For Qualified Engineers Only: This information is intended for Gas Safe registered engineers. Adjusting diagnostic parameters can affect boiler operation and efficiency. Always refer to the manufacturer's installation and servicing instructions.
About S-Codes and D-Codes
Vaillant ecotec boilers use two types of codes for diagnostics and configuration:
- S-Codes (Status Codes): Display the current operating status of the boiler. These are read-only indicators showing what the boiler is currently doing.
- D-Codes (Diagnostic Codes): Adjustable parameters that control various aspects of boiler operation. These can be modified by qualified engineers to optimize performance.
How to Access S-Codes and D-Codes
- Press the info button (i) on the boiler control panel
- Use the + and - buttons to scroll through codes
- For D-codes, press and hold the info button for approximately 5 seconds to enter adjustment mode
- Use + and - to change values, then press OK to confirm
Note: Some newer ecotec models may have slightly different navigation. Always consult the specific manual for your boiler model. See Vaillant Anti-Cycling Time for more configuration information.
S-Codes: Status Codes
S-codes indicate the current operating status of your Vaillant boiler. These are read-only displays that help engineers understand what the boiler is doing at any given moment.
| S-Code | Status Description | Details |
|---|---|---|
| S.0 | Heat Request Off | No demand for heating or hot water |
| S.1 | Heat Request CH | Central heating demand active |
| S.2 | Heat Request CH & DHW | Both heating and hot water demand |
| S.3 | Burner Start CH | Burner starting for central heating |
| S.4 | Burner Operation CH | Burner running for central heating |
| S.5 | Burner Start DHW | Burner starting for hot water |
| S.6 | Burner Operation DHW | Burner running for hot water |
| S.7 | Burner Off | Burner has been switched off |
| S.8 | Burner Post-run After CH | Post-purge cycle after heating |
| S.10 | Heat Request DHW | Hot water demand only |
| S.11 | Burner Post-run After DHW | Post-purge cycle after hot water |
| S.14 | Burner Post-run After Fault | Post-purge after error condition |
| S.20 | Anti-cycling Mode | Boiler in anti-cycling delay (see D.002) |
| S.21 | Anti-cycling Mode DHW | Anti-cycling delay for hot water |
| S.22 | Anti-cycling Mode CH Priority | Anti-cycling with heating priority |
| S.28 | Frost Protection Active | Boiler protecting against freezing |
| S.30 | Room Temperature Setpoint Reached | Target temperature achieved |
| S.31 | Boiler Controlled in Summer Mode | Heating off, DHW only |
| S.34 | Frost Protection Standby | Monitoring for frost conditions |
| S.41 | DHW Setpoint Reached | Hot water at target temperature |
| S.42 | Pump Post-Run | Pump running after burner shutdown |
| S.51 | 24 Hour Pump Cycle Active | Automatic pump exercise function |
| S.53 | Pump Priority Switching | System managing pump priority |
| S.54 | Combi Flow Turbine Signal Fault | Issue with DHW flow sensor |
| S.58 | Flue System Fault | Check flue installation and fan |
| S.96 | Gas Valve Test Mode | System in test/diagnostic mode |
| S.98 | Boiler Under Service Mode | Engineer service mode active |
D-Codes: Diagnostic Parameters
D-codes are adjustable parameters that allow engineers to configure and optimize boiler performance. Each D-code controls a specific function or setting.
Warning: Incorrect adjustment of D-codes can lead to inefficient operation, increased energy consumption, or boiler faults. Only adjust these parameters if you fully understand their function.
