Find the disconnect switch to your air handler or furnace, and switch it off. This will be located near the unit. Shut off the circuit breaker at your home’s electrical panel if you can’t find the ...

Is it because the old twist-knob was a "4-wire" thermostat that cuts power completely while the new RLV3120A is a "2-wire" thermostat that doesn't? I'm an electrical engineer but this is my first time ...