Combi Boiler Hot Water Activation Sequence – Wiring & Voltage Guide

Understanding the hot water activation sequence in a combi boiler is essential for heating engineers, electricians, and DIY enthusiasts. This guide will walk you through the step-by-step process, including voltage readings, wiring connections, and troubleshooting techniques to diagnose faults effectively.

Step 1: Standby Mode (No Hot Water Demand)

When the boiler is in standby mode, the PCB (Printed Circuit Board) remains powered, monitoring system sensors for a demand signal.

Key Voltage Readings in Standby Mode:

• Main Live (L) to Neutral (N): 230V AC (Mains supply voltage)
• PCB Control Circuit: 5V DC & 230V AC (Powering internal sensors)
• Flow Sensor (Hall Effect): 5V DC (Waiting for water flow)
• Gas Valve Circuit: 0V (Remains inactive until ignition)

Step 2: Hot Water Tap Turned On – Flow Sensor Activation

When a hot tap is opened, water movement is detected by the Hall Effect Sensor, sending a 5V DC signal to the PCB.

Voltage Checkpoints:

• Hall Effect Sensor Output: 5V DC
• Hydraulic Pressure Switch: 0V → 5V DC (If flow detected)
• NTC Sensors: 5V DC (Monitoring water temperature)

Step 3: Diverter Valve Activation

The PCB supplies 230V AC to the Diverter Valve Motor (L3 Terminal), shifting it into hot water mode.

Expected Voltage Readings:

• PCB (L3 Output): 230V AC
• Diverter Valve Feedback (N2): 230V AC
• NTC Sensor (Direct Hot Water): 5V DC

Step 4: Fan & Pump Activation

Before ignition, the fan and pump activate to ensure safe operation.

Voltage Checks:

• Fan Live (ERP Fan): 230V AC
• Pump Live: 230V AC
• Fan Pressure Switch Feedback: 5V DC (PCB checks airflow)

Step 5: Ignition & Gas Valve Activation

Once airflow is confirmed, the gas valve opens, allowing ignition.

Voltage Readings:

• Gas Valve Live: 230V AC
• Ignition Electrode: 5V – 15V DC pulse
• Flame Sensor Feedback: 5V DC (Verifies burner ignition)

Step 6: Hot Water Delivered to Tap

Water passes through the plate heat exchanger, transferring heat to the tap.

Final Voltage Checks:

• NTC Sensor (Hot Water Output): 5V DC
• Pump Live: 230V AC
• Gas Valve Live: 230V AC

Common Hot Water Activation Issues & Troubleshooting

1. No Hot Water Flow?

Check the flow sensor (5V DC). If it doesn’t send a signal, replace it.

2. Diverter Valve Stuck?

Check 230V AC output at L3. If present but valve doesn't shift, the motor may be faulty.

3. Fan Not Running?

Check 230V AC supply to the fan. A blocked condensate trap can prevent startup.

4. No Ignition?

Ensure 5V – 15V DC pulse at the ignition electrode. If no voltage, check the PCB.

Conclusion

By following these wiring steps, voltage checks, and troubleshooting techniques, you can diagnose and fix hot water activation issues in a combi boiler. Regular maintenance and correct electrical readings ensure a reliable hot water system.

Important Disclaimer

This guide is intended for educational purposes only and should only be used by qualified heating engineers or competent individuals with experience in boiler servicing and fault-finding. Working with live electrical components (230V AC), gas systems, and water circuits can be dangerous if proper safety procedures are not followed. If you are not qualified, always seek assistance from a Gas Safe registered engineer to avoid potential safety hazards and ensure compliance with UK regulations.

Further Reading

Understanding how a combi boiler activates hot water is only part of the system's functionality. To gain a complete understanding of how the boiler transitions into central heating mode, including the role of the diverter valve, thermostat, and pump activation, visit our in-depth guide on Combi Boiler Central Heating Activation.