8 bar means there is insufficient water in the circuit, which can cause overheating.
- Top up system pressure via the filling loop if needed DIY safe
Locate the filling loop — usually a short braided hose or a small tap beneath the boiler. Slowly open the valve(s) until the gauge reads approximately 1.2 bar, then close the valve fully. Never exceed 1.5 bar. If pressure drops again within a few days, there is likely a leak somewhere in the system.
- Bleed the radiators to remove trapped air DIY safe
Starting with the radiator furthest from the boiler, use a bleed key to open the bleed valve at the top of each radiator until water (not air) starts to trickle out, then close it. Work back towards the boiler. Re-check the pressure gauge afterwards and top up if it has fallen below 1.0 bar.
- Reset the boiler DIY safe
Once you have corrected the pressure and bled the radiators, press the Reset button and allow the boiler to restart. Observe it through at least one full heating cycle. If F.20 returns immediately or within a short time, do not keep resetting — repeated resets on an overheating fault can cause further component damage and should be avoided.
- Check whether the circulation pump is running DIY safe
With the boiler running, place your hand carefully on the pipework near the pump — you should feel a slight vibration and the pipe should warm up as water flows. If the pump appears silent and the pipes stay cold, report this to your engineer. Do not attempt to open the pump or tamper with electrical connections yourself.
- Inspect for visible water leaks around the boiler DIY safe
Look underneath and around the boiler casing for damp patches, drips, or white limescale staining that would indicate a slow leak. If you find one, turn off the boiler and contact a Gas Safe registered engineer — do not attempt to seal pipework joints or replace seals yourself.
- Call a Gas Safe registered engineer if F.20 returns or the cause is unclear Gas Safe engineer
If the fault code reappears after your basic checks, or you could not identify an obvious cause, a Gas Safe registered engineer should carry out a full diagnostic. They can test the NTC temperature sensor with appropriate equipment, inspect and service or replace the circulation pump, perform a chemical flush or power flush to clear sludge and limescale from the heat exchanger and pipework, check and re-pressurise the expansion vessel diaphragm, and — on ThemaFast or Condens models — investigate any PCB communication issues. Only a Gas Safe registered engineer is legally permitted to carry out gas boiler component work in the UK.
Parts you may need
- Circulation pump (circulator) · from £85
- NTC temperature sensor (flow or return) · from £25
- Expansion vessel · from £45
- Expansion vessel diaphragm/bladder · from £15
- Pressure relief valve · from £20
- PCB (main control board) · from £180
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 £120–£400, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler when F.20 appears?
You can try one or two resets after correcting system pressure or bleeding radiators, but you should not repeatedly reset a boiler that keeps locking out on an overheating code. F.20 is a safety lockout designed to protect the heat exchanger and other components from damage caused by excessively high temperatures. Continually overriding it without fixing the root cause risks damaging the NTC sensor, the PCB, or the heat exchanger itself — repairs that can easily cost several hundred pounds. If the code returns within 48 hours of a reset, call a Gas Safe engineer.
How much does it typically cost to fix an F.20 fault in the UK?
Most homeowners pay somewhere between £120 and £400 to resolve an F.20 fault. A straightforward pump service or NTC sensor swap at the lower end, and a circulation pump replacement or expansion vessel job at the higher end, usually fall within that range. A power flush to clear sludge from the system typically costs £350–£500 on top of any component work. If the heat exchanger itself is found to be heavily scaled and needs replacing, costs can reach £600–£800 or more — at which point, on an older boiler, a new boiler installation may be worth considering.
Why does F.20 keep coming back a few days after I reset it?
An F.20 that clears on reset but reappears within a day or two — especially on the ThemaFast — often points to a faulty NTC temperature sensor giving intermittent false readings, or a communication fault between PCB boards rather than a genuine overheating event. On the Condens range, fitting an incompatible replacement part can also trigger this pattern. A Gas Safe engineer can test the sensor resistance with a multimeter and read live temperature data to confirm whether the boiler is truly overheating or whether the sensor is lying to the control board.
Could a water leak cause my Saunier Duval to show F.20?
Yes. A slow leak anywhere in the heating circuit — even a weeping joint behind a wall — gradually reduces system pressure. Once pressure falls low enough, there is insufficient water flowing through the heat exchanger and it overheats, triggering F.20. If your pressure gauge keeps dropping back below 1.0 bar despite regular top-ups, treat this as a strong indicator of a leak and have a Gas Safe engineer trace and repair it before the low pressure causes further damage.