VAV DESIGN

FOR

IMPROVED

INDOOR AIR QUALITY

“Air conditioning is the control of the humidity of air by either increasing or decreasing its moisture content. Added to the control of humidity is the control of temperature by either heating or cooling the air, the purification of the air by washing or filtering the air, and the control of air motion and ventilation.”

Willis H. Carrier

1. COMPLETE AND PERFECT MIXING OCCURS BETWEEN ALL OF THE ROOM AIR AND THE SUPPLY AIR

2. THE RETURN AIR IS LEAVING THE ROOM AT THE TEMPERATURE OF THE ROOM

3. THE INTRODUCTION OF THE SUPPLY AIR GENERATES AN ACCEPTABLE AMBIENT AIR VELOCITY IN THE ROOM

4. EITHER THE FLOW RATE (CFM) OR THE T OR SOME COMBINATION OF BOTH CAN BE VARIED TO SPAN THE ENTIRE RANGE OF CAPACITY REQUIREMENTS

5. ADEQUATE VENTILATION AIR MUST BE CIRCULATED THROUGH THE SPACE AT ALL TIMES TO REMOVE THE CONTAMINENTS

q = CFM (1.08) (tr - ts)

HEAT-COOL-OFF

DUAL STREAM

REHEAT

DUAL STREAM

qs = CFM (1.08) (tr - ts)

CONSTANT CFM, VARIABLE T

EXAMPLES:

DOUBLE DUCTMULTIZONE

TERMINAL REHEAT

qs = CFM (1.08) (tr - ts)

CONSTANT CFM, VARIABLE T

VARIABLE AIR VOLUME (VAV)

qs = CFM (1.08) (tr - ts)

VARIABLE CFM, CONSTANT T

Poor mixing of supply air

Inadequate ambient air circulation

Dumping

Reduced ventilation rates as load decreases

• The dehumidification of the outdoor ventilation air and the space can be separated from the space temperature control

• The ventilation supply need not be integrated with the space temperature control

• The Economizer System is an option

RULES1. The building envelope must be well defined and must

be secure from any opportunity for mass air flow at conditions different from the indoors and must prevent migration of water vapor due to vapor pressure difference

2. The envelope itself must prevent vapor migration by use of continuous vapor barriers properly located

3. The point at which outdoor air is introduced into the space must be well defined

4. No air should be introduced into the space at moisture conditions different from those desired in the space

1. The conditioned makeup or ventilation air can be supplied to and distributed to the space in a separate duct and distribution system, or

2. The makeup air can be introduced into the return side of the recirculating units

3. There could be a single makeup air unit for a building with multiple recirculating units, or

4. There could be a makeup air unit for each recirculating unit

• Cost

• Load dynamics

• Occupancy patterns

• Ratio of Outdoor Air to Recirculated Air

1. In warm humid climates the age-old problem of poor part-load humidity control in the spaces was solved, and

2. The major sources of indoor air quality problems are eliminated. These are

a) Uncontrolled microbial growth

b) Poor air circulation and mixing

c) Inadequate ventilation air quantities

DESIGN GUIDELINES

1. UNDERSTAND THE LOAD AND DESIGN THE SYSTEM TO SERVE THE ANTICIPATED LOAD PLATEAUS

2. CONTROL THE BUILDING AT ALL TIMES

3. PROVIDE A SEPARATE MINIMUM VENTILATION AIR CONDITIONING SYSTEM OR UNIT (PARTICULARLY WITH DIRECT EXPANSION)

4. USE THE SIMPLEST BUT HIGHEST QUALITY VAV TERMINAL POSSIBLE

5. USE FAN POWERED TERMINALS SPARINGLY

6. DESIGN AN AIR DISTRIBUTION SYSTEM THAT WILL PROVIDE EFFECTIVE MIXING WITHOUT DUMPING, DOWN TO THE MINIMUM COOLING LOAD THE SYSTEM WILL EVER SEE ON AN OCCUPIED CYCLE, OR THE MINIMUM DICTATED BY THE EFFECTIVE PERFORMANCE

7. BELOW THE MINIMUM PERFORMANCE QUANTITY, IF THE LOAD CONTINUES TO DROP, REHEAT IS NECESSARY

8. RESET THE COLD DECK IF THIS CAN BE DONE WITH OUT ADDING TOO MUCH COMPLEXITY

9. DESIGN THE AIR HANDLING SYSTEM FOR THE LOWEST TOTAL PRESSSURE THAT IS ECONOMICALLY FEASIBLE (TRY TO HOLD THE UPPER LIMIT AT 4” w.c.)

10. DON’T DEPEND ON DIRECT DIGITAL CONTROL TO SOLVE PROBLEMS THAT THE DESIGNER DOESN’T UNDERSTAND