You are troubleshooting a single-zone hydronic heating loop in a residential home. The thermostat is calling for heat, the boiler is firing, and the circulator pump is running, but the radiators at the end of the loop remain cold. The supply temperature from the boiler is 60°C, but the return temperature is only slightly cooler at 55°C. What is the most probable cause?

Question: You are troubleshooting a single-zone hydronic heating loop in a residential home. The thermostat is calling for heat, the boiler is firing, and the circulator pump is running, but the radiators at the end of the loop remain cold. The supply temperature from the boiler is 60°C, but the return temperature is only slightly cooler at 55°C. What is the most probable cause?

Answer options: ✅ Insufficient flow through the loop, likely due to air in the system or a closed valve.

• The boiler's high-limit temperature setting is too low.
• The pressure relief valve on the hydronic system is leaking.
• The expansion tank pre-charge pressure is incorrect.

Correct answer: Insufficient flow through the loop, likely due to air in the system or a closed valve.

Explanation: A small temperature differential between supply and return (5°C in this case) combined with cold radiators suggests that although the pump is running, there isn't enough effective flow through the radiators to transfer heat. Air binding or a closed isolation valve are common culprits for insufficient flow. A low boiler high-limit wouldn't explain why the return is almost as hot as the supply. A leaking relief valve or incorrect expansion tank pre-charge relate to pressure management, not flow through the heat emitters.