Vaillant F.20 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.20 fault code mean?
The F.20 code appears when your Vaillant boiler detects that the water temperature at the flow or return NTC thermistor has climbed beyond the safe upper limit — typically around 95–97°C. At that point the boiler's safety logic triggers a hard lockout, cutting the burner to protect internal components from heat damage. The fault can be caused by a genuine overheating event (for example, poor water circulation or a scaled-up heat exchanger) or by a faulty temperature sensor that is reporting an inflated reading to the control board. Either way, the boiler will not restart automatically — you must address the underlying cause before resetting. Until it is fixed you will have no central heating or hot water.
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.
Common causes
- Low system pressure Common
When the system pressure drops below roughly 1 bar, there is insufficient water volume circulating through the heat exchanger. The burner keeps firing but the reduced flow cannot carry heat away quickly enough, so water temperature spikes and the safety cut-out activates. Check the boiler's pressure gauge — if it reads below 1 bar when the system is cold, low pressure is the most likely culprit.
- Circulation pump failure or restriction Common
The pump is responsible for moving heated water around the whole system. If it has seized, is running too slowly due to wear, or is clogged with sludge, water movement through the heat exchanger slows dramatically. Heat builds up rapidly and the boiler locks out on F.20. A banging or boiling sound when the boiler fires is a tell-tale sign of pump-related overheating.
- Limescale or sludge in the heat exchanger Common
In hard-water areas or older systems without a magnetic filter, scale and iron oxide sludge accumulate inside the heat exchanger. These deposits act as insulation, creating localised hot spots and causing the familiar kettling noise. The boiler overheats well before it would in a clean system, making F.20 lockouts frequent and recurring until the system is properly flushed.
- Faulty NTC thermistor (temperature sensor) Sometimes
Each NTC thermistor is a small resistive sensor that sends a temperature reading to the PCB. If the sensor's resistance drifts or it develops an internal fault, it can report a falsely high temperature even when the boiler is operating within normal bounds. This triggers the F.20 lockout despite there being no actual overheating. An engineer can confirm this by reading the live sensor values and checking resistance against the manufacturer's specification.
- Trapped air in the system Sometimes
Large air pockets within the heat exchanger or pipework prevent water from making proper contact with the heated surfaces. The portion of water that does circulate heats up very quickly, pushing temperatures past the safety threshold. Bleeding radiators and checking the auto air vent can resolve this, though if the system repeatedly accumulates air there is usually an underlying cause such as a leaking joint or a failing expansion vessel.
- Blocked or restricted flue Sometimes
If the flue outlet is partially obstructed — by a bird's nest, debris, or a poorly installed terminal — combustion gases cannot escape efficiently. This can cause incomplete combustion and elevated heat levels inside the boiler, eventually triggering an overheat lockout alongside F.20.
- Fan fault Rare
The flue fan removes combustion gases and also influences the overall thermal management of the boiler. A fan running below its target speed or failing intermittently can cause heat to build up, contributing to overheating conditions that trigger F.20.
- Expansion vessel or pressure relief valve issue Rare
A waterlogged expansion vessel or a pressure relief valve that is not seating correctly can cause the system pressure to creep up under heat. Excessive pressure restricts flow and can contribute to localised overheating, particularly in sealed systems. This is a less common root cause of F.20 but worth checking if pressure is consistently too high rather than too low.
How to fix it
- Switch the boiler off and let it cool for at least 10–15 minutes DIY safe
Do not attempt a reset immediately. Allowing the boiler to cool gives the thermistors time to return to an accurate baseline reading and reduces the risk of compounding any overheating damage. Use the boiler's power switch or isolate it at the fused spur.
- Check system pressure on the boiler's pressure gauge DIY safe
With the boiler cold, the needle should sit between 1 and 1.5 bar. If it is below 1 bar, locate the filling loop — usually a short flexible braided hose with one or two valves beneath the boiler. Open both valves slowly and watch the gauge rise. Stop at around 1.2–1.3 bar, then close both valves firmly. Never overfill beyond 1.5 bar when cold.
- Bleed the radiators to remove trapped air DIY safe
Starting at the highest radiator in the house, use a radiator bleed key to open the bleed valve a quarter-turn. Hold a cloth beneath it. When water — not air — begins to trickle out, close the valve. Work your way down through all radiators. Check the pressure gauge again afterwards and top up if it has dropped below 1 bar.
