A plumber is commisioning a new hydronic heating system with multiple zones. Upon initial startup, the plumber observes that one of the zones is not heating adequately, even though the zone valve is open and the circulator pump is running. The P/T gauges indicate flow through the loop. What is the most likely cause of this issue?

Question: A plumber is commisioning a new hydronic heating system with multiple zones. Upon initial startup, the plumber observes that one of the zones is not heating adequately, even though the zone valve is open and the circulator pump is running. The P/T gauges indicate flow through the loop. What is the most likely cause of this issue?

Answer options: ✅ Air trapped in the heating loop for that zone.

• The system pressure is too high, preventing proper flow.
• The thermostat for that zone is faulty.
• The boiler's high-limit control is set too low.

Correct answer: Air trapped in the heating loop for that zone.

Explanation: Air trapped in a hydronic loop is a common cause of inadequate heating, as it can create air locks that impede proper water circulation even when pumps are running and zone valves are open. While a faulty thermostat could prevent a call for heat, the question states the zone valve is open and the pump is running, indicating a call for heat is present. High system pressure might affect pump performance in extreme cases, but less likely to completely stop heating a single zone with indicated flow. A low high-limit setting would affect all zones, not typically isolate one.