I'm replacing my old thermostat due to issues with it somehow not staying at my set temperature. It's a Simple Comfort 2001, and I'm replacing it with a newer model, a Simple Clean 2001L. Both of them are programmable thermostats. I live in a 2-story house with one HVAC unit outside of the house for the ground floor, and one unit in the attic for the second floor. In order to change out the thermostat, I killed power in my main breaker, shutting off power to everything. However, even though the HVAC unit was off, I noticed that the thermostat was still displaying the set temperature of 45 degrees. It was also the same with the downstairs thermostat. This shouldn't be possible. They don't run on batteries, so why would the thermostat still be displaying? Is there a second switch in my HVAC unit that I should turn off? I don't want to needlessly try to switch it out in a situation like this and get electrocuted, so I wanted to ask about it first.
One thing to mention is the house. It was built in 2004 I believe, and the original thermostats are still here. It's not one of those systems built in the '90s, so I'd say it's relatively modern as it could be. It runs on gas, not electric, so would it be strange if there was a second breaker that I would have to turn off in order to make sure that I can change out the thermostat?