Vaillant F.20 fault codewhat it means and how to fix it
The overheat thermostat has tripped because the boiler got too hot. It has shut down to protect itself and it should not be restarted repeatedly.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Vaillant F.20
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What are the radiators doing?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Radiators are hot at the top and cold at the bottom
Sludge. Magnetite has settled in the bottom of the radiators and is restricting flow through the whole system, including the heat exchanger.
The fix is a powerflush or at minimum a chemical clean and a magnetic filter, plus fresh inhibitor. Replacing parts without cleaning the system will not hold.
No radiators heat up at all, but the boiler gets hot fast
Water is not leaving the boiler. Pump failure or a closed valve, and the heat exchanger is taking the full burner output with nowhere to send it.
Turn the boiler off at the front panel and leave it off until an engineer has seen it. This is the condition that cracks heat exchangers.
Hot water works but heating trips it
The hot water circuit flows properly and the heating circuit does not, which points at the diverter valve or the heating side of the system rather than the boiler itself.
Mention this to the engineer. It narrows the fault to the diverter valve or heating circuit immediately.
What F.20 means
F.20 is a safety cut-out. Vaillant boilers carry an overheat thermostat that trips when the flow temperature exceeds a fixed limit, typically around 95 to 100 degrees. Under normal operation the boiler modulates well below that, so a trip means heat is being produced faster than it can be carried away. The heat itself is almost never the problem. The problem is flow: water is not moving through the heat exchanger quickly enough, so a small volume of it heats rapidly past the limit.
Pump failure, closed valves, a sludged system and a blocked heat exchanger all produce the same picture. That is the version worth understanding, but Vaillant's own cause list is a useful corrective to it, because most of what they name is electrical rather than thermal. Their table for F.20 lists a defective flow NTC, a defective return NTC, a faulty earth connection, and a stray spark by way of the ignition cable, ignition plug or ignition electrode.
In other words Vaillant expect a good proportion of these to be a sensor lying about the temperature or a spark finding the wrong path, rather than water that genuinely reached 100°C. This matters when an engineer arrives and finds a system that circulates perfectly well: that is not a puzzle, it is the other half of Vaillant's list. Repeatedly resetting an overheating boiler stresses the heat exchanger, which is by far the most expensive component in the appliance.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump not circulating properly | Either failed outright or running at too low a speed for the system. Water sits in the exchanger and boils rather than flowing through it. | Common |
| Sludge restricting flow | Magnetite build-up narrows the heat exchanger waterways. Radiators cold at the bottom are the giveaway. | Common |
| Closed or seized valves | A closed isolation valve, a seized zone valve, or every thermostatic radiator valve shut at once leaves the pump with nowhere to push water. | Occasional |
| Failed overheat thermostat | The thermostat itself has failed closed and is reporting an overheat that is not happening. Diagnosed by measurement, not assumption. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£150–£750
Time on site
2 hours
Pump replacement £280–£450. Powerflush £400–£700. Overheat thermostat £140–£220. A cracked heat exchanger is £600–£1,200 and often means replacing the boiler.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.23Get it checkedVaillant call this a safety shutdown for temperature spread too great. It nearly always means the pump is not moving enough water.Related: overheat and circulation
- F.83Get it checkedThe burner lit but neither sensor saw the temperature rise it should have. Either water is not moving through the boiler, or a sensor is misplaced.Related: overheat and circulation
- F.24Get it checkedTemperature is rising faster than it should when the burner fires. That is a circulation problem caught a step earlier than an overheat.Related: overheat and circulation
- F.25Get it checkedOn a current ecoTEC this is a flue gas temperature safety shutdown. The same number meant a cable break on the 2009 ecoTEC and a water switch fault on the ecoMAX.Related: overheat and circulation
- F.76Get it checkedThe overheating protection built into the primary heat exchanger has responded. A thermal fuse on the heat cell, or its wiring, has opened, and the boiler has stopped.Related: overheat and circulation
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: same brand
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.20
Can I keep resetting F.20 to get through the evening?
Please do not. An overheating boiler is one where water is not carrying heat away from the exchanger. Every cycle you force stresses a component that costs £600 to £1,200 to replace, and on an older boiler that figure writes the appliance off.
Why does it overheat when the heating is set low?
The thermostat setting controls how long the burner runs, not how hot the exchanger gets if water is not moving. With no flow, the small amount of water in the exchanger reaches the cut-out temperature in seconds regardless of what the dial says.
Is a powerflush always the answer?
No, and it is over-sold. It is the right answer when radiators are cold at the bottom and the system is visibly dirty. If the pump has simply failed, a flush is an expensive way to avoid fixing the actual fault. Ask the engineer to show you what they found.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.