Occupancy sensing
Sensors such as PIR, microwave and presence radar that detect whether a room is in use, so lighting, heating and ventilation follow actual occupancy.
What is occupancy sensing in a house?
Occupancy sensing uses a sensor to detect whether a space is in use, so that lighting, heating or ventilation respond to people actually being there instead of to a clock or a manual switch. The sensor reports to a controller, which switches a lamp, starts a fan or changes a heating setpoint, then holds that state for a set time after the last detection. In a family house the most common device is a ceiling or wall unit wired into the lighting circuit, although the same signal can also feed a smart home controller.
The difference from a timer is the trigger. A timer runs on the clock, so it switches the corridor light on at the same hour whether anyone is home or not. A sensor reacts to the room, so the light comes on for a guest at night and stays off on an empty Sunday morning. The practical question is which part of the room to automate, and for how long.
How do the main sensor technologies compare?
Four technologies cover nearly all residential products. They differ in what they detect and where they fail, so the choice follows the room rather than the price list. The pohybový senzor (motion sensor) is usually PIR. The prítomnostný senzor (presence sensor) is the radar-type device that also catches small movements.
| Technology | How it senses | Strength | Weakness |
|---|---|---|---|
| PIR (passive infrared) | Heat radiated by a moving body crossing its field of view | Cheap, low power, well suited to corridors and small rooms | Needs movement; can miss a person who sits still; reacts to heat from vents and direct sun |
| Microwave | Reflected radio waves, no line of sight needed | Covers irregular spaces and detects through thin partitions | Can trigger on fans, curtains and moving objects; can reach into the next room |
| Ultrasonic | Reflected sound waves above human hearing | Good coverage in open rooms; less affected by heat than PIR | Sensitive to air movement and doors; can react to pets |
| Presence radar (mmWave) | Fine radio reflections, including breathing and small motions | Keeps a seated or sleeping person detected | Higher cost; needs careful placement and tuning to avoid triggering from the neighbouring room |
Two limits apply to all four. None of them knows whether the figure in the room is a householder or a pet, although sensitivity settings can reduce pet triggers on some PIR units. And none of them measures how many people are present, so a single unit cannot size a ventilation rate.
Where does occupancy sensing pay off in a house?
Occupancy sensing pays where a space is used briefly and often, and where people forget to switch things off. Corridors, staircases, the WC, the garage, the utility room and the storage room fit that pattern. A PIR unit that brings up a low light in a corridor for a few minutes is simple, reliable and inexpensive to install. In a utility room it removes a common waste: a light and an extract fan left running after the laundry is done. The saving in these rooms depends on how often they sit empty, so estimate it from the household's own routine rather than from a published figure.
| Room | Usual sensor | Verdict |
|---|---|---|
| Corridor, staircase | PIR | Good fit: short stays, no stillness |
| WC, utility room | PIR with fan run-on | Good fit: brief visits, forgotten switches |
| Garage, storage room | PIR | Good fit: enter, use, leave |
| Living room, dining room | Presence radar or manual switch | Poor fit for PIR: long stays |
| Bathroom | Presence radar with long hold time | Hard: long, still baths |
Scene and dimming control sits on top of this. Smart lighting control decides how bright the light is, while occupancy decides whether it is on at all.
Where does occupancy sensing annoy the occupants?
It annoys where people stay still for long periods. A living room is the typical case. A film, a long dinner or a book on the sofa can leave a PIR unit with no movement to detect, so the light drops out in the middle of the scene.
Heating has the same weakness. If a room is on a set-back schedule driven by motion, a sensor that sees no one can lower the temperature while the occupant is still there. The result is a cold room and a complaint, which usually ends with the sensor being disabled.
A bathroom is the hardest case. A long bath is pure stillness, and a motion sensor that switches the light off mid-bath is worse than no sensor. In a bathroom, a presence radar with a long hold time, or a manual switch with an automatic off timer, is the better answer. Set hold times to what the room actually does, not to the sensor's default, and test them with the real routine before the room is handed over.
How does occupancy sensing work with ventilation and heating?
The simplest and most useful ventilation link is a motion sensor that starts the extract fan in a WC or bathroom and runs it on for a set time after the room is vacated.
For whole-house ventilation, motion is the wrong signal. It tells the controller that someone passed through, not how much air the room needs. Demand-controlled ventilation uses CO2 or humidity readings in the occupied rooms, so the fresh air rate follows the actual load.
For heating, occupancy works best with fast-responding emitters, such as panel radiators, in rooms with irregular use. Slow systems such as underfloor heating gain little from being switched by motion, because the floor takes hours to change temperature. Those rooms are better driven by a daily schedule, with occupancy used only to block heating when a room is known to be empty for a long period.
How is occupancy sensing different from demand-controlled ventilation?
Occupancy sensing answers whether someone is in the room now. Demand-controlled ventilation answers how much fresh air the room needs, which depends on how many people have been there and for how long, so it is measured rather than switched. A motion sensor cannot tell two people from one, and a CO2 sensor cannot confirm that a room is empty until the air has had time to clear. In practice a house may use both: presence for lights and short extract runs, and measured air quality for the living space. Neither replaces the other.
How should occupancy sensors be placed and tuned?
Placement decides most of the result. Mount a PIR unit so that the walking path crosses its field of view, not toward the door swing, and keep it away from heat sources, direct sun and supply air outlets. Microwave and radar units need clear space and should not face a neighbouring room or a fan. Tune sensitivity and hold time in the room itself, with the people who use it, and test the worst case: a person reading with no movement, and a person who leaves quickly. A setting that suits a corridor may be wrong for a living room, so do not copy one configuration across the house.
Frequently asked questions
- Can a PIR sensor detect someone sitting still at a desk?
- Often not for long. A PIR needs body movement across its field of view, so a reader who stays still for a long time can be missed. A presence radar sensor holds detection through small movements, at a higher price. Choose the technology for the chair, not for the corridor.
- Will occupancy sensing switch my lights off while I am reading?
- It can, if the sensor type and hold time do not suit the room. Use presence detection in rooms with long stays, set a longer hold time, and keep a manual override on the wall. Test the room in the evening, when the reading actually happens.
- Do I need a smart home system to use occupancy sensing?
- No. A standalone PIR unit can switch a light or a fan directly, with a built-in timer. A smart home controller adds the ability to combine signals, for example motion plus daylight, and to log events. For a typical family house the simplest working setup is often the best one.
- Are occupancy sensors a privacy concern?
- Less than cameras. A PIR or radar sensor reports presence and movement, not images or identity, and the detection usually happens inside the device. Check whether the controller sends data to a cloud service, since that part depends on the vendor and should be understood before installation.
- Where should I start with occupancy sensing in an existing house?
- Start with the corridor, the WC and the utility room, where the rule is simple and a wrong setting causes only mild annoyance. Add rooms with long stays only after testing. If the house is being renovated, fit the sensors while the electrical circuits are still open.