Dew-point temperature setpoint for zone humidity control

design/FY2026/NFP-Dewpoint_Humidistat.md

@@ -0,0 +1,80 @@
+Dew-point Temperature Humidistat
+================
+
+**Jeremy Lerond, Pacific Northwest National Laboratory**
+
+ - Original Date: 03/03/2026
+ - Revision Date: 03/03/2026
+
+
+## Justification for New Feature ##
+
+Currently, EnergyPlus humidistats are used to control zone humidity using relative humidity (RH). However, in practice it is also common for thermostats/humidity controllers to control (or limit) humidity using dew-point temperature. In addition, standards such as ASHRAE 62.1 include requirements—under certain conditions—that the zone (space) dew-point temperature must not exceed a specified limit (see Addendum k to ASHRAE 62.1–2022).
+
+This feature will allow users to specify a maximum/minimum allowable space dew-point temperature (i.e., a dew-point setpoint) to trigger humidity control.
+
+While controlling humidity based on dew-point is currently possible in EnergyPlus using EMS programs, it requires having multiple programs (or a large one) to recalculate each zone's relative humidity setpoint based on the targeted dew-point temperature. This new feature will provide a more streamlined and user-friendly way to control humidity based on dew-point temperature.
+
+## E-mail and Conference Call Conclusions ##
+
+N/A
+
+## Overview ##
+
+The `ZoneControl:Humidistat` object uses both a humidifying and dehumidifying setpoint schedule based on RH. The new features will add a new `Control Variable` input field to specify what variable is described in the schedule. The value for this new input would be `RelativeHumidity` (current approach) and `Dew-point Temperature` (new approach).
+
+## Approach ##
+
+First, the existing schedule fields would be renamed as follows:
+- `Humidifying Relative Humidity Setpoint Schedule Name` -> `Humidifying Setpoint Schedule Name`
+- `Dehumidifying Relative Humidity Setpoint Schedule Name` -> `Dehumidifying Setpoint Schedule Name`
+
+Second, a new input field would be added to the `ZoneControl:Humidistat` object: `Control Variable`.
+
+Third, that control variable would be used to determine In `calcPredictedCorrector` the current schedule inputs are used to calculate `WZoneSetPoint` which is the zone humidity setpoint. The new schedules would be used to also calculate `WZoneSetpoint` but based on a dew-point temperature target rather than a relative humidity target. The user could mix/match schedules to control zone humidity, for instance, use a dew-point-based dehumidification schedule and a relative humidity-based humidification schedule.
+
+## Testing/Validation/Data Sources ##
+
+A unit test will be included to test the new feature.
+
+## Input Output Reference Documentation ##
+
+The following field description would be added to the input/output reference guide:
+
+```latex
+\paragraph{Field: Control Variable}\label{field-humidistat-control-variable}
+
+This field describes if the schedules describe relative humidity or dew-point temperature values.
+```
+
+## Input Description ##
+
+The following fields would be added to the `ZoneControl:Humidistat` object:
+
+```
+ZoneControl:Humidistat,
+[...]
+  A5 ; \field Control Variable
+       \note When using RelativeHumidity, the schedule values should be in percentages.
+       \type choice
+       \key RelativeHumidity
+       \key Dewpoint
+       \default RelativeHumidity
+```
+
+## Outputs Description ##
+
+No new outputs will be added.
+
+## Engineering Reference ##
+
+This new feature doesn't require any engineering reference changes.
+
+## Example File and Transition Changes ##
+
+This new feature doesn't require any transition changes.
+
+## References ##
+
+- ASHRAE Standard 62.1-2022 Addendum k, https://www.ashrae.org/file%20library/technical%20resources/standards%20and%20guidelines/standards%20errata/standards/62_1_2022_k_20240422.pdf
+- Lennox S40 Smart Thermostat, https://www.lennox.com/dA/6003ce4d6f/508223-03b.pdf

