Error D,11

Safety precautions

1. If you smell gas or suspect a leak, stop using the appliance, turn off the gas at the meter if you can do so safely, evacuate the property and call the gas emergency number. Do not attempt repairs.

2. Electrical hazard: isolate the boiler electricity at the fuse or isolation switch before accessing any electrical parts. If you are not competent with electrical work, do not open the boiler casing.

3. Avoid working on or touching hot components or hot water pipes. Let the boiler cool down before doing any visual checks.

4. Note that opening the boiler or attempting internal repairs may void warranty and is not recommended for unqualified people.

Initial checks a homeowner can safely do

1. Read and record the full display message and any additional error codes. Take a photo for the engineer.

2. Reset the boiler: switch the boiler off at the mains or use the reset function, wait a minute, then switch back on and check whether D,11 returns. A single reset can sometimes clear temporary sensor/communication glitches.

3. Check that the central heating is calling for heat: check room thermostat, programmer/timers and any wireless controls. Ensure the boiler is set to heating mode and not inhibited by frost mode or holiday mode.

4. Check boiler pressure on the gauge. Low system pressure (below about 1.0 bar) can cause circulation problems—top up the system only using the correct filling loop procedure for your boiler model if you are confident doing so.

5. Listen and feel: when heating is called for, listen for the external pump operating (hum/vibration). Carefully feel relevant pipes (not the pump electrical terminals) to see whether they warm up after a call for heat. Do not touch electrical connections.

6. Check any external pump isolator switch or fused spur (if the pump has one) to ensure it is turned on and the fuse/trip hasn’t operated.

7. Bleed a radiator to remove an airlock which can prevent flow in the system and mimic a pump fault.

Specific diagnostic and safe actions (if competent) and what they indicate

1. If the pump is silent and you have a fused spur or isolator for the pump, confirm the spur is live (check fuse or RCD). If a fuse has blown or an RCD tripped, replacing/resetting that may restore pump power but underlying fault should be investigated by an engineer.

2. If you can access the external pump safely and the power is isolated, some bonded circulator pumps have a small manual rotor boss you can try to free by hand (turning the rotor). Only do this if you know what you are doing and power is isolated. If the rotor is seized, the pump will need replacing.

3. If the pump appears to run but the system still has no heat, there may be a blockage or severe airlock. Bleeding radiators can help; persistent circulation problems usually require a professional powerflush or pump replacement.

4. Visual inspection: without opening the boiler, check that any visible low-voltage wiring to external accessories is connected and not obviously damaged. Do not open the boiler casing to probe PCBs or internal connectors unless you are a qualified engineer.

5. After you have performed safe checks and basic steps, try another controlled reset and see whether the D,11 returns immediately or after a short time. Intermittent faults point to wiring/communication or failing pump; permanent faults often indicate failed pump or control electronics.

When to call a professional

1. If the simple checks above do not clear the code, the fault persists or you are not comfortable performing any of the checks, call a Gas Safe registered engineer (or your local certified service). This is necessary to safely test pump electrical supply, measure voltage and resistance, check wiring continuity, test the pump motor and impeller, and check the boiler PCB and communication lines.

2. Tell the engineer the exact code (D,11), what you observed, any other error codes and what checks you have already done. If the boiler is under warranty or service contract, contact Vaillant customer support or your installer first.

3. The engineer may need to replace the external pump, repair or replace wiring/harness, or investigate the PCB/eBUS communications and replace components as required.

Useful final notes

1. Do not attempt internal electrical repairs, PCB replacements, or gas-side work yourself. These require a qualified engineer.

2. Take photographs of error screens and of any relevant external pump/fuse/isolator locations to speed diagnosis when the engineer attends.

3. If heating is required urgently and you cannot get an engineer quickly, temporary measures such as electric heaters may be used safely in occupied rooms, but avoid tampering with the boiler beyond the safe checks listed above.