My movement detector is triggering with nobody there
What PIRs actually see, why they get it wrong, and what can be checked safely
Answer first: a PIR does not see people, it sees moving heat. Once you accept that, most mystery activations stop being mysterious. Warm air off a radiator, sun tracking across a floor, a boiler flue, an extractor fan on a timer, a spider on the lens, all present as moving heat and all look like a person to the detector.
The reverse problem, a detector that fails to pick you up, is usually about geometry and age. PIRs are least sensitive to movement straight towards them and most sensitive to movement across their field. Fitted facing down a corridor rather than across it, or fitted a decade ago and now well past its best, a detector can genuinely miss somebody walking through a room.
This guide covers both failure modes, what you can check without opening anything, and where the honest limits of self diagnosis sit. We service and repair intruder alarms across Chatham, Medway, Kent and South East London, and detector work is one of the most frequent jobs we do.
Why It Happens in the First Place
Moving warm air
Radiators cooling, heating cycling, extractor fans and boiler flues all move warm air across the detection field, which is precisely what a PIR is built to notice.
Direct sunlight and reflections
Sun tracking across a floor or reflecting off glass creates a moving heat pattern, and this is why some activations follow the seasons and the time of day.
Insects and cobwebs on the lens
An insect at the lens is extremely close to the sensor, so a small creature produces a very large signal and reliably triggers the zone.
Detector at end of life
PIRs age. Sensitivity drifts, electronics become unstable and a unit fitted many years ago will start falsing or missing regardless of what is in the room.
Poor positioning for the space
A detector aimed along a corridor, over a radiator, or into a conservatory that gets fierce solar gain will always struggle, because the siting is fighting how PIRs work.
Pets in a zone not designed for them
Pet tolerant detectors exist and work well, but a standard PIR in a room a cat now has access to will trigger, and that access often changed after installation.
How It Tends to Show Up
One detector causing all your grief?
Tell us the zone name and what is in that room, and we will bring the right replacement.
Team available now, no call centres, real engineer answers.
Step by Step, Safely, at Home
Look at the room from the detector's point of view
Stand at the detector and look out. Note every heat source and air path in its field, radiators, vents, flues, a fan, a boiler, sunlit floor, a fridge in a garage zone.
Watch for the LED with the system unset
Most PIRs blink an LED when they detect. Watching that in an empty room for a while tells you whether the detector is genuinely firing or whether the panel is misreporting.
Do a walk test the correct way
Walk across the detector's field rather than straight at it. PIRs are far less sensitive to direct approach, and this alone explains many complaints about poor coverage.
Check the age and condition of the housing
Yellowed plastic, a loose mounting or an obvious knock all point to a detector at the end of its life. Note the make and any date printed on the case if you can read it from below.
Clear what you reasonably can
Move a heater, redirect a fan, close a vent, adjust a blind so the sun no longer tracks across the floor. Then give it a few nights and see whether the pattern breaks.
Do not open the detector
Removing the cover puts the tamper circuit into alarm on virtually every system and can set the sounder off. Leave the housing to an engineer.
Straight Answers to Common Questions
My movement detector is triggering with nobody there: common questions
Stop Here and Get an Engineer Out
Keeping it from coming back
Detector siting is worth getting right once rather than living with it. During servicing we walk test every device, clean lenses, and flag units that are old enough to be replaced before they start falsing. If a room has changed use, gained a conservatory or gained a pet, that is the moment to have the zone design revisited rather than waiting for the first three in the morning activation.
Related Guides Worth a Look
- Is my old alarm panel worth keeping?
- Burglar Alarm Low Battery: What To Do Next
- Understanding tamper circuits on Pyronix Enforcer systems and what to check before calling an engineer
- Hikvision NVR Hard Drive Error: What It Means and What to Do
- Hik-Connect Not Working: Fix Remote Viewing Faults
- Sorting a dead credential from a reader problem before calling out an engineer
Need Security or WiFi Help Fast?
Alarm installation, CCTV, access control, WiFi and repairs across Kent & South East London. Free quote in 60 seconds, call, WhatsApp or send a message.