Why "auto-on, manual-off" isn't just a setting preference — it's the exact wording several energy codes use to tell you which one you're legally required to install.
Ask most people the difference between an "occupancy sensor" and a "vacancy sensor" and they'll assume it's two different products — maybe one is more sensitive, maybe one costs more. In the overwhelming majority of installations, it's the exact same PIR, ultrasonic, or dual-technology motion sensor hardware. The only thing that differs is a configuration setting buried in the sensor's programming: whether the automatic-ON function is enabled or disabled. That single toggle is why energy codes like ASHRAE 90.1 and California's Title 24 name one of these configurations explicitly, in specific space types, instead of leaving it up to installer preference.
Every occupancy-based lighting sensor has to do two jobs: notice when a space becomes occupied, and notice when it becomes unoccupied. What varies is which of those two jobs is actually connected to the lights. Configure the sensor as an occupancy sensor and both jobs drive the lighting circuit — motion turns lights ON automatically, and a no-motion timeout turns them OFF automatically. Configure the identical hardware as a vacancy sensorand only the second job is wired to the lights — the occupant has to flip a wall switch to get light, but the sensor still handles turning it back off. Nothing about the sensor's ability to detect motion changes between the two. What changes is which detected event is allowed to act on the lighting load.
Automatic OFF is uncontroversial: nobody argues that lights should stay on in an empty room, so both configurations keep that behavior identical. Automatic ON is where the energy tradeoff actually lives. An occupancy sensor turns lights on for everymotion event — whether the room is flooded with daylight, whether the occupant is just grabbing a folder and leaving, whether artificial light is needed at all in that moment. A vacancy sensor forces a person to make a conscious decision before any lighting energy is spent. That's why ASHRAE 90.1 and energy codes like California's Title 24 specifically mandate manual-ON (vacancy) configuration in certain space types — private offices, classrooms, conference rooms, storage and supply rooms are common examples — rather than leaving the auto-ON vs. manual-ON choice to installer or occupant preference. The code isn't asking which sensor to buy. It's specifying how the identical sensor has to be configured.
False, or at best badly incomplete. In the large majority of installations, the underlying sensor hardware — the same PIR, ultrasonic, or dual-technology motion detector — is identical between the two. What differs is a configuration setting: whether the automatic-ON function is enabled (occupancy sensor behavior) or disabled (vacancy sensor behavior). This is not a minor convenience toggle. Many energy codes specifically require manual-ON (vacancy) configuration in defined space types precisely because auto-ON behavior triggers unnecessary lighting energy use on every motion event, regardless of whether artificial light is actually needed at that moment. Choosing auto-ON where the code requires manual-ON isn't picking a different convenience option — it's a code compliance failure, using the exact same hardware wired the wrong way.
Explains why occupancy sensors and vacancy sensors are, in most installations, the identical motion-sensing hardware configured two different ways — an occupancy sensor is configured 'auto-ON,' turning lighting on and off automatically, while a vacancy sensor is configured 'manual-ON,' requiring a deliberate wall-switch press to turn lighting on while still turning it off automatically after a no-motion timeout. Illustrated with the same sensor and room shown under both configurations.
Most occupancy-based lighting controls use a PIR (passive infrared), ultrasonic, or dual-technology motion sensor. That same physical device can be configured, via a jumper, DIP switch, or programmed setting, to drive the lighting load in either of two modes. In 'auto-ON' mode it is called an occupancy sensor: it turns lights on automatically the moment it detects motion, and turns them off automatically after a configured period with no detected motion. In 'manual-ON' (sometimes called 'vacancy-only') mode the identical hardware is called a vacancy sensor: the automatic-ON function is disabled, so lighting only turns on when an occupant manually operates a wall switch, while the automatic-OFF function remains active exactly as before.
It is critical to understand what is actually turned off in a vacancy-sensor configuration: only the auto-ON behavior. The sensor keeps detecting occupancy exactly as it always did — that detection still drives the auto-OFF timeout. What a vacancy sensor will never do is turn lights on by itself, even in a space that is genuinely occupied with the lights off — for example, a daylit room where someone deliberately switched lights off, or a space someone entered without immediately needing artificial light. That is by design, not a limitation: the point of manual-ON is that a person, not the sensor, decides when artificial lighting energy should be spent.
ASHRAE 90.1 and a number of state energy codes, including California's Title 24, specifically require manual-ON (vacancy sensor) configuration for certain space types rather than allowing auto-ON (occupancy sensor) configuration. The reasoning is straightforward: auto-ON turns lights on for every motion event, regardless of whether the space already has adequate daylight or whether the occupant needs artificial light at that moment, which measurably increases unnecessary lighting energy use. Manual-ON forces a deliberate decision before that energy is spent, while the auto-OFF function — present in both configurations — still guarantees lights don't get left on in an unoccupied space. That is why the configuration choice, not the choice of hardware, is treated as a genuine, code-enforced energy design decision.
Usually not. Most commercial sensor product lines use the same PIR, ultrasonic, or dual-tech detector for both, with a configuration setting (DIP switch, jumper, or firmware setting) that determines whether the sensor is allowed to auto-ON. Some manufacturers do sell dedicated "vacancy-only" models with the auto-ON circuitry omitted entirely, but functionally they behave identically to the same sensor configured for manual-ON.
No. That is specifically the behavior manual-ON configuration disables. If lights are off in a space using a vacancy sensor, they will remain off regardless of how much motion is detected, until an occupant manually operates the switch. This holds even if the space is genuinely occupied.
Yes — the automatic-OFF function, present in both configurations, is what prevents lights from being left on in an unoccupied space, which is typically the larger energy waste in unmanaged lighting. The distinction between auto-ON and manual-ON is about a smaller, additional increment of savings: whether lights turn on automatically for motion that doesn't actually require artificial light.
Requirements vary by code edition and jurisdiction, but commonly cited examples include private offices, classrooms, conference/meeting rooms, break rooms, and storage or supply rooms — spaces where occupants are present long enough and often enough that a switch press is not considered a meaningful burden, and where unnecessary auto-triggered lighting adds up over many short visits. Always confirm the exact list against the locally adopted code edition for a given project.
In most commercial-grade sensors, yes — it is typically a DIP switch, jumper, or programmed parameter accessible during commissioning, without needing to replace the sensor itself. This is exactly why the auto-ON vs. manual-ON choice is treated as a configuration decision made at design and commissioning, not a hardware selection made at time of purchase.
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