Overview
D.5 on a Vaillant EcoTEC is a diagnostic parameter, not a traditional fault code. It shows the boiler's current target temperature — either the flow temperature setpoint or the return temperature setpoint depending on how the boiler is configured (see parameter d.17). In normal operation D.5 tells you the temperature the boiler is trying to produce at the flow pipe (or the target return temperature if return-target control is selected). Seeing D.5 change when the room controller (VRC 430) adjusts the demand is expected; seeing it jump to 0°C or behave unpredictably is not. When the boiler shows the behaviour you described (initial correct heat-up, then repetitive short firings with long waits, flow target briefly reached only for a few seconds, D.5 sometimes switching to 0°C and the boiler reporting S.31), that points to a control/communication or sensor issue rather than a simple user setting. Possible causes include an incorrect control mode (flow vs return target), a fault or intermittent wiring/connection problem between the VRC 430 and the boiler, a defective flow or return NTC thermistor or its wiring, a problem with the PCB/logic that reads D.5 and D.40/D.41, or an actuator/pump/zone valve not circulating correctly. Severity: this is not an immediate gas-safety emergency but it is a significant fault because it prevents the heating from reaching the required room temperature and should be diagnosed promptly. Some basic checks and setting changes can be done by a homeowner; any work involving gas, the combustion chamber, replacing sensors, or PCB-level diagnosis must be done by a Gas Safe certified engineer.
Possible Cause: Set value of flow temperature or target value of return
Troubleshooting Steps
Safety precautions:
- Do not open the boiler’s combustion chamber or any gas-containing parts. Any gas or sealed-component work must be carried out by a Gas Safe engineer.
- Before touching electrical connectors, switch the boiler off at the mains. If you are unsure, stop and call a professional.
- Take care around hot pipes and components; allow the boiler to cool before touching plumbing.
Initial homeowner checks (safe, non-invasive):
1. Check boiler pressure. If pressure is low (typically below 1.0 bar), top up the system using the filling loop following the boiler handbook. Low pressure can cause odd behaviour.
2. Reset the boiler once via the reset button and observe whether the problem clears temporarily. Note any displayed status or fault after reset.
3. Check the VRC 430 controller settings: ensure CH is enabled, Room Temp Control modes, AUTO_OFF and heating curve. Try setting CH to Manual and a fixed flow target to see if behaviour changes. Record what D.5 shows when you change the controller.
4. Bleed radiators to remove trapped air and ensure radiators are full of water. Air can reduce circulation and mimic pump/flow issues.
5. Confirm the circulation pump is running: with the boiler on, carefully feel the flow and return pipes (not the pump casing) for temperature difference and vibration. On your Grundfos 15-50 you can try different speeds and observe any change.
Diagnostics you can read from the boiler (no disassembly):
1. Enter the diagnostic menu (model dependent: usually press two front-panel buttons together, navigate to the installation/diagnostic level). Write down these values while the behaviour is occurring: d.5 (target flow/return setpoint), d.17 (flow vs return control setting), d.40/d.41 (actual flow and return temperatures), and any status codes (for example S.x or F.x codes).
2. Observe whether d.5 changes from the expected target to 0°C when the cycle starts. Note what d.40 (flow) and d.41 (return) read during the short 5s firing and during the long idle periods.
3. If the boiler supports it, check d.71 or equivalent that limits maximum flow temperature, and d.70 if you have a zone valve centre position setting — confirm these are set to expected/default values.
Specific investigative steps and possible fixes you can try (safe):
1. Power-cycle both the boiler and the VRC 430: switch off at mains, wait 60 seconds, power on. Sometimes the controller and boiler lose sync and a full reboot restores normal communications.
2. Change the heating curve away from 1.8 and then back again (as you noted this sometimes temporarily forces a new target). Observe if that gives stable control and if d.5 remains correct.
3. Set the VRC 430 to a fixed/manual flow target (if available) and disable room compensation to see if the boiler holds the flow temperature steadily. This isolates whether the modulation from the VRC is triggering the issue.
4. Temporarily bypass the room controller if you have a simple on/off thermostat or can safely join the CH call terminals (only if you understand low-voltage wiring). If the boiler then maintains stable flow, the VRC 430 or its wiring may be at fault. If this step is beyond your confidence level, skip it and call an engineer.
5. Check all visible low-voltage connectors (on the boiler PCB and thermostat wiring) for secure seating and corrosion. Do not probe or disconnect inside the boiler unless the mains is off and you are competent; simple visual checks are safe.
6. Ensure TRVs are not closed on all radiators and that any motorised zone valves are calling and fully switching (listen for click and feel for valve movement). A closed zone or stuck valve can cause poor flow and rapid cycling.
When to call a professional and what to tell them:
- Call a Gas Safe registered engineer if: the boiler repeatedly shows D.5 = 0°C or S.31 status, D.5 is inconsistent with actual flow/return temps, the boiler cycles and will not maintain set temperature, or you suspect a sensor, PCB or gas/combustion related fault.
- Provide the engineer with the diagnostic values you recorded (d.5, d.17, d.40, d.41 and any S.x or F.x codes), the behaviour timeline, and what you tried (reset, heating curve change, pump speed changes, radiator bleeding). This information speeds diagnosis.
What a professional will check or do (do not attempt unless qualified):
- Test NTC thermistor resistances and sensor placement; repair or replace faulty flow/return sensors.
- Check wiring harness between thermostat, pump, actuators and main PCB; repair loose/intermittent connections.
- Verify operation of pump, motorised valves and end switches under load and replace faulty components.
- Investigate PCB logic/firmware or intermittent faults that cause the target setpoint to be lost (D.5 = 0) and replace PCB or module if needed.
Final note: D.5 itself is a diagnostic readout showing a target temperature. If the value is changing to 0°C or the boiler will not hold the target and cycles frequently, this strongly suggests a control, sensor or wiring fault. Do the safe checks above, record diagnostic numbers, and call a Gas Safe engineer for anything beyond simple setting changes, visual connector checks and radiator bleeding.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Gas Boiler.