hciuart failing on a Pi 3 that also does Modbus on the serial header: I’d bet whoever set up the Modbus put dtoverlay=pi3-disable-bt in /boot/config.txt to get the good UART on GPIO14/15. That overlay switches the BT chip off completely, so hciuart has nothing to talk to.
You can have both. On Stretch use the other overlay, which gives BT the mini UART and keeps the PL011 on the header:
# /boot/config.txt
#dtoverlay=pi3-disable-bt
dtoverlay=pi3-miniuart-bt
core_freq=250core_freq=250 keeps the mini UART’s clock stable, BT gets flaky without it. Also make sure console=serial0,115200 is not in cmdline.txt. Reboot, then sudo systemctl restart hciuart and hciconfig should show hci0.
Your Modbus side keeps /dev/ttyAMA0, so the chiller code shouldn’t notice.