Error D.83

Safety first:

1) If you smell gas, stop immediately: do not operate electrical switches, open windows, turn the gas supply off at the meter if you can do so safely, leave the property and call the emergency gas number. 2) If you see black sooting, persistent flame failure or carbon-monoxide alarm activation, turn the boiler off and call a Gas Safe engineer. 3) Before touching the boiler isolate the mains electricity using the external switch and allow the appliance to cool; if you are unsure stop and call a qualified engineer.

Initial homeowner checks (what you can safely do):

1) Read the boiler display and note every code shown (D.83 and any F.xxx, S.xxx codes). Photograph the display and the label with model and serial number for the engineer. 2) Check the system pressure on the boiler gauge — it should typically be around 1.0–1.5 bar when cold. If it is below 1.0 bar, the boiler may not run correctly. 3) Check that hot-water outlets are demanding hot water (open a hot tap) and that any external cylinder thermostat or programmer is set to call for DHW. 4) Check other gas appliances to confirm gas supply is present. 5) Check condensate discharge (in cold weather condensate can freeze) and visible pipework for leaks.

Simple fixes to try (safe DIY):

1) Repressurise the system if pressure is low: follow the boiler manual to use the filling loop and bring pressure to about 1.0–1.5 bar, then close the filling valves. 2) Bleed a radiator or two to remove trapped air which can affect circulation. 3) Reset the boiler: consult the manual — press the reset button for the prescribed time then wait for the boiler to attempt restart. 4) If D.83 was just a log entry and the boiler now runs normally for hot water, monitor for recurrence and note any other codes.

If the problem persists or other fault codes appear (for example F83 or repeated ignition failures):

1) Pay attention to any F or S codes shown alongside D.83 — F83 in many Vaillant guides relates to return-temperature sensing/temperature differential issues and can cause lockouts. 2) Do not open the boiler or disconnect gas/electrical components yourself. Useful information for the engineer: record when the fault occurs (every hot-water demand or intermittently), show recent pressure history, any radiator bleeding or filling actions, and provide photos of the error codes and the boiler data plate.

When to call a professional (Gas Safe):

1) If the boiler will not ignite or keeps locking out after resets. 2) If you get F-codes (particularly temperature-sensor related codes), persistent low pressure that won’t hold, visible leaks, pump noise/failure, or suspected heat-exchanger/gas-valve faults. 3) For electrical/wiring checks, thermistor replacement, gas-valve adjustment/replacement, heat-exchanger repair or PCB diagnostics. These tasks are hazardous and legally require a qualified engineer.

What the engineer will likely check and do:

1) Confirm the fault history and replicate the condition. 2) Check system pressure, filling loop operation and bleed if needed. 3) Test flow and return temperatures and thermistor readings; check sensor placement and wiring/connectors for damage or poor contact. 4) Check pump operation and ensure correct flow through the heat exchanger. 5) Verify gas supply and gas-valve operation, inspect the heat exchanger for blockages or leaks and test ignition sequence. 6) Replace faulty NTC thermistors, wiring, gas valve, pump or other components as required and clear any lockouts after repair.

Final notes and precautions:

1) D.83 alone is usually an informational entry for a burner start in hot-water mode; only escalate if it’s associated with failures or additional fault codes. 2) Never attempt gas-side repairs, internal wiring changes or part replacements unless you are Gas Safe qualified. 3) When you call a Gas Safe engineer, give them the exact model, serial number and a list of codes and the conditions under which they occur — this speeds diagnosis and repair.