Some larger homes use multi-zone HVAC systems, which typically means they employ multiple furnaces, with each unit responsible for a section of the home. In these cases, each HVAC unit is controlled by its own thermostat. If you have multiple thermostats in your home, that’s a multi-zone system. If that’s the case, keep in mind that ecobee thermostats can only control one zone, so you would need to buy one to replace each of your existing thermostats.