30 November 1998
a. The schematic diagram and ladder diagram additions to incorporate control for this type of
preheat coil are shown in figure 5-8. The steam coil valve, VLV, is controlled from a 2-position
thermostat, TSL, in the incoming outside air duct. The thermostat opens the steam valve when the
outside air temperature drops to its setpoint and remains open as long as the outside air temperature is
at or below the setpoint. The loop consisting of devices TT, TC, IP, and DA controls the air temperature
in the discharge of the coil by modulating the preheat coil face and bypass damper to maintain the
preheat coil discharge air temperature.
Figure 5-8. Control system variations for steam preheat coil with face and bypass dampers.
b. Thermometer TI in the outside air intake is required if there is not already such a device at an
outside air temperature transmitter associated with an economizer controller.
c. The control devices and their parameters must be added to the equipment schedule.
7. CONTROL SYSTEM VARIATION FOR HOT WATER OR GLYCOL PREHEAT COIL. The loop to be
added to control system schematics for modulating control of such preheat coils is as discussed in
paragraph 3-3 and shown in figure 3-2. When required, this loop can be added to any HVAC system by
showing the loop in the control system schematic, showing the loop devices and their parameters in the
equipment schedule, and showing the controller and related devices in the control panel drawings.
8. CONTROL SYSTEM VARIATION FOR COMBINING HYDRONIC SYSTEM AND AIR SYSTEM
CONTROLS IN THE SAME CONTROL PANEL. This variation is shown in Chapter 4 in the single-zone
HVAC system with humidity control. Small buildings generally require a hydronic heating system or a
dual-temperature water system and an air handling system. When appropriate, the designer will
combine such systems into a common HVAC control panel.
9. UNOCCUPIED MODE SPACE TEMPERATURE SETBACK CONTROL FOR TERMINAL UNITS. In
Chapter 4, the control systems for unit heaters and perimeter radiation are shown with room thermostats
capable of one temperature setting. When the hydronic systems serving such units are controlled to
maintain a reduced space temperature in the unoccupied mode, a microprocessor-based room
thermostat will be substituted for the single-temperature thermostat shown in Chapter 4. Examples of
the substitution are shown in figures 5-9 and 5-10.
Figure 5-9. Control system variations for unoccupied mode setback.
Figure 5-10. Control system variations for unoccupied mode setback.
10. CONTROL SYSTEM VARIATIONS FOR 2-WAY SHUTOFF VALVES FOR FAN COIL UNITS. For
dual-temperature hydronic systems with variable flow pumping, the fan coil units will have 2-way shutoff
valves in lieu of 3-way shutoff valves. The designer may show 3-way shutoff valves on selected fan coil
units for pump relief of the variable flow pumping system. The schematic and ladder diagram variations
are shown in figure 5-11.
Figure 5-11. Control system variations for 2-way shutoff valve on fan coil units.