blockFreezeProtection
Freeze protection sequence for single zone air handling unit
Information
Freeze protection sequence for single zone AHU system. It is developed based on Section 5.18.11 of ASHRAE Guideline 36, May 2020.
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If the supply air temperature
TAirSupdrops below 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. -
If the supply air temperature
TAirSupdrops below 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.
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Upon signal from the freeze-stat (if installed),
or if supply air temperature drops below 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.)
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Boolean | have_frePro | true | True: enable freeze protection |
| Buildings.Controls.OBC.ASHRAE.G36.Types.PressureControl | buiPreCon | Buildings.Controls.OBC.ASHRAE.G36.Types.PressureControl.ReliefDamper | Type of building pressure control system |
| Buildings.Controls.OBC.ASHRAE.G36.Types.FreezeStat | freSta | Buildings.Controls.OBC.ASHRAE.G36.Types.FreezeStat.No_freeze_stat | Type of freeze stat |
| Buildings.Controls.OBC.ASHRAE.G36.Types.HeatingCoil | heaCoi | Buildings.Controls.OBC.ASHRAE.G36.Types.HeatingCoil.WaterBased | Heating coil type |
| Integer | minHotWatReq | 2 | Minimum heating hot-water plant request to active the heating plant |
| Heating coil controller | |||
| Buildings.Controls.OBC.CDL.Types.SimpleController | heaCoiCon | Buildings.Controls.OBC.CDL.Types.SimpleController.PI | Heating coil controller |
| Real | k | 1 | Gain of coil controller |
| Real | Ti | 0.5 | Time constant of integrator block |
| Real | Td | 0.1 | Time constant of derivative block |
| Real | yMax | 1 | Upper limit of output |
| Real | yMin | 0 | Lower limit of output |
| Advanced | |||
| Real | THys | 0.25 | Hysteresis for checking temperature difference |
Connectors
Revisions
-
September 26, 2025, by Jianjun Hu:
Renamed the parameterThystoTHys.
This is for issue 4374. -
March 1, 2023, by Michael Wetter:
Changed constants from0to0.0and1to1.0.
This is for #3267. -
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.