Analysis of how HVAC loads differ for different programming layouts

Example simulated hourly cooling load time-series

Example simulated hourly cooling load frequency chart

HVAC system sizing calculations are typically performed with dedicated load calculation software, but BEM analysis is also useful for evaluating the relative peak heating and cooling load impacts for alternative programming layouts. Those results can be used to identify opportunities to minimize HVAC system size and the associated construction costs.

In contrast to typical load calculation methods, BEM also provides information about annual heating and cooling loads and the associated HVAC system energy consumption. And hourly outputs from BEM software provide useful information about the frequency of heating and cooling loads, which can provide insights useful for HVAC design decisions. Those frequency results will often highlight the value of designing for partial-load efficiency.

See also these related space-programming topics:

• Analysis of energy and comfort impacts of different programming layouts
• Analysis of the impact of locations of special zones

Impact of Programming Layouts

The focus in this section is on peak heating and cooling loads, considering 1) the impact of programming layouts on peak loads and 2) the impact of those peak loads on HVAC and architectural design. The impact of space layout on energy and comfort is covered here.

Impact of programming layouts on loads

The loads in each space -- the amount of heat that needs to be added or removed to maintain the air temperature at setpoint -- depend on several factors.

• Internal heat gains. For alternative space layouts, the internal heat gains for each space type will probably remain the same.
• External heat gains or heat losses. The external gains and losses in a space can vary significantly and are influenced by several design factors, each of which might vary depending on space layout.
  • Fenestration. Window area, orientation, SHGC and U-factor are often the most important factors. See define fenestration (glazed constructions) and fenestration and daylighting options.
  • Opaque envelope. Exterior surface area, U-factor and heat capacity. See constructions and thermal mass options.
  • Indoor design conditions. Air temperature requirements for comfort or other needs.
• Space characteristics such as thermal mass. Space characteristics could vary depending on space layout if there are changes, for example, in a space’s exposure to surfaces with high thermal mass. See constructions and thermal mass options.

Impact of peak loads on HVAC design and architectural design

At the individual space level, peak load calculations are used to determine airflow requirements (or required radiant system capacity). That airflow requirement is used to determine the size of ducts and quantity of diffusers needed to deliver air to each space. Therefore, peak loads affect architectural design in terms of space required for ducts, typically above the ceiling, and ceiling space required for diffusers.

At the HVAC system level, peak loads affect the space required for air shafts and other airflow paths. They may also affect the size of mechanical rooms and the size and weight of HVAC system components.

In some cases, the magnitude of peak heating or cooling loads affects the possibility of choosing alternative HVAC system types such as radiant heating and cooling systems, which have practical limits on the rate of heating or cooling they can deliver to a space.

Programming Layout Alternatives

Given the impacts that space layouts can have on peak loads, as described above, there are several strategies to consider that can minimize peak heating and cooling loads.

Minimize unwanted solar heat gain

• Avoid east and west-facing windows by locating spaces that do not need windows, such as restrooms or storage rooms, at the perimeter on those orientations.
• Provide spaces that have view requirements with north-facing windows or appropriately shaded south-facing windows.

Relax the design conditions for indoor air temperature in perimeter zones

• Locate spaces with transient occupancy and relaxed air temperature design conditions, such as corridors, at the perimeter.
• Move occupants away from windows, reducing the comfort impact of window surface temperature and allowing the indoor design air temperature to be lower in winter and higher in summer while maintaining equal comfort.

Take advantage of thermal mass

• For special zones with highly variable loads, such as conference rooms, locate them so the space contains thermal mass to spread out heat gains over time and reduce peak cooling loads. See Analysis of the impact of locations of special zones

See also the space layout alternatives described here: Analysis of energy and comfort impacts of different programming layouts

Guidance on Modeling Approach

Example time-series plot of simulated zone air temperature, useful for model review

In addition to the guidance on this page, here are considerations for evaluating some of the strategies described above.

Cooling and heating temperature setpoints

Choose model inputs for indoor air design temperature and heating and cooling thermostat setpoints that are appropriate for each space type and that account for the comfort impact of the interior surface temperatures of exterior walls and windows.

Accurate construction modeling

Pay attention to thermal mass and thermal bridging in the approach used for opaque constructions as described here: define opaque envelope constructions.

Check that simulated systems meet heating and cooling loads

Check for “unmet hours”. Review hourly output for zone air temperature in a sample of important zones.

Review load results

Review hourly outputs for system heating and cooling loads, and check for unexpected load peaks or patterns that could be a result of input error.

Use future weather data

Consider running the analysis with future weather data in addition to historical weather data to check for potential peak load impacts of changing climate. See weather data selection.

Design day vs. weather file

Be aware that a weather file used for simulation may or may not include outdoor temperature conditions similar to the design day used for HVAC load calculations.

Guidance on Presenting Results

When presenting heating and cooling load results from a BEM analysis, it is often helpful to show both time-series hourly results and a plot of load frequency. These plots can provide insights into how many hours per year the system operates at partial load versus peak load.