Efficiency in HVAC systems increased significantly with new semiconductor technology. 

Phase-based inverter technology took the role of the mechanical relays.

• New equipment vulnerability to voltage fluctuations has increased; surge protection must be installed. 
• A power spike will wipe out any new contemporary variable-speed air conditioner; it would not harm an air conditioner from the 1970s. 
• All equipment of this age is exposed to brief overvoltages; any one of the five types of surge-sensitive electronics included in modern HVAC systems will result in total system failure without surge protection.