ASHRAE Guideline 36 — Redundant Equipment Staging
What it means on the job
Redundant CRAC units must have one designated as lead and one as lag. Setting both as lead causes both units to run at full capacity simultaneously, wasting energy and eliminating redundancy protection. The Liebert vNSA switch must coordinate lead/lag sequencing so only one unit operates at a time under normal conditions.
Tested in 1 CraftIQ puzzle
- whats-wrongCRAC Unit iCOM Controller Setup
Related ASHRAE sections
ASHRAE Guideline 36 — High-Performance Sequences of Operation
The DP sensor is located at the pump discharge instead of at the most remote coil or 2/3 of the way out in the system. Sensing DP at the pump header causes the VFD to maintain artificially high system pressure, defeating the purpose of variable flow. The sensor must be relocated to represent the actual system load point.
ASHRAE Guideline 36 — Monitoring Requirements
Temperature trend data should have at least 0.1 degree resolution. Whole-number-only resolution (e.g., 72 vs. 72.3) loses critical information needed for analyzing control loop performance, setpoint drift, and energy optimization.
ASHRAE Guideline 36 — Redundant Equipment / high-rise office Mechanical Specification
Redundant CRAC units must have automatic failover capability. If the lead CRAC unit fails (compressor fault, high-head pressure, loss of refrigerant), the vNSA switch must automatically start the lag unit without manual intervention. Critical cooling spaces cannot wait for a building engineer to respond, especially during off-hours. The Liebert vNSA network switch is specifically designed for automatic lead/lag coordination and failover — disabling this feature defeats the purpose of redundancy and risks equipment damage from overheating.
ASHRAE Guideline 36 — Section 5.18
The installed sensors meet the specification. ASHRAE Guideline 36 recommends space temperature sensors with accuracy of +/- 0.5 degrees F or better for zone control. The 10K thermistor is a standard sensor type for DDC systems, providing adequate accuracy for HVAC zone control.
ASHRAE Guideline 36 — Sensor Placement
Temperature sensors must not be placed in direct airflow from supply diffusers. The supply air temperature causes false readings, making the controller unable to maintain proper space temperature setpoint.
ASHRAE Guideline 36 / Crestline Controls Specification
Duct temperature probes must be inserted 25-50% of the duct width to obtain a representative reading. Inserting only 10% places the sensor in the boundary layer near the duct wall where temperature is not representative of the airstream, causing inaccurate control.
CraftIQ explanations are original educational commentary written in trade language and verified against the cited standards. Always confirm requirements against the code edition adopted by your jurisdiction — this is training material, not a substitute for the code book or your AHJ.