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I have a bog standard HVAC system with a Nest thermostat. When I tell the theromostat to turn the fan on for 15 minutes the A/C compressor also kicks on. Is this normal?
This is not normal. When the thermostat energizes the G terminal, only the blower fan should come on.
Check for a jumper/short between Y and G either at the thermostat, or the air handler.
Required, but never shown