ASHRAE 90.1 Appendix G PRM Modelling

This section outlines the procedures involved in setting up proposed and baseline building models in accordance with ASHRAE 90.1 Appendix G PRM. In the text below "Appendix G" refers to ASHRAE 90.1 Standard Appendix G.

Note: you will need the DesignBuilder LEED / ASHRAE 90.1 module to access this functionality.

Add new site

To access ASHRAE 90.1 functionality the Mandatory energy code must be set to ASHRAE 90.1 2007 or 2010 . This can be done manually for existing models from the Regions tab at site level, or for new models, follow the instructions in the next step.
When creating a new ASHRAE 90.1 project check the ASHRAE 90.1 App G PRM checkbox on the Add new site dialog and choose one of the 1-ASHRAE 90.1-2007 or 2-ASHRAE 90.1-2010 options.
DesignBuilder uses the ASHRAE climate zone site level setting to identify the climate zone for generating baseline constructions and glazing according to ASHRAE 90.1 building envelope requirements. The default climate zone loaded will have been loaded from the Locations template and that value was originally derived from the value shown on the Hourly weather data dialog which in turn was derived by analysing the .epw hourly weather data. In some cases the climate zone may not be the same as defined in ASHRAE 90.1 Standard Appendix D for the model location. In this case simply select the correct ASHRAE climate zone at site level and the baseline constructions and glazing will be automatically updated to reflect the new climate zone.

Add new building

When the Mandatory energy code is ASHRAE 90.1 2007 or 2010 then on the Add new building dialog the Model type template shows ASHRAE 90.1. This loads some appropriate model data settings such as Detailed HVAC and 6 timesteps per hour to the building model.
On the Add new building dialog you can select either 1-Proposed or 2-Baseline for the ASHRAE 90.1 building type. DesignBuilder does not currently generate different settings based on this selection, however it always generates baseline constructions and construction templates in accordance to building envelope requirements defined in 90.1 Constructions from Standard chapter 5 (detailed in Tables 5.5-1 through 5.5-8) can be selected as required for both proposed and baseline buildings.
For your proposed design you can improve the baseline construction (by copying and editing) or simply replacing them with definitions shown on architectural drawings or as built for existing building envelopes.
Following formal ASHRAE 90.1 PRM modelling procedure, you are strongly advised to create the proposed building first and to create the baseline building from that.

Constructions and Glazing

When a site is created or when the climate zone is changed, baseline constructions and glazing systems as well as construction templates are generated for the selected ASHRAE climate zone ready for assigning to the building.

Opaque construction templates

Glazing templates

Two special opaque construction types required by the ASHRAE 90.1 Standard, are F-factor ground floors and C-factor below-grade walls. These are different from conventional layered construction in that they don't use layers of materials but instead the overall thermal properties are defined as an F-factor (for slab on grade floors) or a C-factor (for below grade walls). EnergyPlus uses these factors to generate equivalent layered constructions behind the scenes. During the simulation these surfaces exchange heat with the FCFactor ground temperatures only.
When running Heating or Cooling design calculations, if the FCfactorMethod ground temperatures source data at site level has 1-Hourly weather file selected, a weather file containing ground temperatures in the header should be selected and will be downloaded (if not yet in your weather file folder on your computer) prior to the calculations. If 2-User defined is selected, the Heating and Cooling design calculations do not require the weather file.
The terminology used in the ASHRAE 90.1 constructions list gives the climate zone, a description of the construction, insulated level(s) in IP units and the U-value in IP units and in brackets the values in SI units. For example construction CZ4 Non-Res, Wall, Steel-Framed, R-13.1+R-7.4 (2.3+1.3), U-.064 (.365) indicates a non-residential steel framed wall for climate zone 4 (CZ4) with internal insulation levels of two layers of minimum 13.1 and 7.4 ft2-°F/(Btu/hr) in IP units (2.3 and 1.3 m2-K/W in SI units) and a U-value of 0.064 Btu/hr-ft2-°F in IP units (0.365 W/m2-K in SI units).

Create 3-D building model

The proposed building model geometry should be set up with reference to ASHRAE 90.1 Standard Appendix G Table G3.1-1.
Define different thermal zone types using the Zone type option on the Activity tab. For ASHRAE 90.1 models additional Zone types can be selected per 90.1 requirements including:
1. 5-Semiheated, for zones that are heated but not to normal comfort levels.
2. When the 2-Unconditioned Zone type is selected choose whether the unconditioned zone is ventilated or unventilated using the Unconditioned zone ventilation setting below the Zone type setting.
Define each zone type as the model requires (checking with rules from Appendix G Table G3.1-7,8,9), and then assign relevant construction/fenestration templates according to their “Space-Conditioning Categories” (construction default is Non-Residential). Activity, lighting templates for each block/zone should be updated with relevant templates as well.

