After-Hours HVAC Waste: A One-Week Schedule Audit

AI-generated cover image for illustration.

An office closes at six, but the air handler runs until ten. A Saturday event ended months ago, yet its override still starts the plant every weekend. Neither problem requires a new chiller to investigate. Both require someone to compare the building's actual needs with what its controls are doing.

A useful after-hours audit connects the people using a building with the equipment serving them. This one-week approach suits ordinary commercial comfort systems. Critical facilities, specialist spaces and process loads need their own approved operating requirements.

Original Nexora schematic. Conceptual only; not a construction or control detail.

Start with an operating map

Choose one air handler or clearly defined area. Record its spaces, normal occupied hours, cleaning times, approved events and who can confirm each exception. Include early arrivals and late work.

Next, identify every schedule that can enable the system. There may be separate calendars for the air handler, terminal units, boiler, chiller and exhaust fans. A local thermostat can also request service while the central calendar says unoccupied. Record holiday calendars, temporary extensions, optimal-start settings and active overrides before changing anything.

ENERGY STAR recommends nighttime audits and startup and shutdown reviews. Observe before changing unfamiliar equipment. Its operations and maintenance checklist provides a useful starting framework.

Monday and Tuesday: Collect a trustworthy baseline

Export an existing representative week if trend history is available. Otherwise begin recording now and use this first week to establish the baseline; a reliable before-and-after comparison will take longer. Include a weekend. Note unusual weather, tenant moves, maintenance work and public holidays so they do not quietly distort the comparison.

Collect the schedule, effective mode, fan command and actual run feedback, outdoor and supply temperatures, representative zone temperatures and relevant humidity readings. Add measured power where available, including plant equipment that may remain enabled after the air side stops. Record meter units and interval length.

Five- to fifteen-minute intervals are a practical starting point for a schedule review, provided the controls and storage support them. Short events may require finer data. Align timestamps and time zones before drawing conclusions. A one-hour clock error can make a correctly scheduled system look like a persistent offender.

PNNL's occupancy scheduling guide uses fan status, duct pressure, outdoor temperature, damper signals and discharge temperature to examine nights and weekends. It also distinguishes unnecessary operation from cycling needed to maintain temperature limits. A fan that runs at night is a finding to explain, not automatically a fault.

Wednesday: Find the cause of each extra hour

Plot actual run feedback against confirmed occupancy. Highlight three periods separately: the morning recovery period, operation after the last regular occupant leaves, and weekend operation. Then select one suspicious interval and trace the enabling request. Avoid adjusting the master schedule until you know which request actually won.

Check for schedules left by former tenants. For an override, ask who applied it, why it remains necessary and whether the underlying fault was repaired. Removing a workaround too soon can restore the original problem.

Also check whether the schedule displayed on the graphic is the effective schedule. A local controller, occupancy extension or supervisory request may have priority. If the command says off but run feedback or measured power says on, investigate the control chain and equipment condition with the responsible technician. Another calendar edit will not repair that mismatch.

Walk the spaces after hours if site procedures allow it. Confirm cleaners, occupied rooms, equipment loads and unrecorded events. Keep an exception log with the time, space, reason and required service.

Thursday: Test a small reversible change

Select one confirmed scheduling error and agree on a change with the facilities team. Save the original settings, record the reason and define the conditions for reversing the test. A modest change to a proven unnecessary operating window is easier to evaluate than a simultaneous rewrite of schedules, setpoints and control sequences.

Where supported, use an expiring after-hours request for the affected area. It should have a named owner and a clear end time. If one small meeting room requires an entire central plant, document that limitation; solving it may need zoning or equipment changes. Do not promise room-level savings when the installed system cannot provide room-level service.

Optimal start begins recovery early enough to meet occupied conditions at arrival. The required lead time changes with weather, thermal mass, overnight drift and capacity. A fixed two-hour start may be excessive on a mild day and insufficient after a cold weekend.

Trend when recovery starts, when representative zones reach their target, and whether the slowest legitimate zone is holding the whole system on. Check that this zone's sensor and equipment are healthy before accepting it as the driver. Adjusting optimal start should follow the installed controller's instructions and an approved sequence, with comfort verified at occupancy.

Protect the services the building still needs

Unoccupied operation can still provide freeze protection, moisture control, process cooling, required exhaust or pressure relationships. A temperature setback alone does not prove that humidity will remain acceptable. Some buildings need nighttime dehumidification even when their sensible cooling load is low. Review the actual environmental limits with the responsible engineer.

Outdoor-air and exhaust schedules must be coordinated with occupancy, applicable ventilation requirements and the building's approved sequence. Avoid a blanket instruction to close all outdoor-air dampers at night. Laboratories, healthcare areas, kitchens and other specialist spaces may have requirements that continue after the main office closes.

Leave life-safety functions, protective interlocks and equipment operating limits intact. If the audit uncovers uncertainty about these functions, stop the dependent change and obtain qualified help. The DOE's re-tuning screening guidance emphasizes comparing operating data with design and control documents and having reliable control components before fine-tuning performance.

Friday through Sunday: Verify the response

Check that the equipment actually stops at the revised time, that associated equipment responds appropriately, and that occupied conditions are achieved the following morning. Review the same checks after the weekend. Ask about comfort rather than treating a lack of complaints as proof that every space is satisfactory.

Check for stable humidity, acceptable arrival temperatures and new alarms. Log unexpected cycling or recovery demand. If conditions breach the agreed limits, restore the previous setting and investigate.

A clean weekend trace confirms the schedule changed, but cannot establish annual savings. Compare suitable days over subsequent weeks and revisit the sequence in another season as recovery and humidity-control needs change.

Put a defensible number on the opportunity

Here is an illustrative fan-only calculation, using assumed values rather than a real project result. Suppose a meter shows an average fan input of 4.8 kW during a confirmed unnecessary three-hour evening period. Removing that period on five weekdays would avoid 4.8 kW × 3 hours × 5 days = 72 kWh of fan electricity per week, if the same operating conditions otherwise apply.

At an assumed marginal energy rate of USD 0.18 per kWh, that is USD 12.96 per week in energy charges. It excludes taxes, demand charges, heating or cooling changes and any additional recovery energy. If the fan modulates, use measured interval power; a motor nameplate is not a measurement of average electrical input.

Do not add guessed chiller, boiler or pump savings. Meter them or model their response with stated assumptions. Whole-building data includes unrelated loads, and recovery may offset some overnight savings. Preserve dates, weather, occupancy and settings so the estimate can be reproduced.

End the audit with one page: confirmed faults, required exceptions, changes made, observed results and unresolved questions. For broader background on HVAC energy use, Nexora's Cool Savings guide is an optional companion. The operating record remains the evidence for what this building can safely save.