By: Scott Lance, EBCP, LEED AP O+M, Senior Project Engineer Horizon Engineering Associates, LLP
Typically, the end of the winter season is the worst time of year in relation to freeze-stat trips. Late February and March have days with large outside air temperature swings, which causes the load of hot water systems to change throughout the day. Also, this time of years extreme cold temperatures at night are usually when these trips may happen. Which causes night calls for after-hour services by the building owner trades groups or perhaps a third party vendor.
Two main reasons for freeze-stat trips and how to solve this issue:
Issue #1: Traditional sequence of operations state to open the control valve for the heating coil when the system is in an unoccupied mode (found in systems that shut down during unoccupied hours). The issue is that the inside of the AHU becomes hotter than required during regular operation when the unit comes back online in the morning for normal operation. The control PID Loop controlling to space, or discharge air temperature, sees this high temperature and reacts accordingly – even though this is a false load. The solution is to control the AHU to an internal setpoint using the mixed air, or supply air, sensor inside the AHU to a setpoint of about 60F. This will not cause the PID control loop to quickly close the control valve on the heating coil allowing the system to best react to the change of bringing in outside air for ventilation purposes. If you continue to have an issue, the addition of a ramp timer to the mixed air dampers may help. The ramp timer will slow down the influx of outside air during start-up and allow the control loop to properly respond. 10 minutes is a good starting point for the initial setup of a ramp timer.
Issue #2: Freeze stat trips during normal operation. This is commonly seen in systems that operate 24/7 (typically for labs and clean rooms). The common case for this issue is that the hot water, or glycol, source is actually too hot. If the heating source to a coil is too hot, the AHU will require less flow at a higher temperature to maintain discharge air temperature. This keeps the coil from flooding. Low flow and high temperature is causing the return water exiting the coil to be colder than typically designed, causing an issue known as stratification. The solution to this is lowering the heating source setpoint. The lower temperature will allow for higher flow to maintain discharge temperature. The higher flow results in better flooding of the coil which leads to the whole surface of the coil to be more evenly heated from supply to return connections. This greatly reduces the effects of stratification. There are many ways to figure out what setpoint is optimal. You can bring the setpoint down manually and verify coil performance and then reprogram the outside air vs. heating source setpoint control tables in the BMS programming. Another solution includes an automatic way; by having the BMS system determine maximum heating valve position on the system and lowering heating source setpoint to maintain the maximum control valve at 80% open. However, there is a manual way that is the easiest to perform by staff that may not be very BMS savvy. The automatic solution will constantly change to conditions and auto-correct itself continuously. The automatic method is also used as an energy savings strategy, BMS contractors refer to this as a demand based heating solution. The interesting thing about these strategies is that energy consumption during normal operation does not change. If the system maintains a 55F discharge setpoint with 180F water at low flow, it requires the same energy to maintain the same discharge setpoint at 160F water at a higher flow. All you are truly doing is finding the best operating condition for your system.