Handbook of air conditioning and refrigeration / Shan K

29.8 CHAPTER TWENTY-NINE
System Characteristics
area and lobby) are primarily under thermostat control. Prior to the improvements, all indoor fans operated
continuously, regardless of compressor operation, to maintain ventilation and air circulation for
occupant comfort. Continuous fan operation resulted in large humidity fluctuations and increased indoor
humidity levels at night when space load was reduced. Continuous fan operation also consumed
more energy and needed additional compressor operation to offset fan heat. If there are wetted surfaces
around the coil and condensate drain pan when the compressors are cycling off, moisture will be evaporated
from these wetted surfaces, extracted by the indoor fan, and supplied to the conditioned space.
An alternative improved control strategy “automatic fan mode” was adopted. The indoor fan
energized only when the compressors and heaters were operating. When the thermostat and
humidistat set points are met, the indoor fan stops and the moisture drains from the packaged unit.
The indoor fan that serves the storage room in the Salvador Dali Museum was changed to automatic
fan mode control. The new control strategy provided a better storage indoor environment with significant
energy savings. For the lobby, daytime continuous fan operation for constant air circulation
was used when the lobby was open to the public. Automatic fan mode was used at nighttime.
System characteristics of a single-zone VAV packaged system are listed in Table 29.1. In Table 29.1,
DCV represents demand-controlled ventilation and MPC indicates mixing plenum pressure control.
29.4 COMPLIANCE WITH STANDARD 90.1–1999 THROUGH SIMPLIFIED
APPROACH OPTION FOR SMALL AND MEDIUM HVAC&R SYSTEMS
Single-zone, constant-volume and single-zone, variable-air-volume packaged systems are the two
widely used air conditioning systems in residential, commercial, and industrial buildings in the
United States. For air conditioning systems in buildings two stories or less in height and with less
than 25,000 ft 2 (11,800 m 2 ) gross floor area shall be considered in compliance with the requirements
of HVAC in ASHRAE/IESNA Standard 90.1-1999 by means of a simplified approach if the
following criteria are met:
1. The system serves a single zone.
2. A single or split-packaged unit is used which is either air-cooled or evaporatively cooled and
meets minimum efficiency requirements listed.
3. An air economizer and control shall be installed as required in Sec. 21.2 with either barometric
or powered relief sized to prevent space overpressurization.
4. Heating shall be provided by packaged heat pumps, PTHPs, gas-fired warm air furnaces, electrical
heaters, or hot water heating systems and boilers with all applicable efficiency requirements
met.
5. Outdoor air supplied by the system shall be equal to or less than 3000 cfm (1420/L/s) and less
than 70 percent of the supply air quantity at minimum outdoor air design conditions unless an
energy recovery ventilation system is provided.
6. The system is controlled by a manual changeover or dual set point thermostat.
7. Controls shall be provided to prevent auxiliary electric heater operation when the heat pump
alone can meet the required heating load.
8. Controls shall not permit reheat or simultaneous heating and cooling.
9. For spaces other than those requiring continuous operation having a heating or cooling capacity
greater than 65,000 Btu/h (30.7 kW), and having a supply fan motor power greater than 3/4 hp
(0.56 kW), a timeclock shall be provided
a. To start and stop the system under different schedules,
b. That is capable of retaining programming and time setting during a loss of power of at least
10 hours
c. That has an accessible manual override allowing temporary operation up to two hours