What the F20 code means
On most ecoTEC models, Vaillant's installation manual describes F20 as a safety switch-off by the temperature limiter. The boiler keeps a constant check on the temperature of the water in and around the heat exchanger. If that temperature climbs beyond a safe limit, the electronics shut off the burner and lock the boiler out. Water that gets too hot can boil inside the heat exchanger, stress the seals and put strain on the rest of the system, so the boiler stops before any of that can happen.
F20 is a lockout, not a passing warning. The boiler stays off until it has cooled down and been reset. A single F20 after an unusual event, such as a power cut while the heating was working hard, is not always a sign of a problem. F20 that keeps coming back, or that appears within minutes of a reset, points to a fault that needs to be found and fixed.
Common causes of F20
Poor water circulation
The boiler depends on the pump pushing water through the heat exchanger and out to the radiators, carrying the heat away. If that flow is restricted, heat builds up inside the boiler instead. Low system pressure, trapped air, most of the radiators being turned off, a pump that is sticking or a partly blocked magnetic filter can all cause this.
Sludge and scale
Over the years, heating systems collect black magnetite sludge from the inside of radiators and pipes. It narrows the waterways in the heat exchanger and creates hot spots. In hard water areas, limescale can build up in a combi's hot water side and cause similar trouble.
A faulty temperature sensor
Vaillant lists a defective flow or return temperature sensor, or a loose connection to one, as a possible cause. If a sensor sends a false high reading, the boiler may shut down even though the water is not actually overheating.
Electrical faults
The manual also mentions a poor earth connection and stray discharge from the ignition lead, plug or electrode. These are internal faults that only an engineer can test safely.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Low pressure or trapped air | Low reading on the gauge, cold tops to radiators | You, by topping up and bleeding |
| Radiators mostly turned off | F20 appears when only one or two rooms are calling for heat | You, by opening more radiator valves |
| Pump sticking or failing | Boiler heats up fast, radiators stay cool | Gas Safe engineer |
| Sludge or blocked filter | Cold patches at the bottom of radiators, noisy boiler | Gas Safe engineer |
| Faulty sensor or wiring | F20 with no sign of real overheating | Gas Safe engineer |
Safe checks you can do yourself
- Leave the boiler to cool for 20 to 30 minutes before you do anything else.
- Check the pressure on the gauge or display. With the system cool it should usually read 1 to 1.5 bar. If it is low, top up through the filling loop. Our guide on how to repressurise a boiler covers each step.
- Walk round the house and make sure most radiators are not switched off at their thermostatic valves. Water needs somewhere to go, and a system with almost every radiator closed can overheat the boiler.
- Run the heating for a while, then feel the radiators. If the tops are cold, there is air in them. Bleed them with the heating off and cool, then check the pressure again.
- Check the thermostat and timer settings, so you know the boiler is not being asked to run in an odd way, such as the heating turned up to maximum with every room shut off.
- Press reset as your manual describes. If F20 returns after two or three attempts, stop resetting and book an engineer.
If the boiler is banging, rumbling loudly or you see steam or hot water coming from the pressure relief pipe outside, switch the boiler off and leave it off. Do not remove the casing. Call a Gas Safe registered engineer.
When to call a Gas Safe engineer
Book an engineer if any of the following apply:
- F20 keeps coming back after two or three resets.
- The pressure is fine and the radiators are open, yet the boiler still overheats.
- You can hear the pump labouring or the boiler making kettle-like noises.
- F20 shows up alongside other codes, such as F83 or F75.
The engineer will usually check that the pump is running at the right speed, test the flow and return sensors, clean the filter and look at the condition of the system water. If the water comes out black, the system probably needs cleaning.
Typical repair costs in the UK
Costs depend on what is found and where you live, so treat these as rough guides only.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Replacing a flow or return temperature sensor | Typically £100 to £200 |
| Replacing the pump | Typically £250 to £450 |
| Fitting a magnetic system filter | Typically £150 to £300 |
| Power flushing the heating system | Typically £400 to £800 |
How to stop F20 coming back
Good circulation is the best protection against overheating. Keep the pressure between 1 and 1.5 bar when cold, bleed radiators if they develop cold tops, and avoid closing every radiator in the house at once. If you have a magnetic filter, have it cleaned at each service. Adding inhibitor to the system water slows down the corrosion that creates sludge in the first place. A yearly service is the best time for an engineer to spot a sluggish pump or dirty water before it causes a lockout, and our boiler service cost guide explains what that normally includes.