Changes in the weather, occupancy, activities, and solar exposure cause heating and cooling loads to vary over time. Spaces having a different use or exposure require a different control to maintain constant temperature. Some areas with special requirements may need individual control or individual systems, independent of the rest of the building. Variations in indoor conditions, which are acceptable in one space, may be unacceptable in other areas of the same building. Each of these can be considered in the HVAC system selection and design as individual zones. The extent of zoning will be affected by the degree of control required in each zone, and by use of the space.
HVAC system selection and design
The selected system is typically designed to meet the intended use of the space at the time it is designed and installed. No matter how efficiently a particular system operates, or how economical it may be to install, it cannot be consider acceptable if it (1) does not maintain the desired interior environment within an acceptable tolerance under all conditions and occupant activities and (2) does not physically fit into the building without being objectionable.
Rarely is thought given to the possible future reuse of the system. There are three major design aspects that may inhibit the reuse or rezoning of any given space. Changing any or all of these to meet new space use requirements can be difficult and costly.
1. Cooling coil depth which establishes the capability to dehumidify the air
2. Air balance in terms of the cfm of supply air to the zone
3. Outdoor air required to meet the ventilation standards
In the latter case, changes in Ventilation Standards may either help or deter reuse. For example, the table below shows ventilation changes for several different space uses.
|Comparison of cfm per person ventilation rates for selected applications|
New Standard 62-1999
|Restaurant dining rooms|
|Cafeteria, fast food, dining halls|
|Bars, cocktail lounges|
|Hotel/motel meeting rooms|
|Office building offices|
|Office building conference rooms|
|Office building reception areas|
|Theatre seating areas|
|Education building classrooms|
|Correction facility cells|
The previous standard 62-89 stipulated a minimum ventilation rate of 15 cfm per person. The principal reason the rates are reduced in the new standard is based on a fundamental perspective shift. The 62-89 was based on studies that indicated 15 cfm per person was the level where 80% of the people entering a space found the room odor acceptable. It should be noted that these same studies indicated once people adapted to the space, 80% of the people found odors acceptable at a much lower rate of 5 cfm per person. While 52-89 used the unadapted rate, the new 62-1999 decided to use the adapted rate modified by carbon dioxide (CO2) concentrations.
The new standard adjustments are largely influenced by carbon dioxide based demand-controlled ventilation and offers the designer options via the prescriptive method who wish to avoid costly over-ventilation of spaces subject to variable or intermittent occupancy. The simplified method can be used for single-zone spaces, and the prescriptive method can be applied to all spaces.
Any reuse or rezoning of existing building spaces must be examined to determine if the new use of the space can be properly served by the existing system components in terms of humidity control, supply air cfm and adequate ventilation.
Links to Related Topics