7.14.7 Economizer Cycle

• Provide outside air "temperature economizer cycle" for comfort air-conditioning or equipment cooling only when humidity control is not required.

• Provide economizer cycle only on systems greater than 3000 CFM (1,416 1) that are operated 8 or more hours per day.

• Enthalpy control for the economizer cycle should not be provided.

7.15 Ductwork design

All ductwork for heating/ventilating-only systems should be insulated where future air-conditioning of the building is anticipated.

• Return air ductwork should be routed into each area isolated by walls, which extend to the roof structure. The designer should not use transfer ducts or openings.

• The use of round or oval prefabricated ductwork is recommended. Round/oval prefabricated ductwork reduces leakage and friction losses and reduces the amount of conditioning and fan energy required. The additional material cost for round/oval prefabricated ductwork would be at least partially offset by cost and time savings.

7.15.1 Variable Air Volume (VAV) Systems

VAV air handling systems and their associated HVAC control systems, due to com­plexity, require more critical and thorough design. When VAV is selected over other types, the following questions must be discussed during design:

• Were other HVAC systems considered and why were they not selected?

• Was a constant volume system with VAV bypass boxes considered?

• How will outside ventilation air be controlled during periods of low cooling loads?

• How will adequate heating be provided along outside walls and perimeter zones, including the need for supplemental baseboard heat?

• Was a multizone system with space discriminator reset of hot and cold deck tem­ peratures or a single zone system with space discriminator control of supply air temperature considered in the design process?

7.15.2 Special Criteria for Humid Areas

The criteria described in this section must be used in the design of air-conditioned facil­ities located in areas where the

• Wet bulb temperature is 67°F (19°C) or higher for over 3000 h and outside design relative humidity of 50% or higher

• Wet bulb temperature is 73°F (23°C) or higher for over 1500 h and the outside design relative humidity is 50% or higher, based on 2.5% dry bulb and 5.0% wet bulb temperatures

Air-conditioning will be provided by an all-air system. The system may consist of a central air-handling unit with chilled water coils or a unitary direct expansion-type unit capable of controlling the dew point of the supply air for all load conditions. The following systems should be considered:

• Variable volume constant temperature

• Bypass variable air volume

• Variable temperature constant volume

• Terminal air blenders

In addition to life-cycle cost considerations, system selection will be based on the capa­bility of the air-conditioning system to control the humidity in the conditioned space con­tinuously under full load and part load conditions.

• System selection should be supported by an energy analysis computer program that will consider the latent heat gain due to

—Vapor flow through the building structure —Air bypassed through cooling coils

—Dehumidification performance of the air-conditioning system under varying external and internal load conditions

• Low sensible loads and high latent loads (relatively cool cloudy days) will, in some cases, raise the inside relative humidity higher than desired. If analysis indi­ cates that this condition will occur, reheat must be used in the design selection.

• Room fan coil units will not be used unless dehumidified ventilation air is sup­ plied to each unit or separately to the space served by the unit and positive pres­ sure is maintained in the space.

• Draw-through type air-handling units will be specified to use the fan energy for reheat. The air distribution system will be designed to prevent infiltration at the highest anticipated sustained prevailing wind.

• Outside air will be conditioned at all times through a continuously operating air- conditioning system.

• The supply air temperature and quantity and chilled water temperature will be based on the

—Sensible heat factor —Coil bypass factor —Apparatus dew point

• The 1% wet bulb temperature will be used in cooling calculations and equipment selections.

• Closets and storage areas should be either directly air-conditioned or provided with exhaust to transfer conditioned air from adjacent spaces.

• Where reheat is required to maintain indoor relative humidity below 60%, heat recovery, such as reclamation of condenser heat, should be considered in life cycle cost analysis.

• Economizer cycles will generally not be used due to the high moisture content of outside air.