.Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.SetPoints.FreezeProtection

Information

Freeze protection sequence for multizone AHU system. It is developed based on Section 5.16.12 of ASHRAE Guideline 36, May 2020.

1. If the supply air temperature TAirSup drops below 4.4 °C (40 °F) for 5 minutes, send two (or more, as required to ensure that heating plant is active, minHotWatReq) heating hot-water plant requests, override the outdoor air damper to the minimum position, and modulate the heating coil to maintain a suppy air temperature of at least 6 °C (42 °F). Disable this function when supply air temperature rises above 7 °C (45 °F) for 5 minutes.
2. If the supply air temperature TAirSup drops below 3.3 °C (38 °F) for 5 minutes, fully close both the economizer damper and the minimum outdoor air damper for 1 hour and set a Level 3 alarm noting that minimum ventilation was interrupted. After 1 hour, the unit shall resume minimum outdoor air ventilation and enter the previous stage of freeze protection.
   • If it is warm enough that the supply air temperature rises above 7 °C (45 °F) with minimum ventilation, the unit will remain in Stage 1 freeze protection for 5 minutes then resume normal operation.
3. Upon signal from the freeze-stat (if installed), or if supply air temperature drops below 3.3 °C (38 °F) for 15 minutes or below 1 °C (34 °F) for 5 minutes, shut down supply and return (or relief) fan(s), close outdoor air damper, open the cooling-coil valve to 100%, and energize the CHW pump system. Also send two (or more, as required to ensure that heating plant is active, minHotWatReq) heating hot-water plant requests, modulate the heating coil to maintain the higher of the supply air temperature or the mixed air temperature at 27 °C (80 °F), and set a Level 2 alarm indicating the unit is shut down by freeze protection.
   • If a freeze-protection shutdown is triggered by a low air temperature sensor reading, it shall remain in effect until it is reset by a software switch from the operator's workstation. (If a freeze-stat with a physical reset switch is used instead, there shall be no software reset switch.)

Revisions

• April 6, 2024, by Michael Wetter:
  Corrected wrong annotation.
• September 18, 2023, by Jianjun Hu:
  Added 2-position relief damper position output.
  This is for issue 3526.
• December 22, 2022, by Jianjun Hu:
  Added flag to disable freeze protection.
  This is for issue 3139.
• July 15, 2021, by Jianjun Hu:
  First implementation.