I'm considering an A-to-A Heat Pump, small 2.3-3kW for on-demand heat.
The metal piping introduces a potential from outside (extraneous conductor), but unless the HVAC external unit is in contact with the ground OR 500V resistance is
I think your reading is correct - however you will have bonding by default via its supply CPC presumably anyway. (assuming the heat pump is not class II and the ducting is not isolated from it)
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