Field Notes
CDU loop air separator vent and fill-log acceptance record
Before CDU loop acceptance, the cooling record should identify the facility loop or TCS boundary, air separator and vent status, makeup-water or glycol feeder log, fill pressure basis, air purge evidence, leak-check locations, pressure trend, alarms, exceptions, corrections, witnesses, and release decision.
Direct answer
Before a chilled-water or technology-cooling loop is accepted for CDU operation, the record should identify the data hall, CDU, facility-water side, technology-cooling side, heat exchanger, pump skid, branch, air separator tag, automatic or manual vent status, vent cap or isolation position, blowdown or drain status where applicable, makeup-water or glycol feeder tag, feeder tank level, fill pressure basis, actual pressure, makeup volume added, fluid type, water-treatment or glycol reference, purge evidence, air complaints, leak-check locations, pressure trend, flow or pump status, leak alarms, exceptions, correction owner, retest evidence, witness, and release decision.
The point is to prove that the loop was not accepted with trapped air, an isolated vent, unexplained makeup-water use, an unverified feeder setting, a wet fitting, or a pressure trend that hides a leak. A useful packet ties air removal, fill behavior, water-risk evidence, and CDU acceptance together.
Use this as documentation guidance only. The CDU manufacturer, hydronic engineer, commissioning script, water-treatment plan, air separator manual, vent manual, makeup feeder manual, backflow and fill assembly requirements, site safety plan, and owner operations team control the actual fill, purge, venting, chemical treatment, pressure test, leak response, and acceptance.
Why this record matters
A CDU loop can look ready because pumps run and the unit is dry at first glance. That does not prove the loop is free of trapped air, makeup-water problems, or slow leaks at vents, drains, strainers, fill hoses, manifolds, and CDU connections.
ASHRAE data center and water-cooled server guidance, Vertiv and Trane CDU documents, Coolcentric commissioning notes, and hydronic air and fill references all point to the same practical issue: liquid-cooling acceptance depends on loop boundary, fluid condition, pressure control, air removal, leak detection, and documented startup evidence.
The weak record says loop filled and no leaks. The strong record shows the air separator and vent condition, feeder log, fill pressure basis, pressure trend, leak-check route, alarms, exceptions, and exact acceptance boundary.
Define the loop boundary
Start by naming the loop being accepted. Separate facility-water side, technology-cooling side, CDU internal circuit, heat exchanger side, pump skid, row manifold, rack manifold, hose kit, temporary fill connection, and owner monitoring points.
Do not let CDU loop stand alone. A vent on the facility chilled-water side does not release the secondary technology-cooling loop. A dry row manifold does not prove the CDU fill hose. A clean feeder log does not prove that a high-point vent was open.
The record should state whether the acceptance covers fill only, purge only, low-speed circulation, leak-check hold, CDU standby, first pump operation, first connected row, or full loop acceptance.
Photograph air separator and vent status
Photograph the air separator tag, flow direction, inlet and outlet piping, isolation valves, top vent, vent cap, vent drain or discharge tube where present, blowdown or drain valve, insulation openings, access clearance, and any wet marks below the device.
Caleffi, Spirotherm, Taco, and Bell and Gossett sources all treat air removal devices as installed hydronic components with orientation, service, vent, drain, and maintenance concerns. The acceptance record should preserve the installed condition instead of only listing the model number.
If the vent is isolated, capped tight, piped to a drain, manually operated, leaking, plugged, under insulation, or not accessible, record that condition and the approved correction or acceptance limit.
Record makeup-water or glycol feeder baseline
Record the feeder tag, feeder type, fluid source, tank level, low-level alarm status, pressure setting or controller setpoint basis, suction hose or fill line condition, isolation valve position, power status where applicable, and the makeup volume or tank level change during the acceptance window.
Axiom feeder instructions describe hydronic system feeders as equipment used to maintain minimum system pressure. Watts and Bell and Gossett fill references show why pressure-reducing or fill assemblies need a documented basis and code-controlled backflow context. The field record should show what device is feeding the loop and what changed during testing.
