The $250 Ecobee3 smart thermostat is available now through the Apple Store. Technically, Ecobee isn't the only thermostat that works with HomeKit, though it's the only one that doesn't require a separate hub device, such as Insteon's thermostat and HomeKit hub.
Like other HomeKit-enabled products, Ecobee3 users can control the thermostat using Siri voice commands. HomeKit can also group multiple actions into "scenes," for instance letting users turn down the heat and switch off connected lighting with a single command. With an Apple TV as a hub, users can control HomeKit devices with Siri from outside the house as well.
Aside from HomeKit support, the new Ecobee3 is identical to the existing version. It has a touch display on the device itself, and offers companion apps for controlling the temperature from a phone, tablet, computer, or Apple Watch. Ecobee also comes with a remote motion and temperature sensor, which adjusts the home's temperature based on which rooms are occupied. (Additional sensors cost $79 each.)
Unfortunately for existing Ecobee users, there's currently no way to add HomeKit support to previous models. A FAQ on Ecobee's website notes that HomeKit requires specific hardware, so upgrading existing models "is not something that is possible with a firmware or software patch." For those with no interest in Apple's platform, Ecobee is selling the non-HomeKit model at a $20 discount.
Why this matters: Thermostats are a good starting point for smart homes, as they don't require as much planning compared to a system of smart light bulbs, outlets, and switches. It's early days for HomeKit, with just a handful of devices supporting Apple's framework, but we're guessing the thermostat will be a popular way for people to get acquainted.