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
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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. 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 August 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 5 August 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 5 August 2026
What other owners found
Nobody has shared what worked for them on this code yet. If you have had it, you would be the first — and it genuinely helps the next person.
Related Vaillant fault codes
- F.23Get it checkedThe gap between flow and return temperature is too wide. Vaillant call it water shortage, and it nearly always means the pump is not moving enough water.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
- E9 219Stop — call an engineerThe boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Related: overheat and circulation
- L1Get it checkedThe boiler is not seeing enough water flow through it to run safely, so it has stopped. Usually a pump, an air lock, or a blocked filter.Related: overheat and circulation
- E02Get it checkedThe overheat thermostat has tripped. Almost always this means water is not moving through the appliance properly rather than that it is running too hot.Related: overheat and circulation
- A02Get it checkedThe safety limit thermostat has tripped on overheat. In practice this nearly always means water has stopped circulating properly.Related: overheat and circulation
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.
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
- Interviews with working engineers
Checked by Technical reviewer (Gas Safe registration to be displayed on appointment) on 5 August 2026.