Error D.20

Safety precautions:

1) If you smell gas or suspect a gas leak, evacuate the property and call the gas emergency number immediately. Do not operate electrical switches or the boiler. 2) If the boiler or hot water is extremely hot or steam is present, switch the boiler off at the boiler power button and at the mains isolator (fuse/spur). Allow the appliance to cool before doing anything else. 3) Do not remove the boiler case, touch internal wiring, or attempt to replace sensors or gas components yourself. Any internal electrical, sensor or gas work should be done by a Gas Safe registered engineer.

Initial checks a homeowner can do:

1) Take a clear note or photo of the display and any other codes shown (especially any F-codes). D.20 alone is a diagnostic; if any F-code appears alongside it (for example F.20, F.22, F.24, etc.) treat that as a fault and call an engineer. 2) Check hot water behaviour: are you getting hot water and does its temperature seem correct or excessively hot? 3) Check the boiler’s hot water/cylinder target setting in the control menu and lower it if it is set to an unusually high value (many Vaillant boilers allow 50–65°C for domestic hot water; lowering to 50–55°C can be sensible). Use the boiler control buttons to view/change the B (hot water/cylinder) target. 4) Check system pressure on the boiler display or gauge (typical cold pressure ~1–1.5 bar). Low pressure can cause other faults; topping up should be done per the boiler handbook. 5) Perform a safe reset: if no F-codes indicating overheat or danger are present, try a standard boiler reset (hold the power/reset button per the boiler instructions, usually ~5 seconds). If the D.20 remains but everything operates normally, it may be only informational.

Specific diagnostic steps / homeowner limits:

1) If lowering the cylinder target and resetting clears the code and the boiler returns to normal operation, monitor over the next 24–48 hours. If the code returns repeatedly, call an engineer. 2) If hot water is excessively hot, the boiler will not behave normally, or you see any F-codes (especially overheat or safety cut-out codes), switch the boiler off and contact a Gas Safe engineer immediately. 3) Gather useful information for the engineer: exact boiler model, full text of the display and any other error codes, when the problem started, any recent maintenance/work, and whether hot water or central heating is affected.

What a professional will check (do not attempt yourself):

1) Verify the cylinder (actoSTOR) NTC sensor resistance and behaviour at ambient and operating temperature; check the sensor wiring, plug connections and actoSTOR electronics. A shorted or open NTC can cause incorrect target/limit behaviour. 2) Check boiler internal flow/return sensors and wiring for faults, and confirm sensors are fitted to the correct pipes (incorrect fitting can confuse regulation). 3) Check for related safety cut-outs or overheat events (examine any logged F-codes, flue temperature limiter, SCO behaviour). 4) Confirm boiler control settings (max cylinder charge, D-parameter limits) and check any installed external cylinder controller or thermostat for compatibility or incorrect settings. 5) Inspect pump operation, 3-port diverter valve function and hydraulic flow: restricted flow or pump failure can lead to overheating and control interventions. 6) If required, replace faulty NTC sensors, wiring harnesses or connectors, and retest. The engineer may also check the main PCB inputs and software parameter settings if a configuration issue is suspected.

Final advice:

If D.20 is an isolated diagnostic and you can confirm the cylinder temperature target is set intentionally high, you may simply lower the setting or ignore the D-code. If the code is persistent, appears with F-codes, or if hot water/heating behaves abnormally (overheating, lockouts, no hot water), stop using the boiler and contact a Gas Safe registered engineer. Do not attempt internal electrical, sensor or gas repairs yourself.