HVAC Control Strategies for Engineers

HVAC Control Strategies for Engineers

HVAC controls regulate temperature, humidity, pressure, and indoor air quality by modulating equipment capacity. Proper control strategy selection and tuning ensures comfort, energy efficiency, and equipment longevity.

PID Control Theory

Proportional-Integral-Derivative (PID) controller:

$$u(t) = K_p e(t) + K_i \int_0^t e(\tau) d\tau + K_d \frac{de(t)}{dt}$$

Where:

• $u(t)$ = control output (0-100%)
• $e(t)$ = error (setpoint - measured value)
• $K_p$ = proportional gain
• $K_i$ = integral gain
• $K_d$ = derivative gain

Proportional Control

Output:

$$u = K_p \times e$$

Characteristics:

• Fast response
• Offset error (does not reach setpoint exactly)
• Gain too high: oscillation
• Gain too low: sluggish response

Typical applications: Discharge air temperature control

Integral Control

Eliminates offset by integrating error over time

Characteristics:

• Eliminates steady-state error
• Slow response
• Can cause instability if gain too high

Typical applications: Space temperature control

Derivative Control

Anticipates future error based on rate of change

Characteristics:

• Improves stability
• Reduces overshoot
• Sensitive to noise

Rarely used alone in HVAC due to sensor noise

Control Tuning

Ziegler-Nichols method:

1. Set $K_i = 0$, $K_d = 0$
2. Increase $K_p$ until sustained oscillation
3. Record ultimate gain $K_u$ and period $P_u$
4. Calculate PID parameters:

$$K_p = 0.6 K_u$$ $$K_i = \frac{1.2 K_u}{P_u}$$ $$K_d = 0.075 K_u P_u$$

Practical tuning:

• Start with P-only control
• Add I to eliminate offset
• Add D only if needed for stability

Reset Schedules

Outdoor air reset:

$$T_{supply} = T_{design} - m \times (T_{OA} - T_{design,OA})$$

Where $m$ = reset ratio (typically 0.5-1.0)

Common reset schedules:

• Chilled water supply temperature vs. outdoor air
• Hot water supply temperature vs. outdoor air
• Duct static pressure vs. VAV damper position
• Discharge air temperature vs. space load

Sequences of Operation

graph TD
    A[Start] --> B{Occupancy?}
    B -->|Occupied| C[Enable ventilation]
    B -->|Unoccupied| D[Minimize ventilation]
    C --> E{Space temp > setpoint + deadband?}
    E -->|Yes| F[Enable cooling]
    E -->|No| G{Space temp < setpoint - deadband?}
    G -->|Yes| H[Enable heating]
    G -->|No| I[Fan only]
    F --> J[Modulate cooling valve/damper]
    H --> K[Modulate heating valve]
    J --> L[End cycle]
    K --> L
    I --> L
    D --> L

Single-zone VAV sequence:

1. Minimum airflow: Maintain outdoor air requirement
2. Cooling: Increase airflow to maximum before enabling mechanical cooling
3. Heating: Reduce airflow to minimum, then enable reheat

Dual-duct sequence:

1. Cooling: Hot deck damper closed, cold deck modulates
2. Deadband: Both dampers at minimum
3. Heating: Cold deck damper closed, hot deck modulates

Advanced Control Strategies

Optimal Start/Stop

Predicts equipment start time to reach setpoint at occupancy

Start time calculation:

$$t_{start} = t_{occupancy} - \frac{T_{setpoint} - T_{current}}{R}$$

Where $R$ = building warm-up/cool-down rate (°F/hour)

Energy savings: 10-30% reduction in operating hours

Demand Control Ventilation (DCV)

Modulates outdoor air based on occupancy (CO₂ sensors)

Outdoor air calculation:

$$OA_{CFM} = \frac{(C_{space} - C_{outdoor}) \times OA_{people}}{C_{limit} - C_{outdoor}}$$

Where:

• $C_{space}$ = space CO₂ concentration (ppm)
• $C_{outdoor}$ = outdoor CO₂ concentration (~400 ppm)
• $C_{limit}$ = design CO₂ limit (typically 1,000 ppm)
• $OA_{people}$ = outdoor air per person (CFM/person)

Economizer Control

graph TD
    A[Cooling required] --> B{OA enthalpy < RA enthalpy?}
    B -->|Yes| C[Open OA damper to 100%]
    B -->|No| D{OA temp < RA temp?}
    D -->|Yes| E[Dry-bulb economizer]
    D -->|No| F[Minimum OA, mechanical cooling]
    C --> G[Reduce/disable mechanical cooling]
    E --> G

Dry-bulb economizer: Compare outdoor vs. return air temperature

Enthalpy economizer: Compare outdoor vs. return air enthalpy (accounts for humidity)

Cascade Control

Master controller sets setpoint for slave controller

Example: Duct static pressure (master) controls fan speed (slave)

Benefits:

• Improved stability
• Faster response
• Better disturbance rejection

Practical Applications

1. Tuning: Use manufacturer defaults as starting point, fine-tune based on performance
2. Deadbands: 2-4°F deadband reduces cycling and energy
3. Sensor placement: Avoid direct sunlight, drafts, heat sources
4. Commissioning: Verify sequences match design intent

Related Technical Guides:

• Variable Flow System Design
• Building Pressurization Control
• Commissioning Procedures

References:

• ASHRAE Handbook of HVAC Applications, Chapter 47: Design and Application of Controls
• ASHRAE Guideline 13: Specifying Building Automation Systems
• ASHRAE Guideline 36: High-Performance Sequences of Operation for HVAC Systems