5.6 Design Criteria

General ParametersHVAC system parameters are provided here for reference, but specific energy performance directives are also listed in 10 CFR 434. Compliance with the latest versions of ASHRAE Standard 90.1 and ASHRAE Standard 62 is required for the elements of the project (architectural, mechanical, and electrical).

Outdoor Design CriteriaOutdoor air design criteria shall be based on weather data tabulated in the latest edition of the ASHRAE Handbook of Fundamentals. Winter design conditions shall be based on the 99.6 percent column dry bulb temperature in the ASHRAE Fundamentals Volume. Summer design conditions for sensible heat load calculations shall be based on the 0.4 percent dry bulb temperature with its mean coincident wet bulb temperature. Design conditions for the summer ventilation load and all dehumidification load calculations shall be based on the 0.4% dew point with its mean coincident dry bulb temperature.

Indoor Design CriteriaIndoor Design Temperatures and Relative Humidity. Indoor design temperatures and relative humidity requirements are stated in Table 5-1. The following spaces shall be kept under negative pressure relative to surrounding building areas: smoking lounge, detention cells, toilets, showers, locker rooms, custodial spaces, battery charging rooms, kitchens and dining areas. Air can be returned from the dining area space. The air from these spaces must be exhausted directly to the outdoors.

Building Pressurization. To keep dry air flowing through building cavities, systems shall be designed with sequence of operations that assure continuous positive pressure with respect to the outdoor environment until the outdoor temperature falls below 4.5°C (40°F), when the building pressure shall be brought to neutral. These building HVAC systems shall have an active means of measuring and maintaining this positive pressure relationship. The BAS shall alarm when the building pressurization drops below a predetermined low limit. In areas where exhaust systems are used or an indoor air quality contaminant source is located, a negative pressure shall be maintained relative to surrounding spaces. Calculations shall be provided that show the minimum outside airflow rate required for pressurization. Minimum outside airflow rates shall be adjusted as necessary to assure building pressurization.

Artwork. In general, it is important to keep within an RH range of 30 to 70%. In a hot and dry geographic region it makes sense to maintain a range that errs on the low side (20 to 40%), while in semitropical climates a range of 55 to 75% may be practical.

Please consult Chapter 4.1, Installation Standards, of the Fine Arts Program Desk Guide for additional information.

Energy Analysis. An energy analysis of building characteristics, the mechanical and electrical components, and all other related energy consumption elements must be performed for each design submission level project as described in Appendix A.3.

Analyses of energy-conserving designs shall include all relevant facets of the building envelope; lighting energy input, domestic water heating, efficient use of local ambient weather conditions, building zoning, efficient part load performance of all major HVAC equipment and the ability of building automation equipment to automatically adjust for building partial occupancies, optimized start-stop times and systems resets. Energy analysis shall utilize public domain DOE-2 programs. Inputs and outputs shall follow ASHRAE 90.1 Standards and 10 CFR 434.

Table 5-1 Indoor Design Conditions

Type of Area Summer
RH2 Winter
General Office 24 (75)   22 (72)  
ADP Rooms9 22 (72) 454 22 (72)  
Corridors 24 (75)   22 (72)  
Building Lobbies10 24 (75)   22 (72)  
Toilets 24 (75)   22 (72)  
Locker Rooms 26 (78)   21 (70)  
Electrical Closets 26 (78)   13 (55)  
Mech. Spaces 35 (95)5   13 (55)8  
Elec. Switchgear 35 (95)5   13 (55)  
Elevator Mach. Room10 26 (78)5   13 (55)  
Emerg. Gen. Room 40 (104)6   18 (65)  
Transformer Vaults 40 (104)5      
Stairwells (none)   18 (65)  
Comm./Tel. Frame Room7 24 (75) 45 22 (72) 3012
Storage Room 30 (85)   18 (65)  
Conference Room11 24 (75)   22 (72)  
Auditorium10 24 (75)   22 (72)  
Kitchen10 24 (75)   22 (72)  
Dining10 24 (75)   22 (72)  
Cafeteria10 24 (75)   22 (72)  
Courtrooms 24 (75)   22 (72) 454*

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Table 5-2 Air Intake Minimum Separation Distances

1 Temperatures are degrees Celsius (Fahrenheit), to be maintained at +/-1 °C (+/-2 °F).
2 Relative humidity is minimum permissible, stated in percent. Maximum permissible relative humidity is 60 percent in conditioned areas.
3 Dry bulb and relative humidity are to be maintained 150 mm (6 inches) to 1800 mm (6 feet) above the floor.
4 Relative humidity should be maintained at +/-5 percent in ADP spaces.
5 Maximum temperature. Space to be mechanically cooled if necessary.
6 Room must not exceed temperature with generator running.
7 Must comply with EIA/TIA Standard 569.
8 Minimum temperature in the building must be 13 °C (55 °F) even when unoccupied.
9 Confirm equipment manufacturer’s requirements as more stringent. Provide in-room display and monitor device (such as wall mounted temperature and humidity chart
10 System shall be designed for process cooling. Cooling system shall be a dedicated independent system.
11 Provide independent temperature control.
12 Minimum relative humidity requirements may be omitted in moderate southern climate zones upon approval of local GSA representatives.


