A lot of smart home gadgets are little more than internet-connected versions of the "dumb" things we already had, but Google's Nest thermostats push the envelope with features like automatic scheduling, geofencing, and various weather-aware energy-saving options. Now, Google is testing a feature that could warn you before a small issue in your HVAC system becomes a major emergency. It's rolling out now, and you probably already have it enabled.
The new "HVAC alerts" can detect potential issues related to your heating, cooling, and ventilation systems thanks to the wealth of historic data Nest thermostats already collect. For example, you might not notice if your house takes slightly longer to heat, but the thermostat can combine weather data with furnace performance to (potentially) spot problems. This isn't a magic bullet, though. HVAC alerts can't tell you if some previously functional component will catastrophically fail, but maybe it'll give you a heads up when parts begin to wear out.
HVAC alerts are bundled with the Nest Home Report feature, which is on by default. If you've got that enabled, you'll get HVAC alerts via email if something looks suspicious to the Nest. The emails will include a very basic description of the problem, along with a link to book an appointment with an HVAC specialist via Handy (in select areas). There will also be a link to opt-out of future HVAC alerts. Google is clear this is just a test, but HVAC alerts could be a real lifesaver if they work.