Overview
The Vaillant F.25 fault means the boiler has detected an abnormally rapid rise in flow/flue gas temperature and has shut down as a safety measure. In practice the boiler watches the difference between flow and return (and the flue gas temperature) and if the flow temperature increases too quickly the control logic triggers a safety cut-out (lockout) to protect the appliance and the heating system. That rapid rise is typically caused by a circulation problem or an incorrect temperature reading from sensors rather than normal operation. Common root causes are restricted water flow (blockages, sludge, limescale or airlocks), a stopped or failing pump, or failed temperature sensors/limiters or the flue thermostat giving incorrect readings. Severity is moderate to high because the boiler is in lockout to prevent overheating; while the boiler protecting itself reduces immediate danger, the underlying fault can cause no heating/hot water and, if left unresolved, may damage heat exchangers or other components. Some simple checks and resets can be done by the homeowner, but most detailed diagnosis and any internal or gas-related repairs must be carried out by a Gas Safe registered engineer.
Possible Cause: Safety switch off flue gas temperature too high
Troubleshooting Steps
Safety first
- If the boiler is showing F.25, do not attempt internal repairs or to work on gas components yourself. Only a Gas Safe engineer should touch gas valves, the burner or internal electrical components. Isolate the boiler from the mains electricity before any visual inspection that requires removing covers. Avoid touching hot pipework or the flue.
Initial checks a homeowner can safely do
1) Record the fault: note the exact code, the conditions when it happened (heating on, hot water only, recent work), and take a photo of the display.
2) Reset the boiler: follow the manufacturer's reset procedure or switch the boiler off at the mains for 30–60 seconds, then restart. If the code returns straight away, do not keep resetting.
3) Check system pressure: look at the pressure gauge on the boiler. Typical cold system pressure is about 1.0–1.5 bar. Do not over-pressurise the system. If pressure is very low, that can cause circulation issues; if you are unsure how to repressurise, stop and call an engineer.
4) Bleed radiators: bleed at least the highest and lowest radiators to remove trapped air which can cause circulation faults. Turn the heating off and allow radiators to cool before bleeding. Close the bleed valves when water appears.
5) Inspect the external flue and terminal: from outside, check that the flue outlet is not blocked by debris, bird nests, or snow. Do not attempt to dismantle the flue or force anything into it.
6) Check for obvious pump noise or lack of circulation: while the boiler is off and cold you can gently feel pipes (careful of hot surfaces when on) or listen when it starts; a silent or seized pump may indicate failure. If you suspect the pump has seized, switch off and call an engineer.
Further diagnostic and fix steps (for a qualified engineer)
1) Reproduce and log the temperature behaviour: measure flow and return temperatures and the rate of rise (engineer-grade thermometer or clamp sensor) to confirm rapid flow temperature rise that triggers F.25.
2) Check thermistors/temperature sensors: test flow and return thermistors for correct resistance/response and replace or recalibrate if out of spec. Check the flue gas thermostat/thermistor for faults or intermittent connections.
3) Inspect limiter and control board inputs: verify the temperature limiter and safety cut-out are functioning correctly and that the control electronics are reading sensors correctly.
4) Check hydraulic circulation: confirm pump speed and operation, verify valves are open and correctly set (including any zone valves), and measure flow rate. Replace or service the pump if flow is inadequate.
5) Inspect for blockages: check the primary heat exchanger, internal pipework and system for sludge, limescale or debris. If blocked, clean or replace the affected parts.
6) System flush and filter: if sludge/limescale is present, perform a chemical flush or powerflush as appropriate, and install or clean a magnetic filter/strainer to prevent recontamination. Consider scale reducer in hard water areas.
7) Flue and exhaust checks: verify flue thermostat and flue gas path are intact and that flue gas temperatures are within manufacturer limits.
8) Replace faulty components: replace failed thermistors, limiter, flue thermostat, pump or other defective parts as required and retest.
When to call a professional now
- If the F.25 code persists after the homeowner checks (reset, bleed radiators, check pressure and flue terminal) you must call a Gas Safe registered engineer. Any diagnostic steps beyond the basic homeowner checks, and all repairs or parts replacements, require a qualified engineer because they involve gas, sealed components, and safety-critical controls.
Prevention and follow-up
- Have the boiler serviced annually by a Gas Safe engineer to catch pump wear, sensor drift and blockages early. Consider magnetic filters and water treatment (inhibitor or scale reducer) to reduce sludge and limescale, which are common causes of circulation faults that lead to F.25.
If you need help finding a qualified engineer, contact Vaillant support or use your local Gas Safe register. Provide the engineer with the fault code, any steps you have already taken, and the boiler model and age to speed diagnosis.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Pro / EcoTEC Plus.