You can obtain a maximum of 20 Heat Points by doing the following actions.

Carjacking by Aiden scores 1 Heat Point.
Civilian casualties scores 2 Heat Points per civilian.
Helicopter destroyed scores 2 Heat Points.
Keep your helicopter within sight of your target to give the sharpshooter a chance to shoot. Each hit will add to your Heat Point total.
Police cars totaled scores 1 Heat Point per car.
Police casualties scores 2 Heat Points per officer.
Police takedowns scores 2 Heat Points per officer.
Sniper killed scores 2 Heat Points.
Targeting the spotlight on Aiden scores 1 Heat Point every four seconds (or every three seconds if the Boiling Point Skill is unlocked).
ctOS incidents triggered by other players will increase your Heat Points. You can help this process by triggering ctOS devices to force other players to commit illegal acts. For example, having other players harm pedestrians. Each successful ctOS attack adds Heat Points to your total.