What F25 means on a Vaillant boiler
F25 tells you the boiler has shut down because the gases leaving it through the flue are hotter than they should be, or because the circuit that monitors that temperature has been broken. Vaillant's ecoTEC plus installation and maintenance instructions (document 0020244997) list F25 as "Safety switch-off: Flue temp. too high", and the 2021 instructions for the open-vent ecoTEC plus (document 0020238195) word it as a "Safety shutdown". Vaillant's UK fault code page puts it more simply: "The flue gas temperature is too high."
A condensing boiler is designed to pull as much heat as possible out of the burner gases before they leave. If the gases are leaving hot, heat is not passing into the water properly or something in the flue route is wrong, so the boiler stops. Treat F25 as a protective shutdown, not a nuisance message.
Which Vaillant boilers show F25
We found F25 with the same meaning in every Vaillant fault table we checked for this page:
- ecoTEC plus VU and VUW ../5-5 regular, system and combi boilers (combi 825 to 838), document 0020244997.
- ecoTEC pro 24 and 28 combis (VUW 246/5-3 and 286/5-3), document 0020244995.
- ecoTEC plus VU ../6-5 OVZ open-vent regular boilers, document 0020238195 (2021).
- ecoTEC plus VU 246/6-5 OVZ LPG boiler, document 0020239015.
The tables we used do not cover the older Vaillant ecoMAX, so if you have one, check F25 against its own manual before assuming the meaning is the same.
What causes F25
Vaillant's tables give two kinds of cause: genuinely hot flue gas, or an electrical break that looks like a tripped monitor.
The flue gas really is too hot
This can happen when too little heat is being carried away by the water in the heat exchanger. Vaillant's checklist for engineers covers the internal air vent and whether there is enough water in the heating circuit, both of which affect how well the water takes heat away from the burner. It also covers the flue gas route itself, including blockages, wind and the length of the flue.
A break in the plug or cable harness
Some ecoTEC boilers have an optional flue gas safety cut-out (SCO). The 2021 instructions list a break in the plug for that cut-out, or a break in the cable harness, as a cause of F25. In that case the flue gas may be perfectly normal and the fault is in the wiring that reports it.
| Possible cause (from Vaillant's tables) | What you might notice | Who deals with it |
|---|---|---|
| Low water or trapped air in the heating circuit | Pressure low on the display, gurgling, radiators cool at the top | You can top up and bleed; an engineer checks the internal air vent |
| Flue route blocked or affected by wind | F25 in stormy weather, or something visibly lodged in the terminal | Gas Safe engineer |
| Break in the safety cut-out plug or cable harness | F25 with no change in weather or pressure | Gas Safe engineer |
| Heating pump cable or connection | Usually no sign you can see from outside the boiler | Gas Safe engineer |
What you can safely check
Vaillant's own advice is that F25 is a job for an expert. These checks stay outside the casing and help the engineer narrow it down.
- Write down the code and whether the boiler was heating or running hot water when it appeared.
- Look at the pressure reading. On the ecoTEC pro and ecoTEC plus ../5-5, Vaillant's operating instructions give 1.0 to 2.0 bar as the range with the system cold and say to top up below 0.8 bar. If it is low, top up with the filling loop as your installer showed you. Our Vaillant boiler pressure guide explains how.
- If the radiators are cold at the top, bleed them and recheck the pressure afterwards.
- From outside, look at the flue terminal without touching it. Note anything that looks blocked, crushed or damaged and pass that on. Our boiler flue guide explains what the terminal should look like.
- Press the reset button once as your manual describes. If F25 comes back, stop there and leave the boiler off.
Never remove the boiler casing or try to clear the flue yourself. A flue problem can let combustion gases into the home. If you smell gas, turn the supply off at the meter if you can do so safely, open windows and call the National Gas Emergency Service on 0800 111 999. If your carbon monoxide alarm sounds, get everyone outside.
A plume of vapour from the flue on a cold day is normal and not, on its own, a sign of F25. Our guide to steam coming from a boiler flue explains when that plume is normal.
What the Gas Safe engineer will test
The ecoTEC plus maintenance instructions set out a clear order of checks for F25, so you can expect the engineer to work through these:
- The main plug connection and the plug for the flue gas safety cut-out, plus the cable harness, looking for a break or a loose connection.
- The cable to the heating pump.
- The boiler's internal air vent.
- The flue gas route for blockages, the effect of wind and whether the flue length is within limits.
- Whether there is enough water in the heating circuit.
Vaillant then has the engineer run check programme P.0, which purges air from the boiler. Tell them if the code appeared in high wind, after the pressure dropped or after work on the flue.
Vaillant's UK customer service line for this kind of fault is 0344 736 0049. If the boiler is still under its guarantee, call Vaillant or your installer before booking anyone else.
Typical repair costs in the UK
Typical ranges for the jobs Vaillant's F25 checklist points to:
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Plug, cable harness or wiring repair | Typically £100 to £180 |
| Flue gas safety cut-out or similar limit thermostat | Typically £100 to £200 |
| Flue repair or adjustment | Typically £150 to £400 |
| Pump replacement, if circulation is the root cause | Typically £150 to £400 |
How F25 differs from other Vaillant codes
F25 is easy to confuse with F20, an overheat shutdown on the water side, and with F28 or F29, which are ignition and flame faults. F25 is specifically about the temperature of what leaves through the flue. If several codes appear, write them all down in order. Our Vaillant fault codes hub lists the other ecoTEC codes we cover.