doc/input-output-reference/src/overview/group-zone-controls-thermostats.tex

@@ -748,7 +748,7 @@ \subsubsection{Inputs}\label{inputs-7-030}
 
 \subsection{ZoneControl:Humidistat}\label{zonecontrolhumidistat}
 
-The humidistat zone control object is used to control a zone to a single relative humidity setpoint schedule, or to dual humidity setpoint schedules (humidifying/ dehumidifying setpoints with deadband). The single setpoint humidistat requires single setpoint input only, and the dual-setpoint humidistat requires inputs of both humidifying and dehumidifying setpoints. The schedules consist of relative humidities, expressed as a percentage (0-100), to be used during the simulation for that zone's moisture prediction calculation. Only one humidistat control statement can be specified for each zone. For a single setpoint humidistat, if the zone relative humidity is below the control relative humidity setpoint and the equipment specified can humidify then that equipment will try and meet the zone's humidification load. The opposite is true if the zone relative humidity is above the control relative humidity setpoint and the equipment can dehumidify. For a dual setpoint humidistat, if the zone relative humidity is below the \textbf{humidifying} relative humidity setpoint and the equipment specified can humidify then that equipment will try and meet the zone's humidification load. The opposite is true if the zone relative humidity is above the \textbf{dehumidifying} relative humidity setpoint and the equipment can dehumidify.
+The humidistat zone control object is used to control a zone to a single relative humidity setpoint schedule, or to dual humidity setpoint schedules (humidifying / dehumidifying setpoints with deadband). The single setpoint humidistat requires single setpoint input only, and the dual-setpoint humidistat requires inputs of both humidifying and dehumidifying setpoints. The schedules consist of relative humidities or dew-point temperatures, expressed as a percentage (0-100) or dew-point temperature, to be used during the simulation for that zone's moisture prediction calculation. Only one humidistat control statement can be specified for each zone. For a single setpoint humidistat, if the zone relative humidity or dew-point temperature is below the control relative humidity or dew-point temperature setpoint and the equipment specified can humidify then that equipment will try and meet the zone's humidification load. The opposite is true if the zone relative humidity or dew-point temperature is above the control relative humidity or dew-point temperature setpoint and the equipment can dehumidify. For a dual setpoint humidistat, if the zone relative humidity or dew-point temperature is below the \textbf{humidifying} relative humidity or dew-point temperature setpoint and the equipment specified can humidify then that equipment will try and meet the zone's humidification load. The opposite is true if the zone relative humidity or dew-point temperature is above the \textbf{dehumidifying} relative humidity or dew-point temperature setpoint and the equipment can dehumidify.
 
 If the ZoneControl:Humidistat is used by a furnace or unitary system (Ref. Furnace and Unitary Systems) no other objects are required. The signal from the humidistat is used directly by that component. If the Zone Control:Humidistat object is used to control a Humidifier or used in conjunction with a \hyperref[controllerwatercoil]{Controller:WaterCoil} object with control variable \textbf{TemperatureAndHumidityRatio} or \textbf{HumidityRatio}, the following objects are required to determine a setpoint for those components for a single setpoint humidistat:
 
@@ -801,24 +801,29 @@ \subsubsection{Inputs}\label{inputs-8-028}
 
 Name of the zone that is being controlled.
 
-\paragraph{Field: Humidifying Relative Humidity Setpoint Schedule Name}\label{field-humidifying-relative-humidity-setpoint-schedule-name}
+\paragraph{Field: Humidifying Setpoint Schedule Name}\label{field-humidifying-setpoint-schedule-name}
 
-Name of a schedule that defines the humidifying relative humidity setpoint, expressed as a percentage (0-100), for each timestep of the simulation.
+Name of a schedule that defines the humidifying relative humidity (expressed as a percentage (0-100)) or dew-point temperature setpoint for each timestep of the simulation.
 
 \textbf{Note}: If only a single setpoint humidistat is desired, then input the single schedule name in the Humidifying Setpoint Schedule Name field (and leave the Dehumidifying Setpoint Schedule Name blank).
 
-\paragraph{Field: Dehumidifying Relative Humidity Setpoint Schedule Name}\label{field-dehumidifying-relative-humidity-setpoint-schedule-name-1}
+\paragraph{Field: Dehumidifying Setpoint Schedule Name}\label{field-dehumidifying-setpoint-schedule-name-1}
 
-Name of a schedule that defines the dehumidifying relative humidity setpoint, expressed as a percentage (0-100), for each timestep of the simulation. This field is optional, only used if a dual setpoint humidistat is to be modeled.
+Name of a schedule that defines the dehumidifying relative humidity setpoint (expressed as a percentage (0-100)) or dew-point temperature for each timestep of the simulation. This field is optional, only used if a dual setpoint humidistat is to be modeled.
+
+\paragraph{Field: Control Variable}\label{field-humidistat-control-variable}
+
+This fields describes if the schedules describe relative humidity or dew-point temperature values.
 