Unexplained makeup-water use is a hold point, not a normal note. If the feeder adds fluid during a leak-check hold, record the time, amount, pressure response, suspected cause, and whether the acceptance was paused.
Preserve the fill pressure basis
The record should not invent a pressure value. It should cite the commissioning script, engineer's fill calculation, CDU manual, feeder setting, expansion tank documentation, water-treatment requirement, or owner standard used as the basis.
Capture actual gauge readings at the feeder, CDU, pump skid, high point where available, low point where available, and BMS or CDU screen. Photograph the gauge face with the tag or location visible so a later reviewer can tell which pressure was used.
If the project uses glycol or treated water, tie the fill pressure record to the fluid label, batch, concentration or chemistry report where required, and the person who accepted the fluid for that loop.
Prove purge and air removal status
Show how the team confirmed that air was removed or controlled under the approved procedure. Evidence can include vent status photos, purge hose photos, air separator vent photos, pump sound or vibration notes, flow trend, pressure trend, strainer status, bleed-point witness, and CDU alarm status.
Vertiv technology-cooling deployment guidance identifies air vents as part of cooling loop deployment, and CDU manuals treat filling, flushing, leaks, pressures, and startup checks as commissioning concerns. Hydronic air separator manuals explain why installed vent condition matters.
If the loop still has air noise, pressure swings, pump cavitation concerns, unstable flow, repeated vent discharge, or unresolved high-point access, do not hide that condition inside an acceptance note. Record the hold and the retest basis.
Run a visible leak check
The leak-check route should include CDU primary and secondary connections, air separator, vent, drain, blowdown, makeup feeder connection, fill hose, expansion tank connection where visible, strainers, filters, pump seals where accessible, valve packing, unions, grooved couplings, manifolds, hoses, low points, drip trays, and leak sensors.
Record the pressure or operating state used for the check, the start and stop time, the person checking, the areas excluded, and the photos that show dry conditions. If the project uses leak detection, record local sensor status and BMS, DCIM, or CDU alarm status.
A dry photo before fill does not prove a post-fill leak check. A post-fill photo at the CDU does not prove the feeder, vent, and high points. Follow the loop path and list what was actually inspected.
Trend pressure, flow, temperature, and alarms
The acceptance record should include trend evidence around the fill, purge, leak-check, and acceptance window. Capture pressure, pump command, pump speed, flow where available, supply temperature, return temperature, differential temperature, CDU alarm status, leak alarm status, feeder operation, and tank level change.
For first CDU loop acceptance, short trend windows can catch unstable pressure, recurring vent discharge, low-flow alarms, leak alarms, and makeup-water events that a single photo misses.
If the trend is not stable enough for the acceptance scope, state the hold. Do not release a connected load because the separator photo is clean while the feeder or alarm trend is unresolved.
Inspection table
Use one table so the mechanical, controls, water-treatment, commissioning, and operations teams review the same acceptance evidence.
| Record field | What to capture | Why it matters |
|---|---|---|
| Loop boundary | CDU, FWS, TCS, branch, row, manifold, fill scope, acceptance step | Prevents one dry component from releasing a different loop |
| Air separator | Tag, flow direction, orientation, isolation, vent, drain, wet marks | Shows installed air-removal condition |
| Vent status | Cap, isolation valve, discharge path, leakage, accessibility, witness | Confirms whether air can be removed under the approved setup |
| Makeup feeder | Tag, source, tank level, pressure basis, setpoint, power, alarms | Makes unexplained fluid addition visible |
| Fill pressure | Basis document, actual gauges, CDU screen, timestamp, operator | Connects acceptance to the approved pressure basis |
| Purge evidence | Bleed point, purge hose, air noise, flow stability, retest result | Separates fill completion from air removal proof |
| Leak check | Locations, state, duration, dry photos, wet findings, sensors | Documents water-risk status before acceptance |
| Trend and release | Pressure, flow, temperature, alarms, exceptions, owner decision | Defines whether the CDU loop is accepted, limited, or held |
Before-acceptance checklist
Run this checklist before representing the CDU loop as accepted for the next operating step.
