F24 means the water in your Vaillant heated up faster than it should once the burner lit, so the boiler shut down to avoid overheating. Very fast heating almost always means too little water is moving through the heat exchanger. Start with the pressure gauge, because that is the one cause on Vaillant's list you can deal with safely.
What F24 means
When the burner lights, the boiler expects the water temperature to climb at a steady rate as water flows past the heat exchanger and carries the heat away. If only a trickle of water is moving, or the heat exchanger is partly full of air, the small amount of water inside heats up very quickly. The control board spots the steep rise on its temperature sensors and stops the burner before the heat exchanger gets too hot.
Vaillant's ecoTEC plus installation and maintenance instructions (document 0020244997) show F24 as "Safety switch-off: Temp. incr. too fast". In the 2021 instructions for open-vent regular ecoTEC plus boilers (document 0020238195) the same code reads "Safety shutdown: Temperature rise too fast". It is a protective lockout, not a sign that the boiler has already been damaged.
Which Vaillant models use F24
F24 has the same meaning across the ecoTEC range covered by the manuals we checked. That includes the ecoTEC plus ../5-5 system, regular and combi boilers (among them the 825, 832, 835 and 838 combis), the ecoTEC pro 24 and 28 combis (document 0020244995), the ecoTEC plus ../6-5 OVZ open-vent regular boilers and the ecoTEC plus LPG regular boiler (document 0020239015). The cause list below comes from the LPG manual, which gives the most detail.
The causes Vaillant lists
- Pump blocked. If the pump has seized or is jammed with debris, water barely moves, so the temperature shoots up.
- Insufficient pump output. A pump that runs but has weakened with age may not move enough water to keep the rise steady.
- Air in the boiler. Air in the heat exchanger or pump means less water is there to absorb the heat. This is common after the system has been drained or worked on.
- System pressure too low. A system short of water circulates poorly and traps air more easily. This is the cause you can check and fix yourself.
- Non-return valve blocked or incorrectly installed. A non-return valve lets water flow only one way. If it is jammed or was fitted the wrong way round, it restricts flow through the boiler. If F24 started straight after new pipework or an installation, this deserves a look.
| Cause | Clue | Who fixes it |
|---|---|---|
| Low pressure | Gauge under 1 bar when cold | You, with the filling loop |
| Air in the boiler | Gurgling, recent drain down | You for radiators, engineer for the boiler |
| Pump blocked or weak | Pressure fine, radiators slow to warm | Gas Safe engineer |
| Non-return valve | F24 since new pipework or installation | Gas Safe engineer or the installer |
Start with the pressure
With the heating off and the system cool, look at the pressure gauge or display. Most ecoTEC boilers like to sit between 1 and 1.5 bar when cold. If yours is lower:
- Find the filling loop under the boiler. Our Vaillant boiler pressure guide shows where it is on the common ecoTEC models.
- Open the valves slowly and watch the gauge until it reaches about 1.5 bar, then close both valves fully.
- Bleed any radiators that are cold at the top, then check the pressure again and top up if it has dropped.
- Reset the boiler. In the 2021 and LPG ecoTEC plus manuals, Vaillant says to press and hold the reset button for more than three seconds.
If the pressure was very low, you may have seen F22 as well. Pressure that keeps falling after topping up points to a leak, which needs finding before the boiler will run reliably.
Stop after two or three resets. If F24 returns even with the pressure correct, the cause is inside the boiler, and repeated resets only put the heat exchanger through more rapid heating cycles.
What the engineer will do
An engineer will usually vent the boiler and pump first, because air is quick to clear and a common trigger. They will then check the pump is turning freely and confirm it is moving enough water. Our guide to the boiler pump explains how a failing pump shows itself. If the pump is sound, the next step is checking any non-return valve in the circuit for a blockage or the wrong orientation. If you have also seen F23, say so, since it shares the pump and air causes.
Do not take the casing off to vent the boiler or free the pump yourself. These parts sit alongside the gas valve and mains wiring. If you smell gas, call the National Gas Emergency Service on 0800 111 999.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Venting the boiler or refitting a non-return valve | Typically £100 to £180 |
| Pump replacement | Typically £150 to £400 |
| Finding and repairing a leak causing low pressure | Typically £100 to £300 |
Patterns worth telling the engineer
A few details help narrow F24 down quickly. Does it appear on the first heating call of the day, or at any time? Did it start after the system was drained, a radiator was added or the boiler was replaced? How often have you needed to top up the pressure in the last month? Does the boiler gurgle as it starts? Answers to these questions point the engineer towards air, the pump or the pipework before they open the boiler.