Legacy Square - Right Sizing HVAC Systems

Santa Ana

Legacy Square is a 91 Unit, affordable, multi-family housing project currently under construction, located in Santa Ana, California.

The Project: Legacy Square is a 91 Unit, affordable, multi-family housing project currently under construction, located in Santa Ana, California.  The project is funded, in large part, by the Affordable Housing, Sustainable Communities (AHSC) Program, which receives proceeds from State of California's Cap and Trade program, which mandates high performance and deep energy efficiency.  The project is "all-electric", utilizes heat pump hot water heaters (one per unit) and will have a rooftop photovoltaic system that will offset approximately 40% of the building's energy loads.   Right-sizing HVAC systems at residential unit level is critically important to reducing the building energy load, reducing the amount of renewable energy needed to hit the project's funding goals.   This is very important for a project where the architectural style and roof forms are dictated by local standards.  

Refining the Conceptual Energy Model for HVAC Analysis:  In a previous IESVE Case Study (see the Iris at San Ysidro), National CORE's (NCORE's) approach to conceptual energy analysis was detailed.   The next step in NCORE’S  process, once the architectural team has refined the building plans, elevations, and window sizes, is to update any geometry changes, confirm window sizes, add shading elements (overhangs, porches, or other local shades), and subdivide larger massing blocks into individual apartments.   Whereas in the conceptual energy analysis, residential units were aggregated floor by floor into zones and glazing aggregated to the zone, during this phase, HVAC loads (envelope, glazing, ventilation, internal gains, etc.) are precisely tabulated so that systems can be sized to within 15% of the calculated load, satisfying the LEED for Homes and Energy Star Homes requirements.  Without detailed analysis and scrutiny, many design teams can fall back into "rule of thumb" design, or can tabulate sizing based on inaccurate loads.  National CORE believes in investing more in higher efficiency equipment, but buying only what is needed, an approach we believe is cost neutral.

Method: NCORE uses IESVE to help them  accurately model the building for each individual apartment unit, allowing  them  to perform visually quality control of construction assemblies, fenestration, and thermal templates, then create ASHRAE level reports that can be used for scrutiny by the design team, or review by others (in their  case by a LEED for Homes Green Rater).  The HVAC system sizing modeling workflow within IESVE includes:

  1. At the end of Design Development (when unit plans, floor plans, and elevations have been fully refined), the conceptual building model (developed previously) is adjusted for design changes and larger building blocks of units are broken down into individual apartments, which are numbered to match architectural plans. DXF files, provided by the design team, are loaded into ModelIT to verify geometries. Window sizes are adjusted, and exterior shading elements added.
  2. Construction assemblies (walls, floors, roofs, fenestration, and storefronts) are adjusted, if needed, from the conceptual model, if any changes were made. While NCORE doesn't see much adjustment to the U-values of opaque surfaces, they do see adjustments in storefront glazing. (Thermally broken storefront glazing is expensive, so they like to limit quantities as much as possible, using NFRC rated windows whenever possible).
  3. HVAC zones are set up for residential units. Preliminarily, NCORE breaks residential zones down by unit size (studios, one-bedroom, two-bedroom, three bedroom, etc). This will allow for a more refined approach to sizing when the ApacheHVAC (ASHRAE) reports are generated. Commercial spaces are zoned separately and they work closely with the mechanical engineer to define the zoning.
  4. Within ApacheHVAC, one system is created for each HVAC Zone. For initial analysis for the residential units, are configured using high-efficiency, ducted minisplits, 1-Ton units for one-bedroom units, 1.25 Ton units for two-bedroom units, and 1.5 Ton units for two-bedroom units. The efficiencies of these units vary slightly. (Later, when HVAC sizing is complete, NCORE will refine HVAC Zone groups and shift units around as needed (i.e. some one-bedroom units may require 1.25 Ton, while the others require one-ton, etc).
  5. Within ApacheHVAC, first run the "Room and Zone Level Sizing", which will help guide decisions about load reduction and provides engineering checks. Within the "ASHRAE Loads" dialogue box, be sure to check the box "Generate Reports", then open the "Room and Zone Loads for ApacheHVAC Options..." then select the reports desired. NCORE likes the "Room & Zone Loads" Excel report. The ASHRAE Loads calculation methodology is the Heat Balance (HB) Method. When this report is run, keep in mind that the "Room and Zone Loads" analysis does not account for code required mechanical ventilation as the delivery mechanism of ventilation air has yet to be defined.
  6. Next, run the "System Equipment and Plant Sizing" analysis. This report includes required mechanical ventilation (which we build into the ApacheHVAC system for each multi-plex zone). For multi-family residential, this is more important as it will contain all information needed to size the HVAC systems. Generate the ASHRAE reports, which will be printed by Zone.
  7. Within the "Room & Zone Loads" Excel report, NCORE adds an additional column, which shows the system loads, which are now higher accounting for the introduction of outside air introduced through the mechanical systems.
  8. If you are the mechanical engineer, these reports and the Excel spreadsheet provide the basis for making final system selections. If you are the energy analyst, or the architect providing high-performance energy analytics to the engineering team (an approach NCORE takes), provide these reports to the mechanical engineer to begin verifying loads assumptions and finalizing system selections. Along with the ASHRAE reports and the Excel spreadsheet, NCORE likes to use the "Input Data Visualization (Model Viewer II) within APACHE to visualize the HVAC loads.

Final Thoughts & Conclusions: Energy analytics, especially for affordable, multi-family housing, is a multi-step process, beginning with conceptual energy analysis (see previous case study), continuing into the HVAC system sizing analysis. There are other steps in this process, including an analysis of common area vs. residential energy loads, renewable energy analysis, operational economics, and code compliance. These steps will be covered in future case studies. The path to cost-contained, high performance affordable housing requires precision, and whenever possible, modeling efficiencies. NCORE relies on IESVE to help pin down critical analysis, providing detailed reports, paving the way for them towards zero net energy and carbon neutral projects.

"Continuing our exploration of how IESVE benefits the affordable housing projects we design, build and operate, Legacy Square is an example of how we can achieve a high-performance, climate-conscious and affordable building design within a limited budget. National Community Renaissance continues to lead the affordable housing industry, telling the real-world story, showing what is possible in affordable housing. When it comes to high performance, zero net energy, and carbon neutral projects:  If we can do this, everyone can."

Tim Kohut, AIA, Director of Sustainable Design at National Community Renaissance, California