If a hydronic terminal unit will not heat, what could be a possible issue?

A hydronic terminal unit that is not heating could indeed be air-bound, which means that air has trapped in the system, preventing water from circulating properly. In a hydronic heating system, water needs to flow through the unit to transfer heat effectively. If air gets trapped in the piping or the terminal unit itself, it blocks the water flow, leading to inadequate heating or no heating at all.

When air-bound, the system may try to activate, but since the hot water cannot reach the terminal unit, it will not warm up. This is a common issue in hydronic systems, especially when they are first filled or if there are leaks which may cause the water level to drop, allowing air to enter the system.

In this scenario, identifying that the unit may be air-bound helps focus on a specific solution, such as bleeding the air from the system to restore proper heat circulation. Addressing air in the system is a vital part of maintaining hydronic systems to ensure they operate efficiently and effectively.