Hydronic System Imbalance and Waterlogged Expansion Tank
A new 5-story office building has a primary-secondary hydronic heating system. The primary pump is rated 400 GPM at 90 ft head; the secondary variable-speed pump serves three zones through a reverse-return header. Design ∆T is 20°F. At full load, the boiler supply is 180°F and return is 150°F. Zone 1 (nearest) is 78°F, zone 3 (farthest) is 65°F. The secondary pump runs at 100% and the VFD never modulates. The air separator is on the suction side of the pump. Differential pressure at the farthest zone is 4 psi; design is 12 psi. The balancing valves at zone 1 are wide open. You notice the expansion tank is waterlogged and the system pressure gauge reads 18 psi cold, 26 psi hot. Determine causes and balancing procedure.
1. Interpret the readings. What do the supply/return temperatures, zone temperatures, and pump operation tell you about the system?
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The 30°F system ∆T (180°F supply, 150°F return) is higher than the 20°F design, which means the system is moving less water than design for the load, not more. But zone 1 is overheated while zone 3 is cold; that is a distribution imbalance, not a global underflow. The far-zone differential pressure of 4 psi versus 12 psi design shows the pump is not producing enough differential at the critical zone. Because the VFD never modulates with the pump at 100%, the DP sensor is probably at the pump discharge or the VFD is in hand. The waterlogged expansion tank explains the 18-to-26 psi pressure swing, which can draw air in and cause pump cavitation. The wide-open zone 1 valve steals flow from zone 3. Root causes are failed expansion tank/air control, incorrect DP sensor location or VFD control, and no balancing.
2. What diagnostic sequence and measurements would you use to confirm the root causes?
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First, check the expansion tank air charge and bladder; drain and recharge or replace it. Second, verify cold fill pressure and purge air at high points and at the air separator. Third, measure pump suction and discharge pressure and compare with the pump curve; calculate flow from the curve or use a flow meter. Fourth, measure differential pressure across each zone coil and at the far zone, and compare to design. Fifth, confirm where the VFD differential-pressure sensor is installed and whether the controller is in auto. Sixth, record temperatures at each zone to calculate zone ∆T. Do not adjust balancing valves until the tank and air are corrected, because pressure swings make any balance unstable.
3. Describe the balancing and repair steps in the correct order.
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Repair and balance in this order: replace or recharge the expansion tank; purge air; relocate the DP sensor to the far zone, or set the VFD to maintain 12 psi at the critical zone; verify the pump curve and trim the impeller or set a maximum speed if the pump is oversized; then balance. Start with the farthest zone valve wide open, then progressively throttle the nearer zones until each zone meets design flow and design ∆T. Use a balancing meter or read coil differential pressure. Recheck system ∆T at full load; it should approach 20°F. Finally, verify the VFD modulates as zone valves close and that the boiler return temperature stays above minimum to avoid flue-gas condensation.