15.1 DDC Fundamentals, Control Loops, and Sequences of Operation

15.1 DDC Fundamentals, Control Loops, and Sequences of Operation

The Building Automation sheet of the HVAC Excellence Competency and Task List opens with a vocabulary requirement: define range, setpoint, dead band, bias, trending, scheduling, digital, analog, input, output, variable, logic, actuator, positioner, Ethernet, fieldbus. It then requires explaining proportional, integral, and derivative control theory, creating flow charts for the operational sequence of VAV boxes, air handlers, cooling towers, and rooftop units, and reading commercial mechanical, electrical, and plumbing prints. Building Automation Systems (BAS) Set-up and Commissioning is a Master Specialist hands-on exam.

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1. What DDC Is

Direct Digital Control replaces mechanical, pneumatic, and electromechanical control with a microprocessor executing a program. The controller:

1. Reads inputs — temperatures, pressures, humidities, flows, switch states.
2. Executes logic — comparisons, control loops, schedules, interlocks, alarms.
3. Drives outputs — relays, valve and damper actuators, variable-speed drives, staged equipment.

Why it displaced pneumatics:

What DDC enables that mechanical controls cannot:

• Scheduling — occupied/unoccupied modes, holidays, optimal start (computing the earliest start time that still reaches setpoint at occupancy).
• Trending — logging point values over time. A trend showing supply air temperature sawtoothing every four minutes diagnoses a hunting loop that no single-point measurement would reveal.
• Resets — supply air temperature reset, duct static pressure reset (Section 12.3), hot water reset by outdoor temperature.
• Demand limiting — shedding load to hold the building's electrical demand below a target.
• Alarming — notifying before a failure becomes a complaint.

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2. Control Loop Vocabulary

The task list names these specifically, and the exam tests the distinctions.

Dead band versus differential is the classic exam confusion. Dead band applies to modulating control between two opposing outputs (heating and cooling) and exists to prevent them fighting. Differential applies to two-position (on/off) control and exists to prevent short cycling.

Digital versus analog:

• Digital (binary) points have two states: a switch, a relay, a status contact. Binary Input (BI) = fan status. Binary Output (BO) = start/stop command.
• Analog points vary continuously: a temperature, a pressure, a damper position command. Analog Input (AI) = space temperature. Analog Output (AO) = 0–10 VDC to a modulating valve actuator.

Data point types the task list names: numeric (a temperature value), Boolean (true/false), enumerated (a mode with a defined list of states: off/heat/cool/auto), and string (text such as an equipment name).

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3. Proportional, Integral, and Derivative Control

Two-position (on/off)

The simplest control. Output is fully on or fully off with a differential to prevent chattering. Appropriate for high-mass, slow-responding loads — a residential furnace, a domestic hot water tank.

Proportional (P)

$\text{Output} = K_p \times \text{Error} + \text{Bias}$ Output varies in proportion to how far the process variable is from setpoint. A narrow throttling range means aggressive response; a wide one means gentle response.

The defining limitation: proportional control always leaves a steady-state offset. Because output is proportional to error, an error of zero produces an output equal only to the bias. If holding setpoint requires 40% valve position and the bias is 50%, the loop settles wherever the error produces 40% — permanently off setpoint. Narrowing the throttling range reduces the offset but eventually drives the loop into oscillation (hunting).

Proportional plus Integral (PI)

$\text{Output} = K_p \times \text{Error} + K_i \int \text{Error}\,dt + \text{Bias}$ The integral term accumulates error over time. As long as any error persists, the integral term keeps growing and keeps pushing the output, so the loop is driven to zero steady-state offset. Integral action is sometimes called reset.

PI is the standard for HVAC. Almost every temperature, pressure, and flow loop in a building runs PI.

Integral windup is its characteristic failure: if the output is saturated (a valve already 100% open) and the error persists, the integral term keeps accumulating. When conditions finally change, the loop takes a long time to unwind and badly overshoots. Modern controllers include anti-windup logic that stops integrating when the output is saturated.

Proportional, Integral, Derivative (PID)

$\text{Output} = K_p E + K_i \int E\,dt + K_d \frac{dE}{dt} + \text{Bias}$ The derivative term responds to the rate of change of error, anticipating where the process is heading and damping overshoot.

Derivative is usually turned off in HVAC. Building thermal processes are slow and sensor signals are noisy; the derivative term amplifies noise into output jitter that wears actuators without improving control. It has a place in fast, clean loops such as duct static pressure on some systems, but a technician who finds derivative gain set on a space-temperature loop should suspect it is the cause of the hunting, not the cure.

Tuning symptoms

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4. Sequences of Operation

A sequence of operation is the written specification of what the system does in every mode. It is the contract between designer, installer, and operator, and it is the reference for both commissioning and troubleshooting.

Example: a VAV air handler

OCCUPIED MODE
 1. Supply fan starts; VFD modulates to hold duct static setpoint
    (setpoint reset by "trim and respond" from VAV damper positions).
 2. Outdoor air damper drives to minimum ventilation position.
 3. Economizer enabled when outdoor enthalpy < return enthalpy.
    Dampers modulate to hold mixed air temperature setpoint.
 4. Cooling coil valve modulates (PI) to hold supply air temperature
    setpoint, reset between 55 F and 62 F based on the warmest zone.
 5. Heating coil valve modulates (PI) to hold supply air temperature
    on a call for heat. Heating and cooling are interlocked - never
    simultaneous - with a dead band between the two setpoints.
 6. Freeze-stat (low limit) trips at 38 F: fan off, OA damper closed,
    heating valve to 100%, alarm to the operator workstation.
 7. Smoke detector or fire alarm interlock: fan off, alarm.

UNOCCUPIED MODE
 8. Fan off, dampers closed.
 9. Night setback: fan cycles on space temperature with wide setpoints
    and OA damper closed.
10. Optimal start computes the start time required to reach occupied
    setpoint at the scheduled occupancy time.

Flow charts — the task list requires creating them for VAV, AHU, cooling tower, and rooftop equipment — are the visual form of the same document, and drawing one is the fastest way to understand an unfamiliar system.

Verify the sequence, do not assume it. The most productive commissioning activity is functional testing: force each input to a value that should trigger a response, and confirm the response occurs. Override the outdoor air temperature to 55°F and verify the economizer enables. Drive the space temperature above setpoint and verify the VAV damper opens. Trip the freeze-stat and verify the fan stops and the alarm annunciates. A sequence that has never been functionally tested is a sequence that probably does not work.