Software Connection

In order to connect a software tool to the OpenStudio-ERI workflow, the software tool must be able to export its building description in HPXML file format.

HPXML Overview

HPXML is an open data standard for collecting and transferring home energy data. Requiring HPXML files as the input to the ERI workflow significantly reduces the complexity and effort for software developers to leverage the EnergyPlus simulation engine. It also simplifies the process of applying the ERI 301 ruleset.

The HPXML Toolbox website provides several resources for software developers, including:

1. An interactive schema validator
2. A data dictionary
3. An implementation guide

ERI Use Case for HPXML

HPXML is an flexible and extensible format, where nearly all fields in the schema are optional and custom fields can be included. Because of this, an ERI Use Case for HPXML has been developed that specifies the HPXML fields or enumeration choices required to run the workflow.

Software developers should use the ERI Use Case (defined as a set of conditional XPath expressions) as well as the HPXML schema to construct valid HPXML files for ERI calculations.

ERI Version

The version of the ERI calculation to be run is specified inside the HPXML file itself at /HPXML/SoftwareInfo/extension/ERICalculation/Version. For example, a value of “2014AE” tells the workflow to use ANSI/RESNET/ICC© 301-2014 with both Addendum A (Amendment on Domestic Hot Water Systems) and Addendum E (House Size Index Adjustment Factors) included.

Note

Valid choices for ERI version can be looked up in the ERI Use Case.

Building Details

The building description is entered in HPXML’s /HPXML/Building/BuildingDetails.

Building Summary

This section describes fields specified in HPXML’s BuildingSummary. It is used for high-level building information needed for an ERI calculation including conditioned floor area, number of bedrooms, number of conditioned floors, etc.

The BuildingSummary/Site/FuelTypesAvailable field is used to determine whether the home has access to natural gas or fossil fuel delivery (specified by any value other than “electricity”). This information may be used for determining the heating system, as specified by the ERI 301 Standard.

Climate and Weather

This section describes fields specified in HPXML’s ClimateandRiskZones.

The ClimateandRiskZones/ClimateZoneIECC element specifies the IECC climate zone(s) for years required by the ERI 301 Standard.

The ClimateandRiskZones/WeatherStation element specifies the EnergyPlus weather file (EPW) to be used in the simulation. The WeatherStation/WMO must be one of the acceptable TMY3 WMO station numbers found in the weather/data.csv file.

In addition to using the TMY3 weather files that are provided, custom weather files can be used if they are in EPW file format. To use custom weather files, first ensure that all weather files have a unique WMO station number (as provided in the first header line of the EPW file). Then place them in the weather directory and call openstudio energy_rating_index.rb --cache-weather. After processing is complete, each EPW file will have a corresponding *.cache file and the WMO station numbers of these weather files will be available in the weather/data.csv file.

Note

In the future, we hope to provide an automated weather file selector based on a building’s address/zipcode or similar information. But for now, each software tool is responsible for providing this information.

Enclosure

This section describes fields specified in HPXML’s Enclosure.

All surfaces that bound different space types in the building (i.e., not just thermal boundary surfaces) must be specified in the HPXML file. For example, an attached garage would generally be defined by walls adjacent to conditioned space, walls adjacent to outdoors, a slab, and a roof or ceiling. For software tools that do not collect sufficient inputs for every required surface, the software developers will need to make assumptions about these surfaces or collect additional input.

The space types used in the HPXML building description are:

Space Type	Notes
living space	Above-grade conditioned floor area.
attic - vented
attic - unvented
basement - conditioned	Below-grade conditioned floor area.
basement - unconditioned
crawlspace - vented
crawlspace - unvented
garage
other housing unit	Used to specify adiabatic surfaces.

Warning

It is the software tool’s responsibility to provide the appropriate building surfaces. While some error-checking is in place, it is not possible to know whether some surfaces are incorrectly missing.

Also note that wall and roof surfaces do not require an azimuth to be specified. Rather, only the windows/skylights themselves require an azimuth. Thus, software tools can use a single wall (or roof) surface to represent multiple wall (or roof) surfaces for the entire building if all their other properties (construction type, interior/exterior adjacency, etc.) are identical.

Air Leakage

Building air leakage characterized by air changes per hour or cfm at 50 pascals pressure difference (ACH50) is entered at Enclosure/AirInfiltration/AirInfiltrationMeasurement/BuildingAirLeakage/AirLeakage. The Enclosure/AirInfiltration/AirInfiltrationMeasurement should be specified with HousePressure='50' and BuildingAirLeakage/UnitofMeasure='ACH' or BuildingAirLeakage/UnitofMeasure='CFM'.