- Check that all radiator valves and lockshield valves are open DIY safe
A partially or fully closed valve anywhere on the system can restrict circulation enough to cause overheating. Walk around the property and confirm that thermostatic radiator valves (TRVs) are turned up and that lockshield valves on both sides of each radiator are open. Pay particular attention to any radiator that was recently worked on.
- Reset the boiler once DIY safe
Press and hold the reset button (on most Vaillant ecoTEC models this is the flame symbol button) for around three seconds until the display changes. Allow the boiler to go through its ignition sequence. If it runs normally and the F.20 does not return within the first few heating cycles, low pressure or trapped air was likely the cause. If F.20 returns promptly, do not keep resetting — repeated lockouts without fixing the root cause can damage the heat exchanger and may affect your warranty.
- Listen for the circulation pump when the boiler fires DIY safe
Stand next to the boiler as it starts up. You should hear a faint hum from the pump within a few seconds of the burner lighting. Silence, grinding, or an absence of water movement through the pipework suggests the pump may be seized or failing. Switch the boiler off and report this to an engineer — do not keep running the boiler with a suspected pump fault.
- Have a Gas Safe registered engineer diagnose and repair the root cause Gas Safe engineer
If the F.20 returns after your basic checks, a Gas Safe engineer should carry out a full inspection. This will typically include: testing the NTC thermistor resistance values and replacing any out-of-spec sensor; inspecting and servicing or replacing the circulation pump; assessing the heat exchanger for scale, sludge, or cracking; performing a power flush of the system if contamination is found and installing a magnetic inline filter to prevent recurrence; checking the expansion vessel charge and the pressure relief valve; and inspecting the flue and fan for blockages or faults.
Parts you may need
- NTC thermistor (flow or return temperature sensor) · from £25
- Circulation pump (e.g. Grundfos UPS2 or equivalent) · from £85
- Expansion vessel (8–12 litre, boiler-mounted) · from £55
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £70
- Pressure relief valve · from £20
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 to clear the F.20?
No — resetting is a temporary measure to confirm whether a one-off event caused the lockout, not a fix. Repeatedly resetting an overheating boiler without resolving the underlying problem puts significant thermal stress on the heat exchanger. Cracks can develop in the aluminium casting, and once the heat exchanger is damaged it must be replaced rather than repaired — a job that typically costs £400–£700 in parts and labour alone, and sometimes makes a boiler replacement the more cost-effective choice. Reset once after completing the basic pressure and bleeding checks; if F.20 comes back, call a Gas Safe engineer.
How much does it typically cost to fix a Vaillant F.20 fault?
Most F.20 repairs fall in the £120–£400 range. Re-pressurising the system and a diagnostic call-out is usually £100–£150. Replacing a faulty thermistor tends to come in at around £180–£250 including labour. A circulation pump replacement is typically £250–£400. A power flush of the full system to clear sludge and scale runs to £300–£500 depending on the number of radiators. If the heat exchanger itself has been damaged by prolonged overheating, replacement costs can reach £500–£700 or more — at that point, the age and condition of the boiler should be weighed against the cost of a new installation.
Why does my Vaillant boiler show F.20 only in cold weather?
Cold snaps put the heating system under its heaviest load. Marginal faults that go unnoticed in mild weather — a slightly sluggish pump, borderline pressure, or a small amount of sludge in the heat exchanger — become acute when the boiler is running continuously at full output. The higher demand means less margin before temperatures hit the safety cut-out threshold. If F.20 only appears in cold weather, treat it as an early warning: get the system serviced before the next winter rather than waiting for a full breakdown.
Does the F.20 fault mean my Vaillant boiler is beyond repair?
Not usually. In the majority of cases F.20 is caused by something relatively straightforward — low pressure, a tired pump, or system sludge — all of which are repairable at a reasonable cost. The fault only indicates that the boiler is overheating; it does not by itself confirm damage. However, if the overheating has been happening repeatedly over a long period without diagnosis, there is a greater risk that the heat exchanger has been stressed. A Gas Safe engineer can assess this during the diagnostic visit. For boilers over 10–12 years old, it is worth asking the engineer to give an honest assessment of overall condition alongside the repair quote.