Heating Coil Freeze Failure: Sequence and Freezestat Protection
AHU-8 is a 12,000-cfm built-up unit with a hot-water heating coil, outdoor and return air dampers, and a factory-mounted freezestat with a 6-ft capillary on the leaving-air face. The unit serves a lab and runs 24/7. In February, the coil froze and split during a weekend. The BAS trend shows the supply fan off for 3 hours due to a VFD fault, the outdoor-air damper command at 0% but the damper blade indicator at 35% open, the hot-water control valve at 0%, and the outdoor air at 8°F. The freezestat did not trip. The low-limit thermostat setpoint is 38°F, and its sensing bulb is in the duct 6 ft downstream of the coil. The owner wants the sequence corrected and a test to prove it will not happen again.
1. Determine the failure sequence and explain why the freezestat did not protect the coil.
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The VFD fault stopped the supply fan, but the outdoor-air damper did not close: it was commanded to 0% while the blade was at 35%. Cold air continued to enter the unit. The hot-water valve was at 0%, so no heat was available. With no airflow, the coil face froze locally. The freezestat did not trip because its sensing bulb is 6 ft downstream in the duct; with the fan off, stagnant air there did not drop to 38°F before the coil froze. The freezestat was protecting the duct, not the coil. A manual-reset device would also require investigation if it had tripped, because a trip without alarm or fan shutdown would not prevent a repeat.
2. Recommend changes to the control sequence and physical installation.
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Install a freezestat with its capillary on the leaving face of the coil in the coldest area, with a setpoint not lower than 35°F. Hardwire it to stop the fan, close the outdoor-air damper, open the hot-water valve, and alarm. Add an auxiliary switch on the outdoor-air damper to prove closed before fan start and to stop the fan if the damper opens unexpectedly. Use a spring-return damper actuator and verify it closes on loss of power. Add low-limit control and a hot-water pump status interlock. Do not rely on the BAS alone for freeze protection, and verify that the hot-water coil has adequate antifreeze protection or drain-down capability.
3. Design a cold-weather functional test to prove the revised protection works.
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With outdoor air below 35°F or with a calibrated sensor simulator, run the fan, then trip the VFD or fan off. Confirm that the outdoor-air damper closes within 30 seconds, the hot-water valve opens, and the freezestat alarm annunciates. Simulate a freezestat trip by cooling the capillary and verify fan stop, damper closure, and valve opening. Simulate power loss and verify spring return. Record coil face temperatures and trend for 24 hours. Acceptance: no negative coil temperature, damper proven closed, alarm logged, and automatic restart only after manual reset and inspection. Document the test with time-stamped trends.