Options to compare:

Nest
Honeywell (Resideo)
Ecobee
Emerson
Johnson
Lux

Pain Points:
Tired of heating your empty house? You can remotely control a smart thermostat from work
Smart thermostats only heat the rooms you actually use during winter months, saving money and the environment
Voice controls allow people with dexterity issues to control their HVAC

Features:
Geofencing – turning off when you leave, turning on when you return
Individual room control – occupancy sensors and door sensors
Humidity and air quality monitoring – as important as temperature for comfort

Ads:
Smart Thermostats
The average homeowner spends thousands of dollars every month on their HVAC system. Think about it: traditional thermostats run on a rigid cycle every day, disregarding whether people are home, heating rooms no one enters or ignoring temperature differentials, and disrupting the air quality and humidity factors of a house. They are essentially on-off switches that cannot detect variation between rooms or make cost-effective decisions without constant monitoring. However, smart thermostats allow homeowners to outsource their time to a computer, one that can make optimal decisions even when people are busy at work or sleeping. Using a wide variety of sensors and machine learning, along with simple apps that allow customization and programming, smart thermostats can potentially save thousands of dollars over the course of a year. Additionally, the Energy Star program reports that widespread usage of smart thermostats could save the planet 13 billion pounds of annual greenhouse gas emissions.

https://www.energystar.gov/sites/default/files/asset/document/SmartThermostat_FactSheet.pdf

The capability of smart thermostats to detect whether people are at home, heat and cool certain rooms independently, and adjust other factors such as humidity and air quality all contribute to a healthy lifestyle and a cleaner planet. They’re so efficient that many electric companies will actually subsidize the cost of purchasing a smart thermostat by as much as $50!

But amid the deluge of smart thermostat products available today, how can you determine which one best suits your HVAC system and personal preferences? Whether you have a single-stage gas furnace or in-floor electric heating, there’s a smart thermostat specifically designed for you. Let’s take a look.

BIG HIGH LEVEL MEGA COMPARISON CHART
https://docs.google.com/spreadsheets/d/1n4SLhYc7lsPBxJRMIkh2evt0In7BMYqZ9bhFky-Awf4/edit?usp=sharing
Ecosystem

Smart thermostats, unlike traditional thermostats, derive most of their usefulness from their connectivity to other components of a smart home system. In practice, there are really 3 big ecosystems that are compatible with most devices: Google Assistant, Amazon Alexa, and Apple HomeKit. Of the three, Amazon’s ecosystem is the most widespread (with over 10,000 compatible devices) and all of the thermostats we recommend are compatible with it. Apple’s HomeKit is more specialized and only the Lux KONO, Ecobee, Sensi, and Mysa thermostats integrate with it. If you have a lot of Apple devices, you should probably consider buying a HomeKit compatible thermostat, but otherwise most devices you buy will work with Amazon and Google.
Power

Unlike a computer, most smart thermostats are powered via a C-wire. This wire is present in most homes, but you should check your existing thermostat to see whether it’s present. It will be plugged into the C port on the back of your thermostat (turn off your electricity before checking!). If it’s not there, you have the option of installing it, calling a professional, or buying a smart thermostat that has an adapter (Honeywell), an independent power brick (Lux GEO-WH) or can be operated via battery power. Operating via battery usually disables some of the more advanced features of smart thermostats that allow it (such as Nest and Sensi), but it’s a decent option if you want to replace your thermostat right away without waiting for a technician to install a C-wire.

I have an electric in-floor or baseboard HVAC system
You’ll want to look at the two Mysa Smart Thermometer options depending on whether you have an in-floor (link) or baseboard (link) system. These thermostats are compatible with the big 3 smart home ecosystems (Google Nest, Amazon Alexa, and Apple HomeKit) in addition to their useful battery backup and 3 year warranty.

I just want the best budget option
For the economical homeowner, the Sensi ST55 or Lux GEO-WH smart thermostats will be strong options. Both work the Nest and Alexa, but only the Sensi is compatible with Apple’s HomeKit and Samsung’s SmartThings. Finally, the Lux GEO-WH is able to be powered via MicroUSB or the Lux Power Bridge if your home doesn’t have a C-wire.

I want to control specific rooms of my home
The Nest Thermostat E, Learning Thermostat, Ecobee ecobee3 lite, and Ecobee SmartThermostat all have the capability to control temperature in specific rooms. For the Ecobee thermostats, you’ll need to purchase a SmartSensor ($79) peripheral for each room. The Ecobee SmartThermostat comes with one SmartSensor already in the box. For the Nest thermostats, you’ll need a Nest Temperature Sensor ($39) for each room.

I want a thermostat that connects to Cortana
Then the Honeywell T9 Smart Thermostat is your best option. None of the other thermostats we’ve checked can integrate with Cortana and Microsoft’s smart home ecosystem. It has a decent 2-year warranty and humidity control capabilities.

What are the different types of HVAC systems?
Heating Elements
Staging
Gas (Forced Air or Gravity)
Oil (Forced Air or Gravity)
Electric (Forced Air or Gravity)
Heat Pump (Radiant)
In-Floor
Baseboard/Radiator
Cooling Elements
Humidity
Fans/Ventilation