Diversity in Sizing calculations

Increase the plant/air system diversity. Lower the peak plant/air system load.

Written by Saurav Subhash
Updated over a week ago

In the context of HVAC system design and sizing, diversity refers to the variation in simultaneous demand for heating or cooling across different spaces or equipment. When we apply the concept of diversity to the different levels of an HVAC system—room, zone, air system, and mechanical plant level—it pertains to the recognition that not all spaces or equipment will need maximum capacity at the same time. Here is what diversity implies at each level:

1. Room Level: At this level, diversity refers to the different heating and cooling loads for each room in a building based on various factors such as room size, number of occupants, and sun exposure. For instance, a room on the sunny side of the building might require more cooling than a room on the shaded side.

1. Room equipment is typically sized based on individual peaks, ignoring diversity. On the platform this is defined by the Winter and Summer Design Day Schedules.

2. Zone Level: A zone usually comprises multiple rooms that are served by a common control system (like a thermostat). Diversity at this level means that even though all rooms in a zone may not need maximum heating or cooling simultaneously, the system should be sized to handle the peak load when it occurs.

1. Zone equipment is typically sized based on the coincident peak of the grouped rooms.

3. Air System Level: An air system serves multiple zones. Diversity at this level recognizes that not all zones will peak simultaneously. This allows for equipment sizing that is less than the sum of the peak loads of all zones, thereby saving on installation and running costs.

1. Similar to Zones, Air Systems are sized based on the coincident or block peaks of the grouped zones.

4. Mechanical Plant Level: At the plant level (like a chiller or boiler plant), diversity accounts for the fact that not all air systems will need maximum heating or cooling at the same time. This means that the total plant capacity can be less than the sum of the peak loads of all the individual systems it serves.

1. Again, similar to Air Systems the Mechanical Plant sizing is based on the coincident or block peaks of the groups Air Systems and coils.

In essence, the concept of diversity allows HVAC designers to optimize system sizing, taking into account variations in demand across different areas at different times, ultimately saving energy and costs.