ASHRAE 90.1-2019, Section 6.5.1.1

ASHRAE 90.1-2019 Section 6.5.5.2 (Heat Rejection Equipment — Fan Speed Control)

ASHRAE 90.1-2019 Section 6.5.5.2 (Fan Speed Control) requires that each fan on heat-rejection equipment (cooling towers, closed-circuit coolers, evaporative condensers) powered by a motor of 7.5 HP or larger have controls that automatically change fan speed — a two-speed motor, a variable-speed drive, or other approved modulating control. The trigger is the fan motor horsepower, not the condenser-water flow rate. This 30 HP single-speed fan is well above the 7.5 HP threshold, so simple on/off contactor control violates the requirement. A single-speed fan cycling on and off also wastes significant energy and causes large swings in condenser water temperature, reducing chiller efficiency. Variable-speed control is preferred because fan power varies with roughly the cube of speed — operating at 80% speed uses only about 51% of full-load power. A VFD on the cooling tower fan typically pays for itself within one to two cooling seasons through reduced energy consumption.

ASHRAE 90.1-2019, Section 6.4.3.3

Night setback/setup is required by ASHRAE 90.1 for buildings with DDC. The heating setback to 55 degrees F prevents freezing while saving energy. The 90 degrees F cooling setup prevents extreme overheating while allowing the cooling system to rest. The 30-minute pre-start is an acceptable fixed lead time, though optimal start is preferred for larger buildings.

ASHRAE 90.1-2019, Section 6.4.3.3.3

Optimal start is not just allowed — it is required by ASHRAE 90.1 for systems with DDC controls. The algorithm must bring the space to occupied setpoint by the scheduled occupancy time using the minimum energy. Using outdoor temperature, space temperature, and historical data is the recommended approach per ASHRAE Guideline 36.

ASHRAE 90.1-2019, Section 6.5.1.1 / ASHRAE 62.1

This economizer setup is code-compliant on multiple fronts. The 75 degrees F dry-bulb lockout aligns with ASHRAE 90.1 Table 6.5.1.1.3 for most climate zones. The enthalpy limit adds humidity protection per ASHRAE 90.1 Section 6.5.1.1. The 20% minimum outdoor air damper position during occupied hours ensures ventilation compliance with ASHRAE 62.1 — the damper never fully closes when people are present. The CO2-based demand-controlled ventilation above 1,000 PPM is an accepted method per ASHRAE 62.1 Section 6.2.7 for adjusting ventilation based on actual occupancy. This is a well-programmed, energy-efficient, code-compliant sequence.

ASHRAE 90.1-2019, Section 6.5.3.2.2

ASHRAE 90.1 requires static pressure reset for VAV systems with DDC. A fixed setpoint wastes fan energy during part-load conditions. The setpoint should reset down based on zone damper positions — when all dampers are less than 90% open, the static pressure setpoint should decrease. ASHRAE Guideline 36 specifies Trim and Respond reset logic for static pressure.

ASHRAE 90.1-2019, Section 6.5.4.4 (temperature reset) / Section 6.5.4.2 (variable-flow DP reset) / Guideline 36

This chiller plant sequence fails ASHRAE 90.1-2019 Section 6.5.4.4 (Chilled- and Hot-Water Temperature Reset Controls), which requires chilled water supply temperature reset for systems with DDC controls. A fixed 44 degrees F supply temperature forces the chillers to work harder than necessary during mild weather and part-load conditions — wasting significant energy (roughly 1-2% per degree F of potential reset). Additionally, the fixed differential pressure setpoint on the variable-flow pumps violates Section 6.5.4.2 (Hydronic Variable Flow Systems), which requires the DP setpoint on DDC variable-flow pumps to be reset downward based on valve positions. Best practice per ASHRAE Guideline 36 likewise resets DP based on the most-open control valve position, reducing pump speed when full design pressure is not needed. Both reset strategies — supply water temperature and differential pressure — are essential for energy code compliance and efficient chiller plant operation.