 An example of this statement in an IDF is:
 
 \begin{lstlisting}
 ZoneControl:Humidistat,
   Zone 2 Humidistat,          !- Humidistat Name
   EAST ZONE,                  !- Zone Name
-  Min Rel Hum Set Sch,        !- Humidifying Relative Humidity Setpoint SCHEDULE Name
-  Max Rel Hum Set Sch;        !- Dehumidifying Relative Humidity Setpoint SCHEDULE Name
+  Min Rel Hum Set Sch,        !- Humidifying Setpoint Schedule Name
+  Max Rel Hum Set Sch,        !- Dehumidifying Setpoint Schedule Name
+  RelativeHumidity;           !- Control Variable
 \end{lstlisting}
 
 An example schedule for the Zone Control:Humidistat

idd/Energy+.idd.in

@@ -35261,15 +35261,21 @@ ZoneControl:Humidistat,
        \required-field
        \type object-list
        \object-list ZoneNames
-  A3 , \field Humidifying Relative Humidity Setpoint Schedule Name
+  A3 , \field Humidifying Setpoint Schedule Name
        \required-field
-       \note hourly schedule values should be in Relative Humidity (percent)
+       \note Hourly schedule values should be in relative humidity (percent) or dew-point temperatures (deg. C).
        \type object-list
        \object-list ScheduleNames
-  A4 ; \field Dehumidifying Relative Humidity Setpoint Schedule Name
-       \note hourly schedule values should be in Relative Humidity (percent)
+  A4 , \field Dehumidifying Setpoint Schedule Name
+       \note Hourly schedule values should be in relative humidity (percent) or dew-point temperatures (deg. C).
        \type object-list
        \object-list ScheduleNames
+  A5 ; \field Control Variable
+       \note When using RelativeHumidity, the schedule values should be in percentages.
+       \type choice
+       \key RelativeHumidity
+       \key Dewpoint
+       \default RelativeHumidity
 
 ZoneControl:Thermostat,
    \memo Define the Thermostat settings for a zone or list of zones.

src/EnergyPlus/DataZoneControls.hh

@@ -88,6 +88,17 @@ namespace DataZoneControls {
     constexpr std::array<std::string_view, (int)TempCtrl::Num> tempCtrlNames = {"None", "Constant", "Scheduled"};
     constexpr std::array<std::string_view, (int)TempCtrl::Num> tempCtrlNamesUC = {"NONE", "CONSTANT", "SCHEDULED"};
 
+    enum class HumidityCtrlVarType
+    {
+        Invalid = -1,
+        DewPoint,
+        RelativeHumidity,
+        Num
+    };
+
+    static constexpr std::array<std::string_view, (int)HumidityCtrlVarType::Num> humidityCtrlVarTypeNames = {"Dewpoint", "RelativeHumidity"};
+    static constexpr std::array<std::string_view, (int)HumidityCtrlVarType::Num> humidityCtrlVarTypeNamesUC = {"DEWPOINT", "RELATIVEHUMIDITY"};
+
     struct TempSetptType
     {
         std::string Name;
@@ -159,13 +170,14 @@ namespace DataZoneControls {
         std::string ControlName; // Name of this humidity controller
         std::string ZoneName;    // Name of the zone
         int ActualZoneNum;
-        Sched::Schedule *humidifyingSched = nullptr;   // humidifying schedule
-        Sched::Schedule *dehumidifyingSched = nullptr; // dehumidifying schedule
-        int ErrorIndex;                                // Error index when LowRH setpoint > HighRH setpoint
-        bool EMSOverrideHumidifySetPointOn;            // EMS is calling to override humidifying setpoint
-        Real64 EMSOverrideHumidifySetPointValue;       // value EMS is directing to use for humidifying setpoint
-        bool EMSOverrideDehumidifySetPointOn;          // EMS is calling to override dehumidifying setpoint
-        Real64 EMSOverrideDehumidifySetPointValue;     // value EMS is directing to use for dehumidifying setpoint
+        Sched::Schedule *humidifyingSched = nullptr;                                             // humidifying schedule
+        Sched::Schedule *dehumidifyingSched = nullptr;                                           // dehumidifying schedule
+        HumidityCtrlVarType humidityControlVariableType = HumidityCtrlVarType::RelativeHumidity; // Control variable type
+        int ErrorIndex;                                                                          // Error index when LowRH setpoint > HighRH setpoint
+        bool EMSOverrideHumidifySetPointOn;                                                      // EMS is calling to override humidifying setpoint
+        Real64 EMSOverrideHumidifySetPointValue;   // value EMS is directing to use for humidifying setpoint
+        bool EMSOverrideDehumidifySetPointOn;      // EMS is calling to override dehumidifying setpoint
+        Real64 EMSOverrideDehumidifySetPointValue; // value EMS is directing to use for dehumidifying setpoint
 
