30 November 1998
I. Cooling energy can be saved by using outside air for cooling when outside air conditions are in
region A. Less energy will be used in cooling outside air than in cooling return air when outside air
conditions are in region D. When outside air conditions are in region B, the outside air dry-bulb
temperature is less than the return air dry-bulb temperature; however, excess cooling energy would be
used if more than the required minimum of outside air is used, because the enthalpy of the outside air is
higher than the design return air condition. When outside air conditions are in region C, there is no
energy saving available from the use of outside air.
j. Figure 3-8 illustrates the method for selection of a setpoint for the DEV contact for economizer
mode switchover in a relatively humid southeastern United States city, based on the published weather
data in TM 5-875. The designer will consult the local weather data for the nearest location of the project.
The method presumes that the location is such that an economizer mode is acceptable in the HVAC
design because it would not place an energy burden on the system due to a requirement for
humidification of more than the minimum quantity of outside air. Using a psychometric chart, the
designer will use the following procedure to determine the setting of the DEV contact:
(1) Plot a constant-enthalpy line (B-C) through the return air design temperature and relative-
humidity condition (A). If the outside air conditions are below this line, the total heat content of the
outside air is less than that of the return air and it can be used for cooling.
(2) Plot an average-weather line (E-F) by using the midpoint of the 2.8 degree C (5 degree F) bin
and the mean coincident wet-bulb temperature for that temperature bin from TM 5-785.
(3) Read the difference in dry-bulb temperature between the design return air temperature and
the outside air temperature where the average-weather line crosses the constant-enthalpy line (D-G).
(4) Use this difference in dry-bulb temperatures as the setting for the DEV contact.
The temperature differential setpoint of the DEV contact is shown as 4.5 degrees C (8 degrees F).
However, the temperature differential determined by this method will vary with: the design return-air
conditions; and the average weather line for the locality. Less-humid climates will tend to shift the
average weather line downward toward the design return air condition, which would result in a smaller
differential. The effect on energy conservation of using this method is shown in figure 3-9.
Figure 3-8. Selecting the economizer switchover point.
Figure 3-9. Effect on energy conservation of selection of the economizer switchover point.
k. Figure 3-9 shows that the dry-bulb temperature line at the intersection of the average weather line
and the constant enthalpy line bisects region B. The area shown as region B-1 represents outside air
conditions when the economizer mode will not save cooling energy even though outside air beyond the
minimum quantity will be used if the control system modulates the dampers open. The area shown as
region A-1 represents outside air conditions when the economizer mode will save cooling energy. The net
effect on energy use depends on how many operating hours per year of the HVAC system are coincident
with the occurrence of the outside air conditions of region B-1.