- Loop boundary, CDU, side of heat exchanger, branch, row, manifold, and acceptance step are named.
- Air separator tag, flow direction, access, isolation valves, vent, drain, and wet marks are photographed.
- Vent status is recorded as automatic, manual, isolated, capped, discharging, leaking, or held as applicable.
- Makeup-water or glycol feeder tag, source, tank level, setpoint basis, power, alarms, and valve lineup are recorded.
- Fill pressure basis is cited from the approved document rather than guessed in the field note.
- Actual gauge, CDU screen, or controls readings are photographed with location and timestamp.
- Purge evidence, air complaints, unstable flow, pump noise, and retest status are documented.
- Leak-check route includes CDU, air separator, vent, feeder, fill hose, valves, drains, strainers, manifolds, and sensors.
- Pressure, flow, temperature, feeder activity, leak alarms, and CDU alarms are trended for the acceptance window.
- Exceptions, correction owner, retest evidence, witness, and release decision are listed.
Weak versus strong record
Weak record: CDU loop filled. Air separator OK. No leaks.
Strong record: CDU-2 technology-cooling loop for Data Hall 3 Row B was reviewed before low-speed circulation acceptance. The basis was commissioning script LC-CX-22, Vertiv CDU submittal V-CDU2, water-treatment report WT-17, and feeder setting sheet MF-2. Photos showed air separator AS-TCS-2 upright with the top vent accessible, vent cap in the approved operating position, isolation open, drain closed, no wet marks, and insulation left open for observation.
The Axiom feeder MF-2 tank level was photographed at 43 gallons before fill and 42.4 gallons after purge, with no added volume during the 45-minute leak-check hold. Gauge photos showed CDU return pressure and feeder discharge pressure under the approved cold-fill condition. Leak check LC-2 covered CDU connections, separator, vent, feeder connection, fill hose, strainer cover, pump skid unions, Row B manifold, and leak sensor rope LDS-B. One wet vent discharge fitting was tightened, dried, photographed again, and observed for 30 minutes. Trend screenshots showed stable pressure, no leak alarms, and no additional feeder run. Row B was accepted for low-speed circulation only; connected rack load remained held.
Common mistakes
The first mistake is treating a feeder that holds pressure as proof that the loop is leak-free. A feeder can mask a small leak if makeup volume, tank level, and run events are not recorded.
The second mistake is photographing the air separator body but missing the vent cap, isolation valve, drain, discharge path, and wet marks below it. Those details decide whether the device was actually available and dry at acceptance.
Other mistakes include no loop boundary, no facility-water versus technology-cooling separation, no fill pressure basis, no purge evidence, no trend window, no leak sensor status, no makeup volume, no correction owner, and no split between fill acceptance and load acceptance.
When to hold CDU loop acceptance
Hold acceptance if the vent is isolated without an approved reason, air separator access is blocked, the vent leaks, drain or blowdown valves are wet or uncertain, purge evidence is missing, pressure is unstable, flow is unstable, leak sensors are in alarm, or the CDU reports unresolved alarms.
Also hold if makeup-water or glycol feeder activity is unexplained, tank level drops during the hold period, fluid label or chemistry signoff is missing, fill pressure basis is missing, or the leak-check route excludes known high-risk fittings.
A hold should name the CDU, loop, location, missing evidence, correction owner, retest requirement, monitoring requirement, and whether fill, circulation, connected load, or full acceptance is held.
Owner handoff
The handoff should include separator and vent photos, feeder baseline photos, fill pressure basis, actual pressure photos, fluid source or treatment reference, purge record, leak-check route, leak sensor status, trend screenshots, exceptions, correction photos, retest evidence, and release decision.
