Presence detection is the single most effective energy-saving input available to KNX lighting and HVAC systems. Turning lights off 10 minutes after the last occupant leaves an office instead of at 18:00 saves 30-50% of lighting energy in real-world installations. But choosing the wrong detector type for the application is the source of the most common commissioning complaints: lights turning off while someone is seated still, lights activating from motion in adjacent spaces, or HVAC setback failing to trigger because the sensor is in the wrong location.

PIR: passive infrared detection

PIR sensors detect the temperature differential between a moving body (37°C) and the background. The sensor sees in detection zones — typically a fan pattern of 5-12 sectors depending on lens design. Movement must cross zone boundaries to trigger detection; stationary presence in a single zone is not detected. PIR is a passive sensor: it draws no RF power and produces no signal interference. Standard ceiling height 2.7-3.5m; lens variants for higher ceilings exist (typically up to 6m with modified lens). Coverage diameter at 3m ceiling: 6-8m for standard ceiling PIR.

PIR limitation: a person sitting still at a desk typing produces minimal detectable movement after 10-15 minutes. This causes the lights-off problem in open-plan offices. If the hold time is extended to compensate, the sensor keeps lights on empty rooms. PIR is appropriate for applications with consistent movement: corridors, toilets, meeting rooms where people move regularly, reception areas.

HF: high-frequency radar detection

HF (microwave radar, typically 5.8 GHz or 24 GHz) detects Doppler shift from any movement — including breathing and micro-movements from a person seated still. The radar signal passes through non-metallic partitions (glass, plasterboard) — this is both the strength and the problem. An HF sensor in a closed office detects a person in the corridor or adjacent room through the partition. Requires careful placement away from partitions facing occupied adjacent spaces.

HF advantage: no false-off with stationary presence. A seated person typing, reading, or on a phone call continues to register. HF sensor coverage: typically 8-10m diameter at 3m ceiling, omnidirectional. MDT SCA-HF55.01, Steinel ECOS 140 SH A KNX, Theben LUXA 102-360 are typical products.

Dual-tech: PIR + HF combined

Dual-technology sensors require both PIR and HF to confirm detection before triggering ON — this reduces false triggers from HF seeing through walls. For light-off logic, only HF confirmation is typically required (HF continues detecting stationary presence). The combination gives: reliable ON trigger (PIR motion detection confirms actual presence in the zone, not leakage from adjacent area), reliable hold (HF maintains hold time while person is seated still). MDT SCA-PD55.01 is the reference dual-tech KNX ceiling sensor: 8Nm 360° coverage at 3m, sends DPT 1.001 (presence bit) and DPT 9.004 (lux value) simultaneously.

ETS6 group objects for presence detection

Standard KNX presence detector group objects: Object 1 — Presence output (DPT 1.001, 1-bit, value 1 = presence detected, 0 = no presence); Object 2 — Brightness/lux (DPT 9.004, 2-byte float, lux value 0-65535); Object 3 — Slave input (DPT 1.001, master-slave grouping between detectors covering the same zone); Object 4 — Disable input (DPT 1.001, 1 = detector disabled, used during cleaning or commissioning); Object 5 — Hold time acknowledgement (some detectors, optional). The presence output object connects to: KNX switch actuator group address (lights on/off) and KNX HVAC controller group address (comfort/standby setpoint).

Hold time configuration by space type

Hold time is the duration the detector maintains presence output after the last detected movement. Too short: lights switch off with occupant still present. Too long: lights stay on in empty rooms. Recommended hold times: Corridor (pass-through): 30-120 seconds. Toilet cubicle: 3-5 minutes (allows for stationary use). Open-plan office: 10-15 minutes (with HF component to maintain hold with seated person). Meeting room: 15-20 minutes (people may sit still for extended periods). Hotel room: 20-30 minutes (guest may be asleep). Server room / storage: 60 minutes (infrequent short visits). Configure hold time in ETS6 parameter page of the detector — not in the actuator.

Lux threshold linking for energy efficiency

The most energy-efficient KNX detector configuration: presence output drives lights ON only when lux drops below threshold. Implementation: presence detector sends DPT 1.001 presence telegram → ETS6 logic block AND gate: (presence = 1) AND (lux < 300 lux) = output 1 → switch actuator ON. The lux input comes from the same detector's DPT 9.004 lux object (most dual-tech detectors include an ambient light sensor). If natural light provides sufficient illumination (lux > 300 lux threshold), occupancy does not trigger artificial lighting. Typical lux thresholds: office task area 300 lux, corridor 100 lux, toilet 150 lux.

HVAC integration: comfort vs standby setpoint

KNX presence detection drives HVAC setpoint mode as well as lighting. HVAC setpoint modes (DPT 20.102): Comfort (22°C heating, 24°C cooling — occupied), Standby (19°C heating, 26°C cooling — short absence), Economy (16°C heating, 28°C cooling — extended absence), Protection (6°C heating, 35°C cooling — frost/heat protection). Presence detector output → KNX HVAC controller mode input: presence = Comfort; no presence for > 15 minutes = Standby; no presence for > 60 minutes = Economy. This saves 15-20% of HVAC energy compared to fixed time-based schedules.

Placement rules for ceiling detectors

Mount in zone centre, not adjacent to heat sources (server, photocopier, kitchen equipment) that cause PIR false triggers. Keep minimum 500mm from air handling vents (airflow disrupts PIR detection). For rooms > 30m², use multiple detectors in master-slave configuration: one detector transmits presence output (master), additional detectors in the same room forward their presence output to the master's slave input group address. Master sends one combined output to lighting/HVAC. Mount height: ceiling mounting gives full 360° coverage; wall-mounting at 1.8-2.2m gives directional detection towards the door — useful for single-entry rooms.