No, Nest thermostats are not inherently bad for your HVAC system, provided they are wired correctly and installed with adequate power supply. However, if a Nest thermostat is installed on an older heating and cooling system without a dedicated C-wire, it attempts to "power steal" electricity by cycling the furnace transformer rapidly. This improper power-stealing can occasionally cause HVAC relays to chatter, short-cycle the compressor, or trigger unexpected system shutdowns. Verification: Ensure a dedicated C-wire is connected to the 'C' terminal on both your furnace control board and your Nest thermostat subbase.
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