ASHRAE 90.1-2019 Section 6.5.5 / CTI Standard 140

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 6.5.4.5 — Variable Flow Requirements

The VFD is in bypass mode despite being fully functional, meaning the pump runs at constant full speed. This eliminates the energy savings from variable flow operation. At 50% flow, a VFD-controlled pump uses only ~12.5% of full-speed power (affinity laws). Running in bypass at full speed wastes substantial energy and creates excessive pressure in the system.

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.