Error D.20

Safety precautions

1) Do not open or work inside the boiler or gas pipework unless you are a Gas Safe registered engineer. Working on gas appliances is dangerous. Isolate electrical power to the boiler only if instructed by a qualified engineer for internal work.

2) Avoid adjusting hot taps while testing hot water because very hot water can scald.

3) If you smell gas, evacuate, ventilate and call the gas emergency number immediately; do not operate the boiler.

Initial checks a homeowner can safely do

1) Note exactly how the boiler is behaving: is the D.20 code steady, intermittent, or accompanied by loss of hot water or heating? Make a note of any other fault codes showing at the same time (e.g. F13, F22, F20 etc.).

2) Try a simple reset: press and hold the boiler power/reset button for 5 seconds to clear transient faults. If the code clears and does not return, monitor for reappearance.

3) Check the hot water temperature setting in the boiler menu and any separate cylinder thermostat or immersion stat setting (if you have an external cylinder thermostat or programmer). If the cylinder target is set very high, reduce it to a safe level (typical usable domestic cylinder temperatures are often 50–60°C; higher increases scald risk).

4) Verify hot water demand: ensure hot taps are off during start-up and confirm whether the cylinder actually has hot water (touch pipework cautiously) and if the boiler is trying to heat.

5) Check for other visible issues: low system water pressure (look at the boiler pressure gauge), other fault lights, and whether any linked controls (room thermostat, timers) are blocking operation.

Specific diagnostic steps and possible fixes

1) If D.20 is simply because the target cylinder temperature is set to its maximum, lower the cylinder temperature target in the boiler menu or via the cylinder thermostat/programmer. Save settings and reset if required.

2) If the code returns immediately or cylinder temperature reading seems inaccurate, check for additional sensor-related fault codes (F13 indicates a cylinder sensor short-circuit/NTC fault). If you see F13 or other NTC codes alongside D.20, this points to a defective cylinder temperature sensor or wiring issue.

3) If you have easy, external access to a separate cylinder thermostat/immersion thermostat (not inside the boiler), check its setpoint and physical condition. Do not open electrical junction boxes or boiler covers. If the external thermostat is set too high or is visibly damaged, replace or adjust it and contact an engineer to fit a replacement.

4) If lowering the setpoint and resetting does not clear the issue and you suspect the cylinder sensor or internal wiring, stop. Do not attempt to remove or replace sensors inside the boiler yourself. These components require a certified engineer to diagnose, test with suitable meters, and replace if necessary.

5) Look for related causes that can make the boiler behave oddly: communication errors between external controls and the boiler, PCB faults, or intermittent wiring connections. These are diagnostic jobs for a Gas Safe engineer.

When to call a professional

1) Call a Gas Safe registered engineer if D.20 returns after simple resets and menu checks, if you see accompanying sensor fault codes (e.g. F13), if hot water is not available, or if you suspect wiring, sensor or PCB faults. Also call an engineer if you are not comfortable changing settings or cannot identify the external cylinder thermostat.

2) Tell the engineer the exact code (D.20), any other codes displayed, what you have already tried (reset, changed temperature setting), and whether hot water is available — this helps speed up diagnosis.

Final notes

D.20 can be a harmless indication that the target cylinder temperature is at its configured maximum, but it can also be a symptom of a sensor or control fault. Simple setting changes and resets may fix the issue; anything involving internal components, gas or electrical wiring must be left to a qualified engineer. Repeatedly resetting the boiler without addressing an underlying fault is not recommended.