Error F.20

Safety precautions:

- If you smell gas, leave the property immediately and call the gas emergency number — do not try to investigate.

- Switch the boiler to standby and isolate the electrical supply before touching any controls. Allow the boiler to cool for at least 20–30 minutes before attempting a reset or any checks.

- Do not remove covers, dismantle the boiler, or attempt gas or flue repairs. Internal electrical and gas work should only be done by a Gas Safe engineer.

Initial checks a homeowner can do:

1) Note exactly when the F.20 appears (after a few minutes of run-time, immediately on demand, intermittently, only on heating or hot water). This helps the engineer diagnose.

2) Allow the boiler to cool fully, then attempt a single reset using the boiler’s reset button as per the manual. Only reset after cooling; if it locks out again promptly, do not keep resetting.

3) Check the system pressure gauge on the front of the boiler. Typical cold pressure is around 1.0–1.5 bar. Record the reading.

4) If pressure is low (well below 1.0 bar) you can consider repressurising using the filling loop if you are comfortable and know the correct procedure from the manual. If unsure, stop and call an engineer. If pressure is high (above approx 2.5 bar) you can try bleeding a radiator to reduce pressure a little—do this carefully and monitor the gauge.

5) Bleed a few radiators to remove trapped air (airlocks impede flow and can cause local overheating). After bleeding, check pressure again and repressurise if necessary.

6) Listen and observe while the boiler runs (if you have got it working momentarily): any kettling/whistling noises suggest scale/build-up in the heat exchanger; no fan noise or unusual mechanical noises can indicate a fan or pump fault.

7) Inspect the external flue and air intake for obvious blockages (bird nest, leaves, snow). Only remove external debris that is safe to access.

8) Check for signs of PRV discharge (wetness or discolouration on the discharge pipe). A stuck or leaking PRV or a blocked discharge can indicate pressure problems.

Specific diagnostic and fix steps for a qualified engineer (do not attempt yourself):

- An engineer will test NTC sensor resistances and wiring to check for faulty temperature sensors and replace them if needed.

- They will test fan operation and motor control and replace the fan if it has failed.

- The pump will be inspected for correct operation and speed; the engineer may clear blockages, change pump settings or replace the pump.

- The heat exchanger may be inspected for limescale or blockages; cleaning, descaling or replacement may be required.

- The expansion vessel, pressure sensor and PRV will be checked and repaired or replaced if faulty.

- The PCB and control electronics will be tested for correct responses to sensor inputs.

- System flushing or chemical cleaning may be carried out to remove sludge/scale and restore correct flow.

- The flue and condensate paths will be inspected for restriction or damage.

When to call a professional:

- If the F.20 returns after a cooldown and reset, or happens repeatedly.

- If you are not confident repressurising the system or bleeding radiators.

- If you smell gas, see ongoing leaks, visible PRV discharge, or cannot restore heating/hot water.

- For any internal inspections, replacement of sensors, fan, pump, heat exchanger work, flue or gas-related repairs.

Always use a Gas Safe registered engineer for diagnosis and repairs that involve gas, the flue, internal electrical components, or dismantling the boiler. Provide the engineer with the recorded behaviour (pressure readings, timings, noises, how often the lockout occurs) — it will speed up diagnosis and resolution.