User Guide

Control Option Comparisons—Duty Cycling (DC)
Siemens Building Technologies, Inc. 2-3
The code for duty cycling HFAN is as follows:
1000 C THIS DC COMMAND DUTY CYCLES HFAN
1010 C ACCORDING TO THE FOLLOWING SCHEDULE:
1020 C
1030 C FIRST 15 MINUTES – ON, OFF, OFF (1)
1040 C SECOND 15 MINUTES – OFF, OFF, OFF (0)
1050 C THIRD 15 MINUTES – OFF, OFF, OFF (0)
1060 C FOURTH 15 MINUTES – ON, ON, ON (7)
1070 C
1100 DC(HFAN,7001)
Duty Cycling Commands
Duty cycling has two commands that can be used independently of
each other: DC and DCR. The DC and DCR commands do not have
to be defined in a specific order.
The DC Command
• The DC command turns points ON and OFF according to a
defined schedule.
• This command is best applied in situations where the
environment is constant or where there are minor deviations in
temperature or humidity. For example, a storeroom, maintenance
shop, building core, or service area.
For more information, see DC in Chapter 3–Syntax.
The DCR Command
• The DCR command turns points ON and OFF according to a
temperature range defined with a dead band (DBSWIT).
• DCR checks the temperature point value every five minutes.
• This command is best used in situations where the environment
fluctuates or where deviations in temperature or humidity are
undesirable. For example, a classroom, meeting room, lobby
area, or an area adjacent to an exterior wall.
For more information, see DCR and DBSWIT in Chapter 3–Syntax.
APOGEE PPCL User’s Manual
2-4 Siemens Building Technologies, Inc.
Enthalpy and Dry Bulb Economizer
Control
Enthalpy is the total amount of heat energy contained within one
pound of air.
When a coil cools the air, it is cooling both the air and the water
vapor contained in the air as humidity. The higher the humidity, the
more water vapor is in the air. Since humid air takes more energy to
cool than dry air, it would make sense to monitor both the
temperature and the humidity of the outside air when we consider
the economizer control.
Conventional economizer control uses only the temperatures of the
outside air and return air to determine when to minimize the damper
position. The total energy content of the return air may or may not be
higher than the outside air.
Example—Economizer Comparison
The cooling coil needs to drop the energy of the air to 23.2 Btu/lb
before it enters the room. With a conventional economizer, the
control system compares the 77°F return temperature and the 85°F
outside air temperature and closes the outside air dampers to
minimum. This results in each pound of recycled air requiring the
removal of 8.4 Btu of energy.
With an economizer that checks the enthalpy, the total enthalpies
would be compared. In this case, the 28.8 Btu/lb would be compared
to the 31.6 Btu/lb and the outside air dampers would be set to full
open. The supply air now only needs to remove 5.6 Btu for each
pound of air.
Using an enthalpy economizer for this situation has reduced the
cooling energy demand by 50%.