If you have a erv central controller or a tstat that controls the erv you can control how you want the unit to run. If it is hot and humid out and cool and dry inside the heat will transfer from the incoming air to the out going air. If its 95 outside and 75 inside you'll prob get 80 degree incoming air. I'd suggest downloading the installation manual and reading it end to end. You really need to understand how it works so you dont sound foolish discussing the entire system with the owners when you are done.
The ERV should run independently of the air handler/A/C, otherwise the amount of ventilation would be determined by how hot or cold it is outdoors.
A desired ventilation rate can be accomplished by the manufacturer's timer control or a thermostat ventilation setting.
The vents should not be routed to the roof, otherwise hot air radiating off the roof and shingle offgassing would be brought inside through the intake.
Most ERVs will have some type of filter, but only the air handler/A/C filter will address air already inside the house as well as fresh air being brought in.
A misconception is that ERVs mix stale air with fresh air to transfer heat or cooling energy. In better ERVs, there is nearly a complete air stream separation. Sensible (heat) transfer generally takes place by conduction, while moisture is transferred by various means.
HVI (Home Ventilating Institute) provides the only independent rating for ERVs, which can range from @ 55-85% Apparent Sensible Effectiveness (HVI term). Latent (moisture) transfer varies from @ 30-60%.