Impact of Diversity Factors on HVAC Load Calculations

Internal heat gain from lights, people and equipment often contribute towards the majority of cooling load in a modern commercial building. Building envelopes have improved in response to more restrictive energy codes in turn making load calculation more sensitive to internal gains. Examples of internal gains as a driving factor for cooling loads include lighting in retail stores, occupancy in auditoriums, and equipment in offices and manufacturing facilities.

The diversity of internal gains has a substantial impact on the peak load and design conditions. Simplified calculation methods cannot keep up with the dynamic nature of heat exchanges and internal loads. This leads to overestimation of loads and thereby oversizing of equipment.  Diversity is defined as the probability that an internal gain will be active at the time of peak building load. An ASHRAE research project^[1] on office buildings noted diversity factors for lighting ranging from 70 to 85% and for receptacles between 42 to 89%.

To illustrate the impact of diversity factors on peak building load, an energy simulation was performed on a 10,000 sq. ft office building in Atlanta, Georgia using IES Virtual Environment (VE). Diversity factors of 75% and 70% were applied to lighting and receptacle gains, respectively.  Two simulations were performed and a difference of 7% was identified on the peak cooling load of the building with and without the use of diversity factors.



Fig 1: Impact of diversity factors on peak cooling load

Using the IESVE Software for Loads Calculations 

IESVE performs load calculations using the ASHRAE heat-balance method and has the ability to incorporate diversity factors for zone/room load and system sizing calculations. The VE provides functionality to include or exclude diversity factors for each sizing step and saves a designer’s preferences for future projects.


Fig 2: HVAC System loads calculation interface in IESVE
 

For example, a designer can turn off diversity factors for zone/room loads calculations. These results can help the designer analyze the impact and contribution of envelope and internal gains on the non-coincident cooling/heating loads. Diversity factors can then be incorporated at system sizing calculations to get optimum equipment sizing.

Learn more about calculating loads for HVAC design using the VE. 

[1] ASHRAE research project RP-1093