Field Notes
Chilled-water valve command and coil delta-T trend record
Before a comfort complaint is closed, trend the chilled-water valve command beside coil entering and leaving air or water temperatures, discharge air, space temperature, setpoints, fan status, chilled-water supply, alarms, timestamps, exceptions, and the closeout decision.
Direct answer
Before closing a comfort complaint involving a chilled-water coil, the record should trend chilled-water valve command, valve feedback where available, coil entering and leaving air temperature, coil entering and leaving water temperature where available, discharge air temperature, space temperature, active setpoint, fan status, occupancy mode, chilled-water supply temperature, cooling enable status, alarms, overrides, timestamps, weather or load context, corrective action, retest window, exceptions, and closeout decision.
The point of the record is not to prove a universal delta-T value. It is to show whether the valve command, coil temperature change, air delivery, space response, and control sequence moved together during the complaint window and retest window.
Use this as documentation guidance only. The controls sequence, design documents, TAB report, commissioning plan, sensor calibration requirements, BAS point list, equipment manual, facility operating policy, and qualified HVAC team control actual testing, setpoint changes, balancing, valve work, and complaint closeout.
Why a single snapshot is weak
A comfort complaint can look closed at one moment while the underlying trend still shows a control mismatch. The space may be satisfied after load changes, the valve may be open without cooling response, or the coil may show temperature change while airflow, fan status, or occupancy mode makes the result hard to interpret.
The weak record says valve open, space OK, complaint closed. The strong record shows the complaint time, valve command, discharge air temperature, coil delta-T, chilled-water supply condition, fan status, space temperature, setpoint, alarms, and retest period on the same timeline.
PNNL re-tuning guidance, UFGS controls sequences, university BAS standards, ASHRAE high-performance sequence material, and chilled-water delta-T references all support the same practical record lesson: trends are more useful than isolated screenshots when a control loop, coil, and comfort outcome need to be reviewed together.
Start with the complaint boundary
The first page of the record should name the complaint, room or zone, occupant report, equipment served, AHU or fan-coil tag, VAV or terminal unit if applicable, BAS graphic, trend window, weather context, operating mode, and whether the closeout covers one room, one zone, one air handler, or a broader chilled-water issue.
Do not let the closeout jump from one cold or warm space to a plant-level conclusion. A bad zone sensor, closed damper, failed actuator, saturated coil, low chilled-water supply temperature, missing airflow, or schedule mismatch can all create different trend shapes.
The record should define the system boundary so the reviewer knows whether the evidence supports the complaint closeout or only one part of it.
Trend valve command beside temperatures
Trend the chilled-water valve command as a percentage or open/closed state, valve feedback where available, coil entering air temperature, coil leaving air temperature, discharge air temperature, space temperature, and active setpoint. If water-side sensors are available, trend chilled-water supply and return at the coil or terminal unit.
Controls sequences commonly use discharge air, leaving air, or zone temperature to modulate cooling coil valves. That means the valve command should be reviewed beside the temperature it is supposed to affect, not as a stand-alone proof of cooling.
If the valve command is high and coil delta-T is low, do not assume the valve is bad from that fact alone. The trend may be showing low load, poor airflow, sensor error, valve leakage, failed actuator feedback, low chilled-water temperature, high flow, a dirty coil, or a sequence issue. The record should preserve the pattern and list the follow-up owner.
Define coil delta-T carefully
State whether coil delta-T means air-side delta-T, water-side delta-T, or both. Air-side delta-T compares entering air to leaving or discharge air at the coil. Water-side delta-T compares chilled-water return to chilled-water supply across the coil where those sensors exist.
The two values answer different questions. Air-side delta-T helps show whether the delivered air changed across the coil. Water-side delta-T helps show whether chilled water picked up heat across the coil. Neither value is complete without airflow, water flow, valve position, load, and sensor context.
Trane, LBNL, Danfoss, PNNL, and NIST-related chilled-water references discuss low delta-T and chilled-water monitoring at the system or coil level. The complaint record should avoid turning design delta-T guidance into a universal pass/fail number for a single room complaint.
Include setpoints and enabling conditions
A trend without setpoints can be misleading. Include active space setpoint, discharge air setpoint, cooling enable, occupancy mode, fan command, fan status, economizer status where relevant, chilled-water availability, and any alarm or override that changes the sequence.
UFGS, CU Anschutz, Virginia Tech, and SMU sequence documents show why this matters: valve command is part of a control sequence. The same valve position can mean different things during occupied cooling, warm-up, unoccupied mode, economizer operation, freeze protection, or alarm response.
