One benefit of hydronic heating is the relative ease of dividing the distribution system into several independently controlled zones. The goal is control--being able to send heat precisely when and where it's needed. Electrically operated zone valves are commonly used to determine which circuits are active at any given time.
Although distribution systems zoned with valves have a well-established track record, they also have the potential to interact with the system's circulator, causing undesirable side effects. The greater the number of zones, and the more powerful the circulator, the greater the likelihood of problems.