You may set your thermostat to 70°F/21ºC, but find that it’s 69°F/20.5ºC inside and the heat isn't turning on. The Zen Thermostat is designed to control temperature within a range of about +/- 1.5°F, which is common to most thermostats and is done to ensure that your heating or cooling equipment isn’t over-stressed by frequently turning on and off when the temperature is very close to the target. This is referred to as a maintenance band and it prevents excess wear and tear to your equipment which can reduce the HVAC system's life.
The Zen Thermostat won’t turn the air conditioning or heat on every time the temperature in the room changes slightly, as this would result in your system turning on and off every few seconds. This would be inefficient, noisy, and uncomfortable in the long run - not to mention expensive.
This maintenance band means that the Zen Thermostat will turn your AC on when it is about 1.5°F away from the set point, and will run your system until the temperature is slightly beyond that set point.
So what happens if you notice the band is more than 1.5°F? This most likely means that your system has a delay built into it, so it takes a few minutes to start cooling or heating after the command is sent. This is intended to protect the equipment and allow refrigerant to cycle through the system properly.
In virtually every home, the difference between the temperature in one room and the next, or between the temperature at head height and at the floor is over 2ºF. The Zen Thermostat balances between heating your home to a comfortable temperature and cycling your system too often.