MiniHPC - DIY Controller, Heat Pump from Air Conditioner

Operating Principle

MiniHPC is a heat pump controller designed to convert a standard supported air-to-air air conditioner into an air-to-water heat pump.

A standard air conditioner consists of two main components: an indoor unit and an outdoor unit, which are connected by a communication cable and refrigerant pipes. When the heating mode is activated on such an air conditioner, the outdoor unit starts the compressor, and high-pressure refrigerant flows into the indoor unit. This refrigerant then condenses inside the indoor unit, transferring heat to the heat exchanger, which, with the help of a fan, heats the air in the room.

Instead of the indoor unit, a plate heat exchanger can be used to transfer this energy to heating water instead of directly heating the room air. The heated water can then be circulated through a standard heating circuit with underfloor heating, radiators, or used to heat domestic hot water (DHW) for showering or bathing.

However, for this to work, the outdoor unit needs to receive information on whether to activate heating or cooling and at what power level. In a standard air conditioner setup, this is handled by the control unit built into the indoor unit. When modifying the air conditioner into a heat pump, it is possible to use this control unit, but it comes with certain limitations and challenges. Therefore, the MiniHPC controller was created to address and resolve these issues.

MiniHPC communicates directly with the outdoor unit, making it unaware that something other than a standard indoor unit is connected on the other end of the communication cable.

MiniHPC performs the following tasks:

• Communicates with the outdoor unit and simulates a connected standard indoor unit
• Regulates the power needed to reach and maintain the set heating water temperature
• Activates the circulation pump in the heating circuit
• Switches the 3-way valve when there is a request to heat DHW
• Switches the 3-way valve when there is a request for ceiling cooling
• Activates supplementary DHW heating via a coil in the tank when a higher temperature is required (above 50°C), e.g., for the anti-legionella function
• Activates supplementary heating for the building when the air conditioner's capacity is insufficient due to extremely low outdoor temperatures
• Measures and monitors water flow in the heating system, calculating heating output and COP based on the flow rate and temperatures
• Displays diagnostic data from the outdoor unit: compressor frequency, fan speed, current, discharge temperatures, suction temperatures, outdoor temperature, etc.
• Displays and records statistics on consumed electrical energy and generated heat
• Provides protection against heat exchanger freezing, prevents 3-way valves and circulation pump from seizing due to prolonged inactivity
• Communicates over Wi-Fi with a local automation system (e.g., HomeAssistant, MQTT server, JSON commands) or with the Cloud over the internet, allowing remote management and online data monitoring