In addition, the building’s volume associated with the air leakage measurement is provided in HPXML’s AirInfiltrationMeasurement/InfiltrationVolume.

Vented Attics/Crawlspaces

The ventilation rate for vented attics (or crawlspaces) can be specified using an Attic (or Foundation) element. First, define the AtticType as Attic[Vented='true'] (or FoundationType as Crawlspace[Vented='true']). Then use the VentilationRate[UnitofMeasure='SLA']/Value element to specify a specific leakage area (SLA). If these elements are not provided, the ERI 301 Standard Reference Home defaults will be used.

Roofs

Pitched or flat roof surfaces that are exposed to ambient conditions should be specified as an Enclosure/Roofs/Roof. For a multifamily building where the dwelling unit has another dwelling unit above it, the surface between the two dwelling units should be considered a Floor and not a Roof.

Beyond the specification of typical heat transfer properties (insulation R-value, solar absorptance, emittance, etc.), note that roofs can be defined as having a radiant barrier.

Walls

Any wall that has no contact with the ground and bounds a space type should be specified as an Enclosure/Walls/Wall. Interior walls (for example, walls solely within the conditioned space of the building) are not required.

Walls are primarily defined by their Insulation/AssemblyEffectiveRValue. The choice of WallType has a secondary effect on heat transfer in that it informs the assumption of wall thermal mass.

Rim Joists

Rim joists, the perimeter of floor joists typically found between stories of a building or on top of a foundation wall, are specified as an Enclosure//RimJoists/RimJoist.

The InteriorAdjacentTo element should typically be “living space” for rim joists between stories of a building and “basement - conditioned”, “basement - unconditioned”, “crawlspace - vented”, or “crawlspace - unvented” for rim joists on top of a foundation wall.

Foundation Walls

Any wall that is in contact with the ground should be specified as an Enclosure/FoundationWalls/FoundationWall. Other walls (e.g., wood framed walls) that are connected to a below-grade space but have no contact with the ground should be specified as Walls and not FoundationWalls.

Exterior foundation walls (i.e., those that fall along the perimeter of the building’s footprint) should use “ground” for ExteriorAdjacentTo and the appropriate space type (e.g., “basement - unconditioned”) for InteriorAdjacentTo.

Interior foundation walls should be specified with two appropriate space types (e.g., “crawlspace - vented” and “garage”, or “basement - unconditioned” and “crawlspace - unvented”) for InteriorAdjacentTo and ExteriorAdjacentTo. Interior foundation walls should never use “ground” for ExteriorAdjacentTo even if the foundation wall has some contact with the ground due to the difference in below-grade depths of the two adjacent space types.

Foundations must include a Height as well as a DepthBelowGrade. For exterior foundation walls, the depth below grade is relative to the ground plane. For interior foundation walls, the depth below grade should not be thought of as relative to the ground plane, but rather as the depth of foundation wall in contact with the ground. For example, an interior foundation wall between an 8 ft conditioned basement and a 3 ft crawlspace has a height of 8 ft and a depth below grade of 5 ft. Alternatively, an interior foundation wall between an 8 ft conditioned basement and an 8 ft unconditioned basement has a height of 8 ft and a depth below grade of 0 ft.

Foundation wall insulation can be described in two ways:

Option 1. A continuous insulation layer with NominalRValue and DistanceToBottomOfInsulation. An insulation layer is useful for describing foundation wall insulation that doesn’t span the entire height (e.g., 4 ft of insulation for an 8 ft conditioned basement). When an insulation layer R-value is specified, it is modeled with a concrete wall (whose Thickness is provided) as well as air film resistances as appropriate.

Option 2. An AssemblyEffectiveRValue. When instead providing an assembly effective R-value, the R-value should include the concrete wall and an interior air film resistance. The exterior air film resistance (for any above-grade exposure) or any soil thermal resistance should not be included.

Frame Floors

Any horizontal floor/ceiling surface that is not in contact with the ground (Slab) nor adjacent to ambient conditions above (Roof) should be specified as an Enclosure/FrameFloors/FrameFloor.

Frame floors are primarily defined by their Insulation/AssemblyEffectiveRValue.

Slabs

Any space type that borders the ground should include an Enclosure/Slabs/Slab surface with the appropriate InteriorAdjacentTo. This includes basements, crawlspaces (even when there are dirt floors – use zero for the Thickness), garages, and slab-on-grade foundations.

