Gathering HVAC input data for detailed design models

Detailed design model HVAC inputs should represent the project's actual system design as specified on the HVAC drawings for design development and construction documents phases. Compliance models should use inputs that represent the final design. Some operational assumptions such as hours of operation and variability of loads due to occupants moving to different spaces may still be needed even during the detailed design phases.

The relevant data sources for HVAC information are the mechanical plans, specifications, and manufacturer data.

Mechanical plan information

The primary source of model input data is gathered from the equipment schedules and HVAC floor plans. Additional information can be found in schematic diagrams, piping diagrams, and control diagrams.

Schedules

The schedules are a set of tables included in the mechanical drawings that include data on system capacities, efficiencies, and operating conditions (e.g., temperatures).

Example HVAC schedule for a packaged rooftop unit. (Source: IBPSA-USA BEM Workshop)

Air handling unit

Here’s an example of a mechanical equipment schedule for a packaged rooftop system.

Schedules for air handling units include data such as the outside airflow, supply airflow, cooling capacity, heating capacity and cooling efficiency.

These schedules often include notes that provide useful information such as whether an economizer is included.

Terminal units

Example HVAC schedule for "VAV box" terminal units. In this example, the first VAV box, has a minimum airflow fraction of 325 cfm / 1404 cfm, equal to 23%. The maximum flow in heating mode is 560 cfm / 1404 cfm, equal to 40% of full flow. (Source: IBPSA-USA BEM Workshop)

For VAV systems, the VAV box schedule provides some useful information about maximum and minimum airflow. In energy models, the minimum airflow setpoint for VAV boxes is an important input that impacts the energy performance of the system.

Terminal units with reheat will also include a capacity for the reheat coil, or a hot water flow rate.

HVAC plans

Example of HVAC floor plan for an office building with a VAV reheat system. (Source: IBPSA-USA BEM Workshop)

Zoomed in view of example HVAC floorplan. (Source: IBPSA-USA BEM Workshop)

HVAC floor plans show the layout for terminal units, ducts and diffusers. The floor plans will show how the systems serve the spaces inside the building. Roof plans will show the locations of rooftop units and other roof-mounted equipment such as exhaust fans. Sometimes, plans will include enlarged views of HVAC equipment rooms to show details of equipment connections.

In the zoomed-in example drawings (see sidebar), colors are added for illustration purposes to make components easier to see.

• In the mechanical floor plans we are seeing the ceiling tiles along with the ducts and devices located above the ceiling.
• Lighting fixtures are also included to help with coordinating positions of the diffusers.
• The red rectangle identifies the vertical supply air shaft that delivers air from the rooftop air handler.
• The main supply air ducts are highlighted in blue.
• VAV boxes are highlighted in green, and the downstream ducts are orange.
• You can identify the layout of the thermal zones by looking at the locations of the diffusers connected to each VAV box.
• Here the boundaries of the zones are highlighted with dashed lines.
• You will also see that each VAV box has a label with a number. That number is referenced in the VAV box schedule..

Even during the detailed design modeling phase, it is important to consider whether you wish to make simplifications for the model relative to the actual zoning in order to reduce complexity. In this example, it might be reasonable to combine some of these interior zones into one larger zone in the model.

Specifications

The project specifications include information that will be needed for the energy modeling process such as:

• Thermostat setpoints for different space types
• System controls schemes and sequences of operation
• Codes and standards for energy and ventilation that form the basis of design and may impact code compliance modeling approaches
• Overall energy (and/or other metric) performance goals for the project.

Manufacturer data

Some system performance information is not included in the drawings or the specifications. The design documents often document the rated system efficiencies, but a BEM model generally requires system efficiency curves or performance maps that define how the system performs in part-load conditions. This information may be available online or in manufacturer's catalogs, but most often it must be requested for a specific unit selected for the project. Some manufacturers may make this information available in system-selection software tools.