If the complaint was closed after a manual override, the record should state that clearly. A manual override can help diagnose a problem, but it is not the same as proving the normal sequence has recovered.
Use a real trend window
Use a trend window long enough to show complaint, correction, and retest. For many BAS complaint records, that means at least the complaint period, the corrective action, and a comparable occupied period after correction. Short-cycle or hunting issues may need higher frequency data than a daily summary.
PNNL re-tuning material points to the value of trend data and practical collection intervals for building analysis. The exact interval belongs to the BAS and commissioning plan, but the closeout record should state the sample interval and any missing data.
A screenshot with no timestamp, no axis labels, and no point names is not a closeout trend. The reviewer needs point names, units, time range, and enough resolution to see the sequence response.
Look for pattern mismatches
The trend review should look for mismatches, not just acceptable end values. Examples include valve command rising while leaving air does not cool, valve command staying high after the space is satisfied, discharge air dropping below setpoint while the complaint remains warm, fan status off while cooling is commanded, or space temperature improving only after a schedule change.
OSTI research on cascaded HVAC control notes that chilled-water valve control can show hunting and operating-condition sensitivity. That does not mean every complaint is a tuning problem, but it supports the need to preserve the command and response pattern.
If the trend shows hunting, saturation, unexplained lag, missing feedback, or sensor disagreement, close the comfort complaint only after the unresolved issue is documented with an owner and retest path.
Separate complaint proof from plant delta-T
A room complaint closeout is not the same as a chilled-water plant delta-T investigation. The room record should show whether the served space recovered under the normal sequence. A plant record should evaluate broader loop temperatures, flow, pump behavior, coil selection, valve authority, and reset strategy.
Low plant delta-T can be related to coils and control valves, but a single comfort complaint trend rarely proves the full cause. Use the complaint record to flag a plant or hydronic issue when the evidence points that way, not to settle the whole issue.
If multiple complaint records show valves open with weak water-side temperature rise or unstable coil response, escalate to a chilled-water system review instead of closing each room in isolation.
Record table
Use a compact table so facilities, controls, TAB, commissioning, and service teams are reading the same trend evidence.
| Record field | What to capture | Why it matters |
|---|---|---|
| Complaint boundary | Room, zone, AHU or FCU, terminal unit, complaint time, closeout scope | Prevents a room complaint from becoming an unsupported system claim |
| Valve command | Command percent, open/closed state, feedback where available, override status | Shows what the control loop asked the coil valve to do |
| Coil delta-T | Air-side delta-T, water-side delta-T where available, units, point names | Shows whether the coil temperature response moved with the command |
| Air and space response | Discharge air, leaving air, space temperature, active setpoint | Connects coil behavior to the comfort outcome |
| Enabling conditions | Fan command/status, occupancy, cooling enable, CHW supply, alarms, economizer | Explains why the same command can mean different things |
| Trend quality | Time range, sample interval, point names, units, missing data | Keeps screenshots from becoming unverifiable |
| Exceptions | Sensor fault, override, hunting, low delta-T, missing feedback, fan off | Makes unresolved issues visible |
| Closeout decision | Closed, closed with monitoring, held, escalated to controls, TAB, or plant review | Defines what the complaint evidence actually supports |
Before-closeout checklist
Run this checklist before the comfort complaint is marked closed.
- Complaint room, zone, equipment tag, and closeout scope are identified.
- Trend includes the complaint window and a retest window after correction.
- Chilled-water valve command is trended with point name and units.
- Valve feedback is included where available.
- Air-side coil delta-T or discharge air response is included.
- Water-side coil delta-T is included where sensors are available.
- Space temperature and active setpoint are on the same timeline.
- Fan status, occupancy mode, cooling enable, and chilled-water availability are recorded.
- Alarms, overrides, missing data, and sensor concerns are exception-listed.
- Closeout decision names the owner for any remaining retest, controls, TAB, or plant review.
Weak versus strong record
Weak record: Room 214 is comfortable now. CHW valve opened. Complaint closed.
Strong record: Room 214 complaint was trended from 8:00 a.m. to 1:00 p.m. on an occupied cooling day and retested the next morning. AHU-2 chilled-water valve command rose from 18 percent to 62 percent as discharge air moved from 64.8 degrees F to 55.6 degrees F. Room 214 temperature fell from 76.4 degrees F to 72.6 degrees F against a 72 degrees F setpoint after the VAV schedule was corrected. Fan status was on, chilled-water supply was stable, no cooling alarms were active, and the only remaining exception was to monitor valve hunting during low-load mornings for one week.
