If your Vaillant ecoTEC Pro, ecoTEC Plus, ecoFIT Pure or turboMAX is flashing F25 (red light locked on, no heating, no hot water), the boiler has deliberately shut itself down because it got far too hot somewhere inside.
F25 means “Safety temperature limiter has tripped” – a hard-wired overheat thermostat has opened and will not let the boiler run again until it’s manually reset by an engineer.
This is a serious safety lockout, not a minor glitch.
What Actually Causes F25 on Vaillant Boilers
Circulating Pump Failed or Seized
The pump isn’t moving water fast enough (or at all), so the heat has nowhere to go – the most common cause on boilers 8–15 years old.
Major Airlock or Blockage in the System
Sludge, closed valves, or a completely air-locked return pipe stops circulation.
Heat Exchanger Completely Blocked
Years of limescale (hard-water areas) or black iron-oxide sludge create a “hot spot” that trips the limiter.
Faulty or Mis-wired Overheat Stat Itself
Rare, but after 12+ years the stat can fail closed or the wiring burns.
Running with Extremely Low Pressure
Below 0.3 bar the pump can’t circulate properly (you’ll usually see F22 first, then F25 if ignored).
Safe DIY Checks (Very Limited)
Turn the power off for 10 minutes, then back on and attempt a reset (hold the spanner button 5 seconds).
Check the pressure gauge is between 1.2–1.8 bar – top up via the filling loop if low.
Feel all radiators – if some are stone cold top and bottom, you have a circulation problem.
That’s it. Do NOT try to bridge wires or force anything.
When You Must Call a Gas Safe Engineer (100 % of F25 Cases)
F25 is a non-resettable safety device by design. Only a registered engineer can:
- Manually push the red reset pin on the overheat stat (usually on the front of the heat exchanger)
- Test pump operation and replace if seized
- Chemically flush or powerflush the system
- Replace the overheat stat or wiring if faulty
- Carry out combustion check after repair
Never accept a “quick reset” without fixing the cause – the boiler will trip again within minutes.
Check they’re Gas Safe at www.gassaferegister.co.uk.
Typical 2025 Repair Costs for Vaillant F25
- Pump replacement (Grundfos or Wilo): £280–£420 fitted
- System powerflush + inhibitor: £350–£550
- New overheat stat + labour: £140–£220
- Full “F25 circulation fault” fixed price (most Vaillant engineers now offer): £380–£680 depending on what’s needed
How to Prevent F25 Happening Again
Fit a MagnaClean or Vaillant magnetic filter and proper inhibitor.
Annual service – the engineer checks pump speed and system cleanliness.
In hard-water areas, install an in-line scale reducer.
Frequently Asked Questions About Vaillant F25
Will resetting clear F25 permanently?
No – the overheat stat has to be physically pushed back in by an engineer.
Is F25 the same as F24?
F24 = temperature rise too fast (early circulation issue). F25 = full overheat trip (problem got worse).
Is F25 covered by warranty?
Only if the boiler is still in warranty AND the system has been kept clean with inhibitor – most claims are rejected if sludge is found.
My boiler is only 6 years old – can I still get F25?
Yes – usually a seized pump or closed lock-shield valve somewhere.
Heating works but no hot water – can it still be F25?
No – F25 kills everything. Hot-water-only issues are usually diverter or plate exchanger.
Staring at F25 right now? Drop your exact model and how old it is in the comments – we’ll tell you whether you’re looking at a pump, a flush, or both.
Stay warm (and safe),
The Boilers2Go Team
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.