Smart Thermostats
There are many different types of smart thermostats.
The Nest thermostat is popular (due to its sleek appearance at ~$350 unfitted). Alternatives start from $180 and are wired directly to your existing air conditioning system by a professional.
WiFi Controllers
Wifi controllers are used when a smart thermostat isn't required. A number of major brands produce Wifi controllers that can be purchased and installed by heating/cooling specialists. This leaves customers with their original wall mounted thermostat and adds the convenience of control via Siri or HomeKit.
HomeKit interface
Whether you choose a smart thermostat or go the Wifi controller route, HomeKit™ renders the thermostat to look like a stylish dial that can be controlled by your finger, or alternatively by voice. "Hey Siri, set thermostat to 30 degrees".