The strong record ties complaint time, command, coil response, delivered air, space recovery, enabling conditions, and remaining monitoring together.
Common mistakes
The most common mistake is closing the complaint from a current space temperature alone. Comfort can recover because the sun moved, occupancy changed, the schedule shifted, or an override masked the issue.
Another mistake is plotting valve command without the temperature it is supposed to affect. A 100 percent command is not proof of cooling, and a 20 percent command is not proof of no problem unless the sequence and load support that conclusion.
Other mistakes include unlabeled trend screenshots, missing setpoints, missing fan status, no complaint timestamp, no retest period, confusing air-side and water-side delta-T, ignoring overrides, and closing a room issue while plant-level delta-T remains unresolved.
When to hold complaint closeout
Hold closeout if the trend does not include the complaint window, the valve command is missing, the active setpoint is unknown, fan status is missing, the BAS point names are unclear, or the space only recovered during a manual override.
Also hold if the trend shows valve hunting, valve saturation, missing feedback, a sensor fault, fan off with cooling command, chilled-water unavailable, discharge air not responding, water-side delta-T inconsistent with the claim, or unresolved alarms.
A hold should name the complaint, point names, missing evidence, required trend window, responsible team, correction, retest time, and whether the issue is a room-level, controls, TAB, equipment, or plant escalation.
Owner handoff and monitoring
The owner handoff should include trend screenshots or exported trend data, point names, units, sample interval, complaint timeline, corrective action, retest result, alarms, overrides, exceptions, and any continuing monitoring period.
If the owner will continue monitoring valve command, coil delta-T, discharge air, space temperature, or chilled-water supply, preserve the exact BAS point names and normal trend values from the closeout period.
Keep the record with the complaint work order and controls notes, not only in a technician's local screenshots.
Questions before closeout
What temperature was the occupant actually reporting? What equipment serves the space? Was the system occupied, enabled, and in cooling during the complaint window? Which point is valve command, and which point is feedback?
Is coil delta-T being measured on the air side or water side? Are the sensors calibrated enough for the decision? Did the space recover under normal sequence or under override? Is the problem isolated to one room or repeated across similar zones?
Answer those questions before the work order says comfort complaint closed.
Compliance and safety limits
This article does not tune a control loop, change setpoints, balance water flow, calibrate sensors, approve a sequence, or diagnose every comfort complaint. It is a record structure for preserving trend evidence before complaint closeout.
The controls sequence, design documents, TAB report, commissioning plan, BAS point list, equipment manual, facility operating policy, and qualified HVAC team control the work. If those documents conflict with this checklist, use the controlling project document and record the decision.
Do not adjust valves, override controls, open equipment, or perform water-side work outside the qualified team's authority. Electrical safety, lockout, ladder access, mechanical-room safety, ceiling access, and facility procedures control the field work.
Sources checked
- PNNL, Trending for Re-TuningUsed for BAS trend data, point selection, and practical trend collection context.
- PNNL, Army Re-tuning Implementation GuidesUsed for re-tuning and trend review context for HVAC complaints and AHU operation.
- PNNL, Building Re-Tuning Training Guide for Central Utility Plant Cooling ControlUsed for chilled-water loop trend and delta-T monitoring context.
- UFGS, Sequences of Operation for HVAC ControlUsed for cooling coil valve command, zone temperature, and HVAC control sequence context.
- University of Colorado Anschutz, Sequence of Operations for HVAC ControlsUsed for discharge air temperature and cooling coil valve sequence context.
- Virginia Tech, BAS Standards Sequence of OperationsUsed for BAS setpoints, alarms, chilled-water temperature, and valve sequence context.
- SMU, BAS Master Sequence of OperationsUsed for discharge air temperature, fan status, cooling enable, and BAS point context.
- ASHRAE, Guideline 36 Addendum qUsed for commanded heating and cooling coil valve operating-state context.
- Danfoss, Low Delta-T Syndrome Application GuideUsed for chilled-water delta-T and control valve context.
- Trane, ASHRAE 90.1's 15 degrees F Delta T RequirementUsed for chilled-water coil delta-T design context and limits on applying design guidance to field closeout.
- NIST and CSTB, Performance Monitoring of Chilled-Water Distribution Systems Using HVAC-CxUsed for chilled-water distribution monitoring and automated commissioning context.
- LBNL, Chilled Water Plant Design GuideUsed for chilled-water plant delta-T, cooling coil, and control valve context.
- OSTI, Cascaded Control for Building HVAC Systems in PracticeUsed for chilled-water valve control, hunting, and operating-condition sensitivity context.