A primary input for a slab is its ExposedPerimeter. The exposed perimeter should include any slab length that falls along the perimeter of the building’s footprint (i.e., is exposed to ambient conditions). So, a basement slab edge adjacent to a garage or crawlspace, for example, should not be included.

Vertical insulation adjacent to the slab can be described by a PerimeterInsulation/Layer/NominalRValue and a PerimeterInsulationDepth.

Horizontal insulation under the slab can be described by a UnderSlabInsulation/Layer/NominalRValue. The insulation can either have a depth (UnderSlabInsulationWidth) or can span the entire slab (UnderSlabInsulationSpansEntireSlab).

For foundation types without walls, the DepthBelowGrade field must be provided. For foundation types with walls, the slab’s position relative to grade is determined by the FoundationWall/DepthBelowGrade values.

Windows

Any window or glass door area should be specified as an Enclosure/Windows/Window.

Windows are defined by full-assembly NFRC UFactor and SHGC, as well as Area. Windows must reference a HPXML Enclosures/Walls/Wall element via the AttachedToWall. Windows must also have an Azimuth specified, even if the attached wall does not.

Overhangs can optionally be defined for a window by specifying a Window/Overhangs element. Overhangs are defined by the vertical distance between the overhang and the top of the window (DistanceToTopOfWindow), and the vertical distance between the overhang and the bottom of the window (DistanceToBottomOfWindow). The difference between these two values equals the height of the window.

Skylights

Any skylight should be specified as an Enclosure/Skylights/Skylight.

Skylights are defined by full-assembly NFRC UFactor and SHGC, as well as Area. Skylights must reference a HPXML Enclosures/Roofs/Roof element via the AttachedToRoof. Skylights must also have an Azimuth specified, even if the attached roof does not.

Doors

Any opaque doors should be specified as an Enclosure/Doors/Door.

Doors are defined by RValue and Area. Doors must reference a HPXML Enclosures/Walls/Wall element via the AttachedToWall. Doors must also have an Azimuth specified, even if the attached wall does not.

Systems

This section describes fields specified in HPXML’s Systems.

If any HVAC systems are entered that provide heating, the sum of all their FractionHeatLoadServed values must equal 1. The same holds true for FractionCoolLoadServeds for HVAC systems that provide cooling and FractionDHWLoadServed for water heating systems.

Heating Systems

Each heating system (other than heat pumps) should be entered as a Systems/HVAC/HVACPlant/HeatingSystem. Inputs including HeatingSystemType, HeatingCapacity, and FractionHeatLoadServed must be provided.

Depending on the type of heating system specified, additional elements are required:

HeatingSystemType	DistributionSystem	HeatingSystemFuel	AnnualHeatingEfficiency
ElectricResistance		electricity	Percent
Furnace	AirDistribution or DSE	<any>	AFUE
WallFurnace		<any>	AFUE
Boiler	HydronicDistribution or DSE	<any>	AFUE
Stove		<any>	Percent

If a non-electric heating system is specified, the ElectricAuxiliaryEnergy element may be provided if available.

Cooling Systems

Each cooling system (other than heat pumps) should be entered as a Systems/HVAC/HVACPlant/CoolingSystem. Inputs including CoolingSystemType, CoolingCapacity, and FractionCoolLoadServed must be provided.

Depending on the type of cooling system specified, additional elements are required/available:

CoolingSystemType	DistributionSystem	CoolingSystemFuel	AnnualCoolingEfficiency	SensibleHeatFraction
central air conditioner	AirDistribution or DSE	electricity	SEER	optional
room air conditioner		electricity	EER	optional

Heat Pumps

Each heat pump should be entered as a Systems/HVAC/HVACPlant/HeatPump. Inputs including HeatPumpType, CoolingCapacity, HeatingCapacity, FractionHeatLoadServed, and FractionCoolLoadServed must be provided. Note that heat pumps are allowed to provide only heating (FractionCoolLoadServed = 0) or cooling (FractionHeatLoadServed = 0) if appropriate.

Depending on the type of heat pump specified, additional elements are required/available:

HeatPumpType	DistributionSystem	HeatPumpFuel	AnnualCoolingEfficiency	AnnualHeatingEfficiency	CoolingSensibleHeatFraction	HeatingCapacity17F
air-to-air	AirDistribution or DSE	electricity	SEER	HSPF	optional	optional
mini-split	AirDistribution or DSE (optional)	electricity	SEER	HSPF	optional	optional
ground-to-air	AirDistribution or DSE	electricity	EER	COP	optional

If the heat pump has integrated backup heating, it can be specified with BackupSystemFuel (currently only “electricity” is allowed), BackupAnnualHeatingEfficiency (percent), and BackupHeatingCapacity.

