blockFreezeProtection
Freeze protection sequence for multizone air handling unit
Information
Freeze protection sequence for multizone AHU system. It is developed based on Section 5.16.12 of ASHRAE Guideline 36, May 2020.
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If the supply air temperature
TAirSupdrops 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. -
If the supply air temperature
TAirSupdrops 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.
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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.)
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.OutdoorAirSection | minOADes | Buildings.Controls.OBC.ASHRAE.G36.Types.OutdoorAirSection.DedicatedDampersAirflow | Design of minimum outdoor air and economizer function |
| 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 |
| Buildings.Controls.OBC.ASHRAE.G36.Types.CoolingCoil | cooCoi | Buildings.Controls.OBC.ASHRAE.G36.Types.CoolingCoil.WaterBased | Cooling 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. -
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.