I have a drawer full of these, so here’s how they differ in practice.
HC-05: classic Bluetooth with the serial port profile, can be master or slave, configured with AT commands at 38400 when you hold the KEY/EN pin high at power-up. HC-06: classic SPP too, slave only, fewer AT commands, default 9600. HM-10: Bluetooth Low Energy, the “serial” data goes through characteristic FFE1. Lower power, but the Pi side has to speak BLE.
All three have 3.3 V logic on RX. Their breakout boards take 5 V on VCC, but put a divider on the Uno’s TX line (1k from the Uno, 2k to ground) before it goes into the module’s RX.
For your case the HC-06 is the least work. On the Pi:
sudo rfcomm bind 0 98:D3:31:F5:A2:10 1 # HC-06 address, channel 1
# /dev/rfcomm0 now behaves like a serial port
python3 -c "import serial; s=serial.Serial('/dev/rfcomm0', 9600); print(s.readline())"Uno side, one line per reading so the Pi can split it easily:
Serial.print("flow_c="); Serial.print(tFlow, 2);
Serial.print(";return_c="); Serial.println(tReturn, 2);If the Pi is already your gateway to Hermetry, report the heat pump as its own child device on v1/gw/telemetry rather than adding the keys to the Pi itself, then it gets its own page and alarms.