Thermostat

A Systems/HVAC/HVACControl must be provided if any HVAC systems are specified. Its ControlType specifies whether there is a manual or programmable thermostat.

HVAC Distribution

Each separate HVAC distribution system should be specified as a Systems/HVAC/HVACDistribution. There should be at most one heating system and one cooling system attached to a distribution system. See the sections on Heating Systems, Cooling Systems, and Heat Pumps for information on which DistributionSystemType is allowed for which HVAC system. Also, note that some HVAC systems are not allowed to be attached to a distribution system.

AirDistribution systems are defined by:

• Supply & return leakages in CFM25 to the outside (DuctLeakageMeasurement/DuctLeakage/Value)
• One or more supply & return ducts (Ducts)

For each duct, DuctInsulationRValue, DuctLocation, and DuctSurfaceArea must be provided.

HydronicDistribution systems do not require any additional inputs.

DSE systems are defined by a AnnualHeatingDistributionSystemEfficiency and AnnualCoolingDistributionSystemEfficiency elements.

Mechanical Ventilation

A single whole-house mechanical ventilation system may be specified as a Systems/MechanicalVentilation/VentilationFans/VentilationFan with UsedForWholeBuildingVentilation='true'. Inputs including FanType, TestedFlowRate, HoursInOperation, and FanPower must be provided.

Depending on the type of mechanical ventilation specified, additional elements are required:

FanType	SensibleRecoveryEfficiency	TotalRecoveryEfficiency	AttachedToHVACDistributionSystem
energy recovery ventilator	required	required
heat recovery ventilator	required
exhaust only
supply only
balanced
central fan integrated supply (CFIS)			required

Note that AdjustedSensibleRecoveryEfficiency and AdjustedTotalRecoveryEfficiency can be provided instead.

In many situations, the rated flow rate should be the value derived from actual testing of the system. For a CFIS system, the rated flow rate should equal the amount of outdoor air provided to the distribution system.

Water Heaters

Each water heater should be entered as a Systems/WaterHeating/WaterHeatingSystem. Inputs including WaterHeaterType, Location, and FractionDHWLoadServed must be provided.

Depending on the type of water heater specified, additional elements are required/available:

WaterHeaterType	UniformEnergyFactor or EnergyFactor	FuelType	TankVolume	HeatingCapacity	RecoveryEfficiency	RelatedHVACSystem	WaterHeaterInsulation/Jacket/JacketRValue
storage water heater	required	<any>	required	<optional>	required if non-electric		<optional>
instantaneous water heater	required	<any>
heat pump water heater	required	electricity	required				<optional>
space-heating boiler with storage tank			required			required	<optional>
space-heating boiler with tankless coil						required

For combi boiler systems, the RelatedHVACSystem must point to a HeatingSystem of type “Boiler”.

For water heaters that are connected to a desuperheater, UsesDesuperheater must be set and the RelatedHVACSystem must either point to a HeatPump or a CoolingSystem.

Hot Water Distribution

A Systems/WaterHeating/HotWaterDistribution must be provided if any water heating systems are specified. Inputs including SystemType and PipeInsulation/PipeRValue must be provided.

For a SystemType/Standard (non-recirculating) system, the following field is required:

• PipingLength: Measured length of hot water piping from the hot water heater to the farthest hot water fixture, measured longitudinally from plans, assuming the hot water piping does not run diagonally, plus 10 feet of piping for each floor level, plus 5 feet of piping for unconditioned basements (if any)

For a SystemType/Recirculation system, the following fields are required:

• ControlType
• RecirculationPipingLoopLength: Measured recirculation loop length including both supply and return sides, measured longitudinally from plans, assuming the hot water piping does not run diagonally, plus 20 feet of piping for each floor level greater than one plus 10 feet of piping for unconditioned basements
• BranchPipingLoopLength: Measured length of the branch hot water piping from the recirculation loop to the farthest hot water fixture from the recirculation loop, measured longitudinally from plans, assuming the branch hot water piping does not run diagonally
• PumpPower

In addition, a HotWaterDistribution/DrainWaterHeatRecovery (DWHR) may be specified. The DWHR system is defined by:

• FacilitiesConnected: ‘one’ if there are multiple showers and only one of them is connected to a DWHR; ‘all’ if there is one shower and it’s connected to a DWHR or there are two or more showers connected to a DWHR
• EqualFlow: ‘true’ if the DWHR supplies pre-heated water to both the fixture cold water piping and the hot water heater potable supply piping
• Efficiency: As rated and labeled in accordance with CSA 55.1

Water Fixtures

Water fixtures should be entered as Systems/WaterHeating/WaterFixture elements. Each fixture must have WaterFixtureType and LowFlow elements provided. Fixtures should be specified as low flow if they are <= 2.0 gpm.