Geometry convention

It is important for ASHRAE 90.1 based-work that the proposed and baselines buildings have identical floor areas and zone volumes. The easiest way to ensure this is to use the Simple geometry convention for both buildings. With the Simple template the block geometry drawn gives the exact surface geometry and zone volumes to be used in the simulation. This means that when you copy the proposed building to create the baseline building the zone geometry will be correct even after loading the baseline constructions.

If instead you used the External measurements template for example you would find that the baseline building surface areas, floor area and zone volumes would be different after loading the baseline constructions because these generally have a different thickness to those selected for the proposed building.

Lighting

Lighting settings shall be either building area method or space-by-space method, but shall not the mix these two schemes. The Building Area Method is generally used for early stage analysis where the exact function of every zone in the building has not yet been determined. The Space-by-Space Method is used more often as it is more flexible, allowing the user to use more detail/specific lighting settings.

Note: When loading an activity template, it will update the lighting schedule.

HVAC

A set of Appendix G HVAC systems for loading to baseline building models is provided in the Detailed HVAC templates list. The proposed building HVAC system should meet the requirements shown in Appendix G Table G3.1-10. A modified copy of a baseline HVAC system can be used for the proposed building.
For the proposed building design, where no heating or cooling systems exist or have been specified, the heating or cooling systems shall be identical to the systems modelled in the baseline building design.
Define the DHW system based on Appendix G Table G3.1-11.

Utility costs

Define the utility costs for the model to generate energy cost outputs based on purchased energy rates using settings on the Tariff analysis dialog on the Economics tab at building level. LEED/ASHRAE 90.1 App G PRM method may only require tariffs object to calculate operational costs.

Outputs

EnergyPlus provides a "LEED Summary" report which is displayed on the Summary tab after the simulation if the appropriate option is checked on the Output tab of the Simulation options dialog under the Summary Tables header.

Convert an Existing Simulation Model to an ASHRAE 90.1 Model

To convert a standard EnergyPlus model to be treated as a proposed building go to the Regions tab at site level and set the Mandatory energy code to one of the ASHRAE 90.1 energy codes. You will be warned to check that the ASHRAE climate zone is correctly set. The default climate zone can be overridden at site level under the Location header.

Unmet hours

"Unmet hours" of a building are the summation of the number of hours when the heating or the cooling set point temperature of a zone is not met either by the HVAC system or by the plant. The data is provided in the Summary report as:

Time Setpoint Not Met During Occupied Heating
Time Setpoint Not Met During Occupied Cooling

Understanding/Interpreting/Calculating the number of unmet hours:

There will be an "unmet hour" for "Time Setpoint Not Met During Occupied Cooling" in a particular zone when the zone indoor temperature is higher than the cooling set point specified for that hour. Likewise for heating when the zone heating setpoint temperature is not met.
The number or the percentage of unmet hours in a building is usually given as one of the outputs of the simulation.
Zone wise, unmet hours can be read from the EnergyPlus: Output Variable, Time Cooling Set point Not met. When two zones are unmet at the same hour, this will count to one unmet hour for the building.
When two zones have unmet hours during different non overlapping times of a day, the total number of unmet hours in that day is the summation of these unmet hours of each zone. This total for the year should be considered as the total unmet hours of the building.

Example

When each zone is unmet in the specified hours as beside

Zone 1 unmet during hours: 6 8 14 16

Zone 2 unmet during hours: 6 8 12 16

Zone 3 unmet during hours: 7 8 12 13

Total number of unmet hours of the building: 7 hrs and not 12 hrs.

6 7 8 12 13 14 16

When percentage of unmet is specified, then this is the percentage of total number of hours (1 year- 8760 hours) for which the simulation is performed (not just the occupied hours).
As per ASHRAE 90.1-2004, the unmet load hours of the total building should be less than or equal to 300 hours and unmet load hours for the proposed design shall not exceed the number of load hours for the baseline building design by more than 50.
If unmet load hours in the proposed building exceed the unmet load hours in the baseline building by more than 50, then the size of equipment in the baseline building shall be reduced incrementally, until the condition is satisfied.
ASHRAE and USGBC say that it is OK if the baseline unmet load hours exceed the proposed unmet load hours by more than 50, provided they are both below 300.

The above information on Unmet hours is sourced from a post by Ashu Gupta (and subsequent posts) on the bldg-sim mailing list