30 November 1998
and very early morning, require practically no domestic hot water. It is this period that defines the top end of
the offset temperature range. The P-mode control alone does not control the water temperature very close
to the controller setpoint in this kind of application. The addition of the I mode to the P mode makes the
offset range much narrower than would occur with P mode alone. However, PI modes alone cannot
handle the unpredictable diversity of demand as the peak periods start and end. What happens during the
period of light demand for hot water use is that the turn-on of a shower or the startup of a dishwasher
produces an upset in equilibrium that has a greater effect than the same event would have during a period
of heavy demand. Periods of heavy demand tend to filter out some of the effect of a single turn-on.
(3) In the control loop applications where manual tuning is prescribed, the proportional mode
constant is set as the result of a calculation. These applications cannot use the integral or derivative control
modes. Self tuning is prescribed for these applications because finding the optimum settings manually is
difficult and time consuming. When controllers self-tune, these settings are automatically optimized, and
an optimized derivative-mode setting is selected due to the controller's self-tune feature. The effects of
adding the I and D modes to the P mode is illustrated in figure 2-15, which shows the results that would be
expected with a step change in setpoint. Step changes in setpoints rarely occur in HVAC control system
applications. The illustration of the step change in setpoint is used to graphically explain the actions of P, I,
and D modes.
Figure 2-15. Comparison of P mode, PI mode and PID mode for a step change in setpoint, and the
contributions of each mode to controller output signal.
9. STANDARD HVAC SYSTEM CONTROL LOOPS.
a. Standard HVAC system control loops will be used for the design of HVAC control systems. The
standard HVAC system control loops use the single-loop digital controller and additional components.
These components are collectively called the control loop logic. The logic varies with the loop
requirements, and its purposes are to interface the loops with the operational mode signals, to modify
signals, and to interface with EMCS. The control loop logic is implemented by the use of combinations of
relays and function modules.
b. Relay coils are activated by occupied-unoccupied, ventilation-delay, safety shutdown, EMCS-
override, and other signals external to the HVAC control system. The contacts of the relays interrupt
analog signals of controllers and function modules, provide inputs to on/off control loops (such as starter
circuits), and operate HVAC control panel pilot lights.
c. All the relays, contacts, and function modules are defined for each loop. The relative physical
locations of the relays and function modules will be assigned in the HVAC control panels.
d. Each of the standard HVAC control loops is described in this Technical Manual as the designer will
show them on the contract drawings. Each control loop on the drawings will show the elements of the
control loop, such as sensor/transmitters, controller, function modules, relay contacts, current to pneumatic
signal converter (IP), and final actuator. The control loops will show all the field-mounted and panel-
mounted devices for a standard loop. Each control loop will show function modules and relay contacts that
are defined for the interfacing of analog control loops with on/off control loops.
10. SIZING AND SELECTION OF CONTROL SYSTEM DEVICES.
a. The designer will estimate the required motor horsepower of the HVAC control system air
compressor, to provide the proper requirements for incorporation into the electrical power design.