Photovoltaics

Each solar electric (photovoltaic) system should be entered as a Systems/Photovoltaics/PVSystem. The following fields, some adopted from the PVWatts model, are required for each PV system:

• Location: ‘ground’ or ‘roof’ mounted
• ModuleType: ‘standard’, ‘premium’, or ‘thin film’
• Tracking: ‘fixed’ or ‘1-axis’ or ‘1-axis backtracked’ or ‘2-axis’
• ArrayAzimuth
• ArrayTilt
• MaxPowerOutput
• InverterEfficiency: Default is 0.96.
• SystemLossesFraction: Default is 0.14. System losses include soiling, shading, snow, mismatch, wiring, degradation, etc.

Appliances

This section describes fields specified in HPXML’s Appliances. Many of the appliances’ inputs are derived from EnergyGuide labels.

The Location for clothes washers, clothes dryers, and refrigerators can be provided, while dishwashers and cooking ranges are assumed to be in the living space.

Clothes Washer

An Appliances/ClothesWasher element must be specified. The efficiency of the clothes washer can either be entered as a ModifiedEnergyFactor or an IntegratedModifiedEnergyFactor. Several other inputs from the EnergyGuide label must be provided as well.

Clothes Dryer

An Appliances/ClothesDryer element must be specified. The dryer’s FuelType and ControlType (“timer” or “moisture”) must be provided. The efficiency of the clothes dryer can either be entered as an EnergyFactor or CombinedEnergyFactor.

Dishwasher

An Appliances/Dishwasher element must be specified. The dishwasher’s PlaceSettingCapacity must be provided. The efficiency of the dishwasher can either be entered as an EnergyFactor or RatedAnnualkWh.

Refrigerator

An Appliances/Refrigerator element must be specified. The efficiency of the refrigerator must be entered as RatedAnnualkWh.

Cooking Range/Oven

Appliances/CookingRange and Appliances/Oven elements must be specified. The FuelType of the range and whether it IsInduction, as well as whether the oven IsConvection, must be provided.

Lighting

The building’s lighting is described by six Lighting/LightingGroup elements, each of which is the combination of:

• LightingGroup/ThirdPartyCertification: ‘ERI Tier I’ (fluorescent) and ‘ERI Tier II’ (LEDs, outdoor lamps controlled by photocells, or indoor lamps controlled by motion sensor)
• LightingGroup/Location: ‘interior’, ‘garage’, and ‘exterior’

The fraction of lamps of the given type in the given location are provided as the LightingGroup/FractionofUnitsInLocation. The fractions for a given location cannot sum to greater than 1. Garage lighting values are ignored if the building has no garage.

Ceiling Fans

Each ceiling fan (or set of identical ceiling fans) should be entered as a Lighting/CeilingFan. The Airflow/Efficiency (at medium speed) and Quantity must be provided.

Validating & Debugging Errors

When running HPXML files, errors may occur because:

1. An HPXML file provided is invalid (either relative to the HPXML schema or the ERI Use Case).
2. An unexpected error occurred in the workflow (e.g., applying the ERI 301 ruleset).
3. An unexpected EnergyPlus simulation error occurred.

If, for example, the Rated Home is unsuccessful, first look in the ERIRatedHome/run.log for details. If there are no errors in that log file, then the error may be in the EnergyPlus simulation – see ERIRatedHome/eplusout.err.

Contact us if you can’t figure out the cause of an error.

Sample Files

Dozens of sample HPXML files are included in the workflow/sample_files directory. The sample files help to illustrate how different building components are described in HPXML.

Each sample file generally makes one isolated change relative to the base HPXML (base.xml) building. For example, the base-dhw-dwhr.xml file adds a DrainWaterHeatRecovery element to the building.

You may find it useful to search through the files for certain HPXML elements or compare (diff) a sample file to the base.xml file.