Occupancy-based HVAC control

Most commercial HVAC systems are controlled by a time schedule: the building is "occupied" from 7:00 to 18:00 on weekdays and "unoccupied" the rest of the time. Within those hours every zone is conditioned and ventilated as if it were full. In reality, meeting rooms sit empty for much of the day, private offices are vacant during meetings and travel, and hybrid work has made whole floors unpredictable.

Occupancy-based control closes that gap. By sensing whether a space is actually in use, the HVAC system can relax temperature setpoints, reduce airflow and cut ventilation in empty zones, then return to normal operation as soon as someone arrives. Done well, it saves fan, heating and cooling energy with no noticeable effect on comfort.

Levels of occupancy control

Occupancy-based control works at several levels, and most buildings use a combination.

This guide focuses on zone-level control, which is where most of the additional savings beyond scheduling come from.

Operating modes

ASHRAE Guideline 36, the industry reference for high-performance sequences, defines zone modes that are widely used as a model:

• Occupied: normal comfort setpoints and full design ventilation.
• Occupied standby: the building is in its scheduled occupied period, but the zone is unoccupied. Setpoints widen by a small amount, airflow minimums can drop, and ventilation can be reduced or shut off where the ventilation standard allows.
• Unoccupied: the building is outside its scheduled hours. Setpoints set back further and the system runs only to maintain setback limits.

The occupied standby mode is the key to zone-level savings. A typical approach widens the heating and cooling setpoints by 1 to 2 K, which avoids reheat and cooling in empty rooms while allowing a quick recovery when someone enters. See ASHRAE Guideline 36 high-performance sequences for more on these modes.

Ventilation in unoccupied zones

ASHRAE Standard 62.1 allows the ventilation rate to be reduced, and in specified occupancy categories reduced to zero, when a zone is unoccupied during the occupied period. The space types eligible for this "occupied standby" reduction are identified in the standard's tables and include common categories such as offices, conference rooms and classrooms. Requirements vary by edition, so the design team should confirm the version referenced by the applicable code.

In Canada, ventilation requirements are set through the National Building Code and provincial codes, which reference ASHRAE 62.1 for most non-residential buildings, with health care facilities governed by CSA Z317.2. Health care spaces with pressure relationships or minimum air change rates generally cannot use occupancy-based reductions without careful review. The authority having jurisdiction has the final say.

Occupancy-based ventilation complements demand-controlled ventilation with CO2 sensors. Occupancy sensors detect presence quickly but not how many people are present. CO2 sensors respond to the number of occupants but react slowly. Many designs use occupancy sensors for small spaces and CO2 for larger, variably occupied spaces such as conference rooms and lecture halls.

Sensor types

Passive infrared (PIR)

PIR sensors detect movement of warm bodies. They are inexpensive and reliable for detecting someone entering, but they can miss occupants who sit still at a desk, leading to false "vacant" signals.

Ultrasonic and microwave

These sensors detect small movements and are better at catching still occupants, but can be triggered by air movement or movement in adjacent spaces.

Dual technology

Combining PIR with ultrasonic or microwave sensing reduces false readings in both directions. Dual-technology sensors are a common choice for offices and meeting rooms.

Other sources

Lighting control sensors, access control badge data, Wi-Fi device counts and camera-based people counters can also provide occupancy information. Sharing lighting sensors with HVAC is often the most economical route, provided the integration is reliable. See integrating HVAC with lighting and access control.

Design and commissioning considerations

1. Time delays. A sensor should hold "occupied" for a set period after the last detected movement, commonly 15 to 30 minutes for HVAC purposes. Too short and zones cycle constantly; too long and savings shrink.
2. Sensor placement. The sensor must see the occupied area of the zone, not just the doorway or a corridor. One sensor per zone may not be enough in large or irregular spaces.
3. Zone and sensor mapping. HVAC zones and lighting zones often do not match. The design must define which sensors drive which terminal unit.
4. Minimum airflow. Reducing a VAV box minimum in standby must still satisfy any heating requirement and any pressurization relationships with adjacent spaces.
5. Fail-safe behaviour. If a sensor fails or communication is lost, the zone should default to occupied, not unoccupied.
6. Recovery. Setpoint offsets should be small enough that the zone recovers quickly when occupied.

During commissioning, each sensor should be tested to confirm it switches the correct zone, the time delay is set as specified and the terminal unit responds with the correct setpoints and airflow. Trending occupancy status alongside airflow and temperature after turnover confirms that the system behaves as expected over a range of conditions.

Where savings come from

Savings depend on how often zones are empty during occupied hours. They come from:

• Lower fan energy as VAV boxes reduce airflow in standby.
• Less reheat because minimum airflows drop.
• Less outdoor air to heat in winter and cool in summer, which is significant in Canadian climates.
• Less heating and cooling to maintain tighter setpoints in empty rooms.

Buildings with many private offices, meeting rooms and classrooms tend to benefit most. Open-plan spaces with steady occupancy benefit less.

Frequently asked questions

Can occupancy control cause comfort complaints?

It can if setpoint offsets are too large or time delays too short. Keeping standby offsets modest and verifying sensor coverage avoids most problems.

Does occupancy control work with older VAV systems?

Often yes. Many existing digital VAV controllers can accept an occupancy input and change setpoints. The work involves adding sensors, wiring or integrating with lighting controls and updating the programming.

Key takeaways

• Schedules treat every zone as fully occupied during business hours; occupancy-based control responds to actual use.
• Occupied standby mode, as defined in ASHRAE Guideline 36, relaxes setpoints and airflow in empty zones during the occupied period.
• ASHRAE 62.1 permits reduced or zero ventilation in eligible unoccupied zones, subject to the edition adopted and the AHJ.
• Dual-technology sensors, sensible time delays and fail-safe defaults prevent comfort problems.
• Commissioning should test every sensor-to-zone link. A control system verification can confirm these sequences are working in an existing building.