ASHRAE 90.1-2019 Section 6.5.4.5 — Variable Flow Requirements

ASHRAE 90.1-2019 Section 6.5.3.4 (supply-air-temperature reset)

This passes. ASHRAE 90.1-2019 Section 6.5.3.4 requires multiple-zone HVAC systems to reset supply air temperature in response to either representative building loads OR outdoor air temperature, by at least 25 percent of the difference between the design SAT and the design room temperature. The scheduled reset from 55 degrees F at 90 degrees F OAT up to 65 degrees F at 60 degrees F OAT is a 10-degree reset driven by outdoor air temperature — an explicitly permitted method that meets the code minimum. Zone-based Trim-and-Respond reset (ASHRAE Guideline 36) is a voluntary best-practice guideline, not adopted code, so its preference cannot make an otherwise-compliant outdoor-air reset fail inspection. Only if the project specification specifically mandated G36 zone-based reset would the OAT method be non-compliant — and the spec would have to state that.

ASHRAE 90.1-2019 Section 6.5.4.2 (Hydronic Variable Flow Systems)

ASHRAE 90.1-2019 Section 6.5.4.2 (Hydronic Variable Flow Systems) requires that any HVAC pumping system with a total pump system power exceeding 10 HP that uses modulating control valves be designed for variable flow — capable of reducing pump flow to 50% or less of design — and three-way valves are explicitly not acceptable. The trigger is pump system power, not chiller tonnage, but a 500-ton plant's chilled water pumps are far above 10 HP, so the requirement plainly applies. The constant-flow arrangement with 3-way valves and no pump VFDs violates it. The system must be redesigned with 2-way control valves at all coils, variable frequency drives on chilled water pumps, differential pressure sensors for pump speed control, and a minimum flow bypass to protect chiller evaporators at low loads. Constant-flow systems at this scale waste enormous pumping energy, particularly at the part-load conditions that represent the vast majority of annual operating hours.

ASHRAE 90.1-2019 Section 6.5.4.2 (Hydronic Variable Flow Systems) / ASHRAE Handbook — HVAC Systems and Equipment

This is a fully code-compliant primary-only variable flow chilled water design that meets ASHRAE 90.1-2019 requirements. The 2-way control valves allow flow to vary with load, the VFDs on pumps reduce energy consumption at part load, and the differential pressure sensor at the most remote coil ensures the pumps maintain only the minimum pressure needed to serve all loads. The minimum flow bypass valve set at 30% of design flow protects the chiller evaporators from low-flow conditions that could cause freezing or laminar flow issues. This design is the current industry-preferred approach as it eliminates the need for separate primary and secondary pumping loops, reducing both first cost and operating energy compared to traditional primary-secondary systems.

ASHRAE 90.1-2019 Section 6.5.5 / CTI Standard 140

Damaged drift eliminators allow excessive water loss through carryover of water droplets into the discharge airstream. This wastes water and chemical treatment, and can create safety hazards from ice formation on nearby surfaces in winter. Drift loss should be limited to 0.001-0.005% of circulating flow per ASHRAE.

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.