Air Conditioning Zoning: Understanding How to Design for Efficiency and Comfort

Zoning is an important aspect of air conditioning design that refers to the areas served by an AC unit. When a single unit serves multiple zones, they may get hotter or cooler at different rates due to varying internal and external heat loads throughout the day. This is where zoning design comes in, as it ensures that each zone is cooled or heated adequately and efficiently.

One key factor in zoning design is solar gain, which is the solar radiation that passes through windows and heats up the walls and floors of a building. If a large office building's east and west zones are served by the same unit, the office floor could be in a constant state of being "too cold" or "too hot," depending on the sun's position. That's why zoning designers take into account the relative location of each zone, as well as the expected solar gain, occupancy density, and floor area. This is especially important in office spaces, where good zoning is the hallmark of good design.

For perimeter zones (those that have significant variation in heat load due to solar gain), floor area is a key factor in ensuring that the zone catches any solar radiation that enters the building. In a typical office building with full-height windows, the floor area is usually no greater than 200 sqm and no less than 50 sqm, and the zone is typically sized so that the associated center zones (those that have consistent loads) will experience no variation at all.

However, in center zones (those without solar radiation gains), load variations are typically caused by changes in occupancy. Humans generate heat and require fresh air, so in galleries, lecture theaters, or cinemas where the number of people in a space is expected to change significantly, varying AC output can lead to significant energy savings and ensure occupant comfort.

Figure 1 - Office Zoning

Floor area is key to insuring that a perimeter zone will catch any solar radiation which enters the building. In a typical office building with full height windows this is normally four meters (around 13 feet). On top of this the zone should be no greater than 200 sqm, and no less than 50 sqm, this ensures that the item of HVAC equipment which is controlling this zone is responsive enough to ensure comfort conditions.

While most of these factors relate to perimeter zones, center zones (typically zones without solar radiation gains) are rarely affected by anything other than variation in occupancy. Humans generate heat and must be provided with fresh air associated with their presence. In a well defined center zone, the load associated with human presence can make up well over 50% of the total. As such, in galleries, lecture theaters, or cinemas, anywhere the number of people in a space is expected to significantly change, varying AC output can present significant energy savings as well as ensuring occupant comfort.

In residential settings, zoning is typically separated into bedrooms and living spaces. The ratio of windows to floor area is usually higher in a home than in an office, so there's no value in attempting to create perimeter zones. Instead, the zones are defined by their occupancy.

Bedrooms present relatively small volumes of constant heat loads, so a proper heat load assessment can determine whether any number of bedrooms can be served by a single unit. Lounges and dining rooms, on the other hand, may see a variable number of people entering them at any point in time and typically have higher window-to-floor ratios than bedrooms. For this reason, any home fitted with ducted air conditioning will typically be fitted with at least two indoor units: one for the bedrooms and one for the living spaces.

Figure 2 - Residential Zoning

For these reasons, any home fitted with ducted air conditioning will typically be fitted with at least two indoor units, one for the bedrooms and one for the living spaces.