You are commissioning a new radiant floor heating system in a residential basement. After filling and purging the system, you notice that the pressure gauge reads 100 kPa (15 psi) when the system is cold, but climbs to 240 kPa (35 psi) when the system reaches its operating temperature of 45°C. The system is equipped with an air separator and pressure relief valve. What action should you take?

Question: You are commissioning a new radiant floor heating system in a residential basement. After filling and purging the system, you notice that the pressure gauge reads 100 kPa (15 psi) when the system is cold, but climbs to 240 kPa (35 psi) when the system reaches its operating temperature of 45°C. The system is equipped with an air separator and pressure relief valve. What action should you take?

Answer options: ✅ Check the pre-charge pressure of the hydronic expansion tank.

• Bleed more air from the highest points in the system.
• Adjust the boiler's high-limit temperature setting.
• Increase the flow rate of the circulator pump.

Correct answer: Check the pre-charge pressure of the hydronic expansion tank.

Explanation: A significant increase in system pressure from cold to hot indicates that the expansion tank is not properly accommodating the thermal expansion of the water. The most common cause is an incorrect or lost pre-charge pressure in the diaphragm-style expansion tank. Adding more air would not solve the pressure change issue directly. Adjusting the boiler temperature or pump flow would not address the underlying expansion issue.