8.2 Energy Control Strategies: Temperature/Pressure Resets, Night Setback, and Deadbands

Energy Control Strategies: Temperature/Pressure Resets, Night Setback, and Deadbands

Implementing advanced control strategies through a Building Automation System (BAS) is one of the most cost-effective methods to achieve deep energy savings. Unlike capital-intensive equipment replacements, control optimizations leverage existing hardware by rewriting the software logic to ensure equipment only works as hard as absolutely necessary. The core philosophy of these strategies involves abandoning fixed setpoints in favor of dynamic setpoints that adapt to real-time building loads and ambient conditions.

Temperature and Pressure Reset Strategies

HVAC systems are traditionally designed to handle the absolute peak cooling or heating load (e.g., the hottest summer afternoon with full occupancy). Consequently, during the vast majority of operating hours (part-load conditions), the system is oversized. Reset strategies adjust operational setpoints during these part-load conditions to improve efficiency.

Chilled Water Supply Temperature (CHWST) Reset

Chillers are extremely energy-intensive. They typically provide chilled water at a fixed setpoint of 42°F to 44°F to handle peak dehumidification and cooling loads. However, when the outdoor temperature is moderate (e.g., 65°F), producing 44°F water is unnecessary.

A CHWST reset strategy dynamically raises the chilled water setpoint based on either the Outside Air Temperature (OAT) or the valve positions of the cooling coils in the building.

The Golden Rule of Chillers: Raising the chilled water supply temperature increases the chiller's efficiency. As a rule of thumb, chiller efficiency improves by 1% to 2% for every 1°F increase in the CHWST setpoint.

• OAT-Based Reset Example:
  • When OAT > 85°F, CHWST Setpoint = 44°F (Peak load, maximum cooling capacity needed).
  • When OAT < 60°F, CHWST Setpoint = 54°F (Low load, warmer water is sufficient).
  • Between 60°F and 85°F OAT, the CHWST scales linearly between 54°F and 44°F.

Supply Air Temperature (SAT) Reset

Air Handling Units (AHUs) typically cool air to a fixed 55°F to satisfy building loads. Similar to the chiller reset, a SAT reset strategy raises the supply air temperature when cooling demand is low. This provides two massive benefits:

1. It reduces the cooling energy required at the AHU cooling coil.
2. It significantly reduces reheat energy. In many VAV systems, overcooled 55°F air must be reheated at the terminal box before entering a room to prevent overcooling the occupants. Raising the SAT to 60°F or 65°F during mild weather slashes this reheat penalty.

Duct Static Pressure Reset

Variable Air Volume (VAV) AHUs use Variable Frequency Drives (VFDs) to control fan speed, maintaining a fixed static pressure in the supply duct (e.g., 1.5 inches of water column, "w.c."). This fixed setpoint is chosen to ensure the most distant VAV box receives enough airflow during peak conditions.

A static pressure reset strategy polls the damper positions of all VAV boxes served by the AHU. If all VAV box dampers are less than 85% open, it means the system has excess pressure. The BAS slowly resets the static pressure setpoint downward (e.g., from 1.5" w.c. to 1.4" w.c., then 1.3" w.c.) until at least one VAV box damper opens to nearly 100%. By continually "starving" the system just enough to satisfy the most demanding zone, fan speed is minimized. Due to the Affinity Laws, even a small reduction in fan speed yields cubic reductions in fan power consumption.

Unoccupied Control Strategies

Buildings spend a massive percentage of their life unoccupied (nights, weekends, holidays). Optimizing controls during these periods is critical.

Night Setback and Night Setup

During unoccupied hours, there is no need to maintain strict occupant comfort standards.

• Night Setback (Heating): The heating setpoint is lowered significantly (e.g., from 70°F to 60°F). The heating system remains off until the building temperature drops below 60°F.
• Night Setup (Cooling): The cooling setpoint is raised significantly (e.g., from 74°F to 82°F). The cooling system remains off until the temperature exceeds 82°F.

Energy Savings Calculation Example: Consider a building operating 50 hours a week. It is unoccupied for 118 hours a week. Implementing a 10°F night setback reduces the temperature differential (\Delta T) between the inside and outside, drastically reducing heat loss through the building envelope. Savings from setback strategies typically range from 5% to 15% of total heating/cooling energy, depending on the severity of the climate, the mass of the building, and the length of the unoccupied period.

Optimum Start / Stop

Traditional scheduling starts the HVAC equipment at a fixed time (e.g., 5:00 AM) to ensure the building is comfortable by 8:00 AM. In mild weather, 3 hours of "warm-up" is wasteful.

Optimum Start is an intelligent algorithm that calculates exactly when to turn on the equipment based on the current indoor temperature, the outside temperature, and the historical thermal mass response of the building. On a mild day, it might start at 7:15 AM; on a freezing day, it might start at 4:30 AM. Optimum Stop performs the reverse, shutting down mechanical cooling/heating before the occupants leave, allowing the building to "coast" on its thermal inertia for the final 30-60 minutes of the day.

Zone Temperature Control: The Deadband

A deadband is a temperature range in which neither heating nor mechanical cooling is activated. It is the neutral zone separating the heating setpoint and the cooling setpoint.

Without a deadband (or with a very narrow one), a system might heat a room to 72°F, overshoot slightly to 73°F, immediately trigger the cooling system to bring it back to 72°F, and oscillate endlessly. This is known as simultaneous heating and cooling, and it is one of the worst forms of energy waste.

• Typical Occupied Setpoints: Heating = 70°F, Cooling = 74°F.
• Deadband: 4°F (from 70°F to 74°F).

When the space temperature is 72°F (inside the deadband), the VAV box damper modulates to its minimum position to provide ventilation air only, and no mechanical heating or cooling is applied. Widening the deadband (e.g., to 69°F heating and 75°F cooling) significantly reduces energy consumption by allowing the building temperature to float freely over a wider range before expending energy.