         // Default Constructor
         ZoneHumidityControls()

src/EnergyPlus/ZoneTempPredictorCorrector.cc

@@ -869,6 +869,10 @@ void GetZoneAirSetPoints(EnergyPlusData &state)
             ErrorsFound = true;
         }
 
+        // Control variable
+        humidControlledZone.humidityControlVariableType =
+            static_cast<DataZoneControls::HumidityCtrlVarType>(getEnumValue(DataZoneControls::humidityCtrlVarTypeNamesUC, s_ipsc->cAlphaArgs(5)));
+
     } // HumidControlledZoneNum
 
     // Start to read Thermal comfort control objects
@@ -3491,8 +3495,24 @@ void ZoneSpaceHeatBalanceData::calcPredictedHumidityRatio(EnergyPlusData &state,
     if (thisZone.humidityControlZoneIndex > 0) {
         auto &humidityControlZone = state.dataZoneCtrls->HumidityControlZone(thisZone.humidityControlZoneIndex);
         assert(humidityControlZone.ActualZoneNum == zoneNum);
-        ZoneRHHumidifyingSetPoint = humidityControlZone.humidifyingSched->getCurrentVal();
-        ZoneRHDehumidifyingSetPoint = humidityControlZone.dehumidifyingSched->getCurrentVal();
+        if (humidityControlZone.humidityControlVariableType == DataZoneControls::HumidityCtrlVarType::RelativeHumidity) {
+            ZoneRHHumidifyingSetPoint = humidityControlZone.humidifyingSched->getCurrentVal();
+            ZoneRHDehumidifyingSetPoint = humidityControlZone.dehumidifyingSched->getCurrentVal();
+        } else if (humidityControlZone.humidityControlVariableType ==
+                   DataZoneControls::HumidityCtrlVarType::DewPoint) { // Recalculate RH setpoint from DP
+            ZoneRHHumidifyingSetPoint =
+                100 * Psychrometrics::PsyRhFnTdbWPb(
+                          state,
+                          this->ZT,
+                          Psychrometrics::PsyWFnTdpPb(state, humidityControlZone.humidifyingSched->getCurrentVal(), state.dataEnvrn->OutBaroPress),
+                          state.dataEnvrn->OutBaroPress);
+            ZoneRHDehumidifyingSetPoint =
+                100 * Psychrometrics::PsyRhFnTdbWPb(
+                          state,
+                          this->ZT,
+                          Psychrometrics::PsyWFnTdpPb(state, humidityControlZone.dehumidifyingSched->getCurrentVal(), state.dataEnvrn->OutBaroPress),
+                          state.dataEnvrn->OutBaroPress);
+        }
 
         // Apply EMS values to overwrite the humidistat values
         if (humidityControlZone.EMSOverrideHumidifySetPointOn) {

testfiles/CMakeLists.txt

@@ -384,6 +384,7 @@ add_simulation_test(IDF_FILE HeatPumpWaterToAirEquationFit.idf EPW_FILE USA_IL_C
 add_simulation_test(IDF_FILE HeatPumpWaterToAirWithAntifreezeAndLatentModel.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw)
 add_simulation_test(IDF_FILE HeatPumpWaterToAirWithAntifreezeAndLatentModel2.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw)
 add_simulation_test(IDF_FILE HeatPumpWaterToAirWithRHControl.idf EPW_FILE USA_FL_Miami.Intl.AP.722020_TMY3.epw)
+add_simulation_test(IDF_FILE HeatPumpWaterToAirWithHumControlDewPoint.idf EPW_FILE USA_FL_Miami.Intl.AP.722020_TMY3.epw)
 add_simulation_test(IDF_FILE HeatPumpwithBiquadraticCurves.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw)
 add_simulation_test(IDF_FILE HeatRecoveryElectricChiller.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw)
 add_simulation_test(IDF_FILE HeatRecoveryPlantLoop.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw)