I was wondering if you have had to upgrade the 240 Volt circuits serving heat pump systems from 3 wire to 4 wire even though the circuit had the required capacity. The 240v circuits in our house, built in 1983 (there are many) are 3 wire, but apparently the standard changed a number of years ago to require 4 wire. When we had our oven replaced, we were required to replace the 3 wire 240v with a 4 wire 240v with a separate ground and neutral wire. The new circuit actually has less amperage than the old one. On the other hand, when we replaced our electric water heater, they just hooked it up to the existing wiring.
I think part of the answer is when the device includes electronics that work off 110 volts split within the new ovens, there is an imbalance in the hot wires and current would flow through the bare wire which is not allowed. Older equipment used 240v only. What is your experience?