ASHRAE Guideline 36 — High-Performance Sequences of Operation
What it means on the job
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.
Tested in 1 CraftIQ puzzle
- whats-wrongVariable Flow Chilled Water System
Related ASHRAE sections
ASHRAE Guideline 36 — Cooling Coil Control
Cooling coil control valves should fail closed (spring return to closed) on loss of signal or power. Failing open causes uncontrolled cooling and potential freezing of downstream coils or spaces.
ASHRAE Guideline 36 — Damper/Fan Interlocks
An end switch or position feedback must confirm the motorized outside air damper is fully open before the evaporator fan is allowed to start. Starting the fan with the damper closed creates high negative pressure in the ductwork and starves the unit of ventilation air, potentially damaging ductwork and causing inadequate outdoor air delivery.
ASHRAE Guideline 36 — Freeze Protection
A freeze stat alarm must automatically shut down the AHU fan, close the outdoor air damper, and open the heating valve. An alarm-only response without automatic action risks coil freeze-up and catastrophic water damage.
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 — Redundant Equipment Staging
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.
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.