Table 5-2 Air Intake Minimum Separation Distances

Object m ft
Property line 1 3
Garage entry, loading dock 7 25
Driveway, street or public way 3 10
Limited access highway 7 25
Grade 14 50
Roof* 0.5 1
Cooling tower or evaporative condensers 5 15
Exhaust fans and plumbing vents 3 10
Kitchen supply and exhaust air 7 25
* Roof intakes must be at least 0.2 m (8 inches) above the average maximum snow depth and the potential for drifts at the intake location must be considered. Outdoor intakes should be covered by 13 mm (0.5 inch) mesh screen. The screen should be of corrosion-resistant material and located outside of or no more that 0.2 m (8 inches) inside of the outside face of the intake grille, louver, or rain hood entry. On buildings of more than four stories the outside air supply louvers shall be located on the fourth level of the building or higher. On buildings of three stories or less, locate the intakes on the roof or as high as possible. Locating intakes high on the exterior wall is preferred to a roof location. Outside air intake is not permitted within seven meters (twenty-five feet) of loading dock or any other fume producing areas.

Air Intake and Exhaust. The placement and location of outside air intakes is critical to the safety of the occupants inside a building and must be in compliance with the security requirements of the building, as described in Chapter 8, “Security Design.” Table 5-2 provides a guide for minimum separation distances between ventilation air intakes and other building features.

Indoor Air Quality. When a building is new, volatile compounds (VOC) may be released in large quantities from materials, such as adhesives, vinyl and carpets. An outside air purge cycle shall be provided to air-handling equipment enabling evening removal of VOC build-ups during the first weeks of occupancy.

GSA recognizes the importance of adequate ventilation to maintain indoor air quality. The outside air and ventilation rates of ASHRAE Standard 62 are the minimum acceptable in GSA buildings. Instrumentation and controls shall be provided to assure outdoor air intake rates are maintained within 90 percent of required levels during occupied hours.

Where occupancy requirements are likely to generate high levels of airborne particles, special air filtration shall be provided on the return air system or dedicated and localized exhaust systems shall be utilized to contain airborne particulates.

Dilution with outside air is the primary method of maintaining acceptable indoor air quality. The site shall be surveyed to determine if there are airborne sources of contaminants that may be unacceptable for use indoors with respect to odor and sensory irritation.

Internal Heat GainOccupancy Levels. For office spaces, the average density of the occupiable floor area of a GSA building is one person per 9.3 usable square meters (100 usable square feet). Within areas occupied by workstations, the occupancy load can be as dense as one person per 7 usable square meters (75 usable square feet) in local areas. Block loads and room loads should be calculated accordingly. Sensible and latent loads per person should be based on the latest edition the ASHRAE Handbook of Fundamentals.

For dining areas, auditoriums and other high occupancy spaces, occupancy loads should represent the number of seats available. Areas such as storage rooms or mechanical rooms do not have occupancy loads.

Equipment Densities. Internal heat gain from all appliances—electrical, gas, or steam—should be taken into account. When available, manufacturer-provided heat gain and usage schedules should be utilized to determine the block and peak cooling loads. Typical rate of heat gain from selected office equipment should be based on the latest edition of the ASHRAE Handbook of Fundamentals. The cooling load estimated for the connected electrical load should be based on the electrical load analysis, and the estimated receptacle demand load outlined in Chapter 6, “Electrical Engineering, Electrical Load Analysis,” and anticipated needs of GSA’s Office of Chief Information Officer. For printers and personal computers, 80 percent diversity shall be considered.

Lighting Levels. For preliminary design loads, heat gain from lighting levels described in Chapter 6: “Electrical Engineering, Lighting, Interior Lighting, Illumination Levels,” shall be used.

Zoning CriteriaSeparate systems shall be provided for interior and perimeter zones where simultaneous heating and cooling operations may occur.

Single air handling units shall not serve multiple floors or scattered building loads. Multiple air handling units or floor-by-floor systems shall be considered as baseline. Systems designed for federal courthouses shall be limited to having no more than two courtrooms served by any single air handling unit, and that air handling unit shall be dedicated to serving only those two courtrooms.

Interior control zones must not exceed 180 m2 (1,500 sf) per zone for open office areas or a maximum of three offices per zone for closed office areas. Corner offices shall be a dedicated zone. Perimeter zones shall be no more than 4.7 meters (15 feet) from an outside wall along a common exposure. Independent zones should be provided for spaces such as conference rooms, entrance lobbies, atriums, kitchen areas, dining areas, child care centers and physical fitness areas. Perimeter zones shall not exceed 30m2 (300 sf).

If a building program shows that an office building will have an open plan layout or if the program does not state a preference, it may be assumed that up to 40 percent of the floor plan will be occupied by closed offices at some point in the future.

The supply of zone cooling and heating shall be sequenced to prevent (or at the very least, minimize) the simultaneous operation of heating and cooling systems for the same zone. Supply air temperature reset control shall be utilized to extend economizer operations and to reduce the magnitude of reheating, recooling or mixing of supply air streams.

Office of the Chief Architect,
Public Building Service,
Washington, DC

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Last Reviewed 2016-08-02