A circuit which is labeled "N/O" means that whatever is connected to it will not run unless the relay is energized. In other words, your fan is also set up on a N/O relay switch. It closes when the fan relay is energized.
Placing a device on the N/O and the incoming hot lead on COM will assure that the device receives power when the realy is on. The other leg of your device goes to Neutral or L2, depending on voltage. I have no idea what a HRV is.