Key D-Codes for Central Heating
| D-Code | Parameter | Range | Default | Description |
|---|---|---|---|---|
| D.0 | CH Max Flow Temperature | 20-85°C | 75°C | Maximum flow temperature for central heating |
| D.1 | CH Minimum Output | 0-100% | 0% | Minimum burner power for heating mode |
| D.2 | Anti-cycling Time | 2-60 min | 20 min | Wait time before burner restarts (see Anti-Cycling Guide) |
| D.16 | Pump Post-run CH | 0-30 min | 3 min | Pump continues after heating burner stops |
| D.17 | Pump Post-run DHW | 0-30 min | 3 min | Pump continues after DHW burner stops |
| D.18 | Burner Block Time | 0-30 min | 2 min | Minimum time between ignition attempts |
| D.19 | Burner Run-on Time | 0-15 min | 2 min | Minimum burner run time once started |
| D.20 | System Type | 0-4 | 0 | 0=Combi, 1=System, 2=Regular, 3/4=Special |
| D.27 | Pump Speed CH Mode | 0-100% | 70% | Pump speed during heating operation |
| D.40 | DHW Maximum Temperature | 35-65°C | 60°C | Maximum hot water temperature |
| D.41 | DHW Flow Rate | 0-20 l/min | 11 l/min | Target DHW flow rate (combis) |
| D.47 | DHW Comfort Setting | 0-3 | 0 | 0=Eco, 1-3=Comfort levels |
| D.49 | Pump Speed DHW Mode | 0-100% | 70% | Pump speed during hot water operation |
| D.52 | Pump Overrun Function | 0-1 | 1 | 0=Off, 1=On (pump exercise) |
| D.53 | Pump Control Mode | 0-3 | 1 | 0=Fixed, 1=ΔP-c, 2=ΔP-v, 3=ΔP-3 |
| D.55 | Burner On Gradient | 1-120 sec | 20 sec | Burner ramp-up time on ignition |
| D.71 | CH Maximum Output Power | 0-100% | 100% | Maximum burner power for heating |
| D.72 | DHW Maximum Output Power | 0-100% | 100% | Maximum burner power for hot water |
| D.82 | Condensate Trap Heating | 0-1 | 1 | 0=Off, 1=On (frost protection) |
| D.84 | Heat Exchanger Type | 0-7 | Model | Factory set for specific boiler model |
| D.91 | Fan Speed Minimum | 1000-7000 rpm | Model | Minimum fan speed (factory calibrated) |
| D.92 | Fan Speed Maximum | 1000-7000 rpm | Model | Maximum fan speed (factory calibrated) |
| D.96 | Partial Load Setting | 0-100% | 50% | Burner power at partial load |
Advanced Configuration D-Codes
| D-Code | Parameter | Range | Default | Description |
|---|---|---|---|---|
| D.100 | Solar Mode Enable | 0-1 | 0 | 0=Off, 1=Solar thermal integration |
| D.101 | Solar Priority | 0-1 | 0 | 0=Normal, 1=Solar takes priority |
| D.102 | Solar Cylinder Temperature | 0-95°C | 60°C | Target for solar heated cylinder |
| D.110 | Weather Compensation Curve | 0.1-4.0 | 1.5 | Heating curve gradient |
| D.111 | Weather Compensation Offset | -20 to +40°C | 0°C | Parallel shift of heating curve |
| D.112 | Room Influence | 0-100% | 0% | Room stat influence on WC curve |
| D.120 | Cascade Mode | 0-1 | 0 | 0=Single, 1=Cascade configuration |
| D.121 | Cascade Address | 0-16 | 0 | Boiler number in cascade system |
| D.122 | Cascade Total Boilers | 1-16 | 1 | Number of boilers in cascade |
Common D-Code Adjustments
Optimizing for Efficiency
For condensing operation and maximum efficiency:
- D.0: Set to 55-60°C for well-insulated properties with larger radiators
- D.2: Increase to 30+ minutes to reduce cycling
- D.71: Range rate to match actual heat loss calculation
- D.110/D.111: Use weather compensation for optimal efficiency
Reducing Cycling
To minimize short cycling:
- Increase D.2 (anti-cycling time) to 30-40 minutes
- Adjust D.1 (minimum output) to match system requirements
- Increase D.19 (burner run-on) to ensure longer burn times
- Consider reducing D.71 if system is oversized
DHW Performance
For optimal hot water delivery:
- D.40: Set to 60°C for legionella protection with stored hot water
- D.41: Adjust flow rate to suit property demands (8-11 l/min typical)
- D.47: Use Eco mode (0) to prevent pre-heat losses
- D.72: Range rate DHW if oversized
Reading Current Values
Many D-codes also show current operating values when in read mode:
- Flow temperature (actual)
- Return temperature (actual)
- DHW temperature (actual)
- Current burner power (%)
- Current fan speed (rpm)
- Current pump speed (%)
Documentation: Always document any changes made to D-codes in the benchmark log. This helps future engineers understand system configuration and troubleshoot issues.
Safety and Compliance
- Only Gas Safe registered engineers should adjust D-codes
- Some parameters require manufacturer authorization
- Changes may affect warranty if not properly documented
- Always perform a full functional test after adjustments
- Verify flue gas analysis meets Building Regulations
- Complete benchmark commissioning sheet
Further Resources
For additional Vaillant technical guidance: