The short:
​When you’re not actively using a room, you have the option to mark it as vacant. A room marked as vacant is controlled by your vacant room range as set in your Ecovent Settings in the app. If your room falls above this range while cooling, or below this range while heating, your HVAC will run to get this room into the temp range regardless of your other system settings.

We will try to keep vacant room vents closed whenever possible to limit wasted energy in the rooms you are not currently in, however sometimes we will need to open these vents to allow other vents to close to keep your occupied rooms comfortable.

The details:
​When you set a room to Vacant on the Room Details screen of the app, you are telling the Hub that you are not currently using this room. This will tell the Hub to close the vents in this room whenever possible to redirect conditioned air to your occupied rooms.

In some cases the vent(s) may open partially or fully in a room that is marked as Vacant due to the necessity of maintaining safe pressure levels in the ducts. This will occur when the system is attempting to get rooms that are occupied to their target temperature, but the Control Hub detects excessive pressure because too few rooms are calling for conditioned air.

A "Vacant" room can also trigger your HVAC system to activate if this room falls outside of your "Vacant temperature range" (previously know as the Away temperature range) as set in the Ecovent Settings. If the sensor in a room you have marked as vacant reads outside of the range, your HVAC system will run until the sensor in that room reads as within the set range.