Mk2 Heat Pump Interface Wiring

This article helps diagnose and resolve issues with the Mk2 HP Interface Unit when integrating with a heat pump’s domestic hot water (DHW) sensor input.

Symptoms This Article Covers 

• Heat pump not triggering DHW mode when expected
• Incorrect or fluctuating temperature readings at the heat pump
• Heat pump continues heating past cylinder target temperature 

Recommended Wiring Setup (Check First) 

Before troubleshooting further, confirm the wiring matches the standard configuration: 

1. DHW Temperature Sensor Location
   1. The Heat Pump DHW temperature probe must be installed on the pipe between the circulator pump and heat exchanger.
   2. This sensor should be connected to the SENSOR1 port on the Mk2 HP interface unit.
2. Wiring to Heat Pump Sensor Input
   1. Use a 2-core cable to connect the HEATPUMP1 terminal on the HP interface to the heat pump's DHW temperature sensor input.
   2. Polarity does not matter (either core can go to either terminal).
3. Expected System Behaviour
   1. When heating is required: The DHW temperature probe is live, and the heat pump reads the true water temperature.
   2. When the target temperature is reached: The HP interface switches the heat pump sensor input to a simulated value using a HOT resistor ladder, indicating approximately 65°C to stop further heating. 

Troubleshooting Steps 

Step 1: Confirm Sensor Wiring 

• Open the HP interface unit and verify all connections are secure. 
• Check that the temperature probe is firmly fixed to the pipe and making good thermal contact. 
• Ensure there is no visible damage to the 2-core cable between HEATPUMP1 and the heat pump input. 

Step 2: Check Sensor Behaviour at Temperature Setpoint 

• Run a test cycle and monitor the switch-over behaviour at the target temperature: 
  • Does the heat pump stop heating once the target is reached?
  • If not, the HOT resistor ladder may not be engaging correctly. 

Step 3: Inspect Switching Logic 

• If the probe appears correctly installed and the cable is intact, verify: 
  • The HP interface controller is receiving the correct call-for-heat signal.
  • The relay that switches between the probe and HOT resistor ladder is functioning (you may hear a click or see a change on a diagnostic LED, if available). 

Common Causes of Fault 

• Heat pump overheats water: Resistor ladder not switching in; probe always active. 
• Heat pump doesn’t start: Sensor signal not reaching heat pump (broken cable, loose terminal).
• Incorrect temperature readings: Poor probe contact or cable interference.

Ensure the above image matches your setup exactly. Differences in sensor input configuration may require OEM guidance.

Further Support 

If issues persist after these checks: 

• Confirm that the heat pump supports simulated DHW inputs. 
• Contact Mixergy Technical Support with your wiring photo and heat pump make/model.