NOTE: The original ecobee3 isn't HomeKit compatible. Make sure that if you decide to purchase the ecobee3 that the product has the HomeKit compatible badge.
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Pro Satellite sensor provides extended sensing range
Instead of a single point of sensing which can be influenced based on its location, the ecobee3 also utilizes a satellite sensor. This satellite sensor provides another perspective on the house, and between the two will provide smarter heating and cooling.
For example, if the main thermostat is located near a heat source (kitchen, full living room, etc) or cold source (near a door in winter) it can be influenced by temporary temperature fluctuations. The satellite sensor can help detect these temporary changes, and won't over-heat (or over-cool) the house when it isn't actually required.
The range on this extra sensor is about 45 feet, which should be plenty for most houses.
Pro Important information is displayed at a glance
The display shows the current temperature, as well as which mode you're in (off, auto, cooling or heating), as well as humidity and the current weather. There is also a temperature slider handy for quick adjustments.
Pro Full web support allows monitoring and adjusting from anywhere
In addition to the phone app, you can access the web portal for (almost) full control of the system. This is very handy if your phone's battery dies, as you can login from any internet connected device.
Pro Straightforward install for 5 wire systems
If you already have the common wire (ie your system uses 5 wires), the installation is very quick and easy.
Pro Integrates with numerous other systems
In addition to HomeKit, the ecobee3 can hook up with IFTTT (IF This Then That - an automation tool based on custom recopies), Alexa (Amazon's voice activated personal assistant found in it's Echo line of products), Wink (a stand-alone ecosystem) as well as SmartThings (Samsung's smart home ecosystem). This allows integration with a wider variety of smart home products outside of the HomeKit ecosystem.
Con Installation can be tricky for 4 wire systems
If your system lacks the common (C) wire and is only a 4 wire system, you will need to use the included Power Extender Kit (5-wire systems don't require this). Then, additional steps are required depending if your HVAC system has a controller board or not.
Con The external sensor can lose connection
The satellite sensor is handy, but it adds another connection point which can disconnect (and based on user reviews, it can do so frequently even when installed within 5 meters of the main thermostat).
Con Extra satellite sensors cost extra
One satellite sensor is included in the box, but if you want more you'll have to buy them. They come in packs of 2, and are sold at $79. It would also be convenient if you could buy just 1 additional sensor as well, but packs of 2 seem to be the only way they come.