AirCompare performs …

two main functions:

1) Compares your indoor conditions to your local weather and reports which air, indoor or outdoor, is more efficient to have in your house.
2) Monitors your electric rate and suggests a thermostat setting to save money while respecting your comfort preferences.

Both recommendations, to open or close the windows and the temperature
target for your thermostat, appear on the Main opening screen. You can then manually adjust your home HVAC system or let AirCompare control HomeKit-enabled fans/windows and thermostats to automate these adjustments. (A HomeKit hub such as an AppleTV or iPad is required for out-of-home-network control of HomeKit appliances. Some HomeKit appliances can be controlled remotely without a hub using their proprietary apps, but Air Compare requires the hub.)

AirCompare is not an interactive app, like a game or web browser. It’s an automation app and is designed to do as much as possible without user interaction. It is intended to be running all the time in the background and uses minimal resources. Once your settings are made and preferences are dialed in, the app will operate with very little additional input.

However it has limits. AirCompare cannot do anything if it’s not running, for instance if you force-quit it by swiping it away after a double-click of the Home button. While running in the background, it will periodically (at times dictated by iOS, details below) check for fresh data from the internet and will send you notifications of noteworthy changes. The app must be brought forward in order to report suggested settings, and to make any changes to your HVAC settings over HomeKit.

So start it, set it, and forget it.