Store the packet with CDU startup files, chilled-water commissioning files, water-treatment records, BMS or DCIM point checks, leak-response records, valve lineup records, TAB notes, and operations turnover documents.
Make the record searchable by CDU, loop, separator tag, feeder tag, row, date, and acceptance step so future pressure loss, air noise, low-flow, or leak events can be compared to the baseline.
Questions before release
Which loop is being accepted, and which side of the CDU or heat exchanger does it cover? Is the air separator installed, accessible, vented, dry, and in the approved status? What feeder or fill assembly supplied the loop?
What pressure basis was used? What actual readings were captured? Did the feeder add fluid during the hold? What leak-check route was walked? What alarms, trends, or sensor points support the decision?
Answer those questions before the acceptance note moves the CDU loop to the next operating step.
Compliance and safety limits
This article does not design hydronic systems, size air separators, select vent locations, set fill pressure, set glycol concentration, approve water chemistry, authorize backflow connections, write CDU startup procedures, or approve connected IT load. It is a record structure for preserving air separator, vent, makeup-water, pressure, leak-check, and acceptance evidence.
The approved drawings, engineer, CDU manufacturer, hydronic equipment manuals, water-treatment provider, commissioning authority, controls integrator, owner operations team, AHJ, and site safety plan control the work. If those documents conflict with this checklist, use the controlling document and record the decision.
Do not open vents, pressurize loops, change feeder settings, add chemicals, bypass leak alarms, operate pumps, remove panels, disturb insulation, enter white-space areas, or release CDU operation outside the qualified team's authority and approved procedures.
Sources checked
- ASHRAE TC 9.9, Water-Cooled Servers Common Designs, Components, and ProcessesUsed for water-cooled server, facility-water system, technology-cooling system, CDU, coolant distribution, water quality, filtration, and service context.
- Vertiv, Understanding Coolant Distribution Units for Liquid CoolingUsed for CDU function, liquid-cooling loop context, pressure, flow, filtration, and alarm context.
- Vertiv, Guidelines for Deployment of Single Phase Technology Cooling SystemUsed for TCS deployment, air vents, isolation valves, strainers, leak sensors, filling, flushing, and commissioning context.
- Vertiv, CoolPhase CDU Installer/User GuideUsed for CDU installation, secondary circuit filling and flushing, leak checks before commissioning, leak detection, and startup context.
- Trane Technologies, Coolant Distribution Unit Installation Operation and MaintenanceUsed for CDU piping, FWS and TCS connections, fill points, valves, filling procedure, pressure example tied to that product, and no-leak checklist context.
- Coolcentric, Data Center Cooling Installation and CommissioningUsed for practical commissioning activities such as filling reservoirs, releasing air from bleed points, checking fluid condition, and logging CDU data.
- Caleffi, DISCAL Air Separator Installation, Commissioning, and Servicing InstructionsUsed for air separator installation, commissioning, servicing, vent, drain, and installed-condition context.
- Caleffi, Automatic Air Vent Installation InstructionsUsed for automatic air vent operation, cap position, hydronic venting, service, and maintenance context.
- Spirotherm, Spirovent Installation and Operation InstructionsUsed for air eliminator location, upright installation, piping context, and expansion tank connection context.
- Taco, ACT and ACTF Air Separator Installation Operation and MaintenanceUsed for ASME air separator vessel, chilled-water closed-loop application, installation, vent, drain, and maintenance context.
- Xylem Bell and Gossett, Air ManagementUsed for air management, makeup water, pressure-reducing valves, backflow context, low-water alarm, and hydronic fill context.
- Watts, Series 1156F Feed Water Pressure Regulator InstructionsUsed for pressure regulator setup, fill pressure basis, fast fill, adjustment, and hydronic feed-water context.
- Axiom Industries, MF200 and MF300 Hydronic System Feeder IOMUsed for hydronic system feeder purpose, tank level, isolation, system pressure, water or glycol mix, low-level alarm, and feeder operation context.