PIR Detector Siting: Coverage Angles, Pet Immunity, Anti-Masking and Avoiding False Alarms

Quick Answer: A standard PIR detector should be mounted at 2.0–2.4 m height with its detection curtains covering the likely approach path of an intruder across the room, not aimed at windows or heat sources. Pet-immune detectors (up to 25 kg) work by ignoring targets below 50–60 cm; they fail if pets can climb above this height. Anti-masking is mandatory for Grade 2 and above under BS EN 50131-2-2. The most common causes of false alarms are thermal interference, draughts, vibration, and RF interference — all preventable at installation.

Summary

PIR (Passive Infrared) detectors are the workhorses of the UK intruder alarm industry. They detect the change in infrared energy caused by a warm body moving across their detection curtains. When sited correctly they are extremely reliable; when sited poorly they generate false alarms, nuisance activations, and gaps in coverage that a burglar can exploit.

The detection pattern of a PIR is not a solid cone — it is a series of discrete detection curtains arranged in a fan. Movement must cross these curtains to be detected. This means siting matters: a detector aimed straight at an approach path (longitudinal movement) will detect an intruder far later, and with less reliability, than one positioned so the intruder walks across the curtains.

False alarms cost the industry money, erode customer confidence, and — most importantly — risk cancellation of police response. NPCC policy allows police forces to withdraw response from systems that generate a defined number of false alarms per year. Correct siting at installation is the most effective way to prevent this.

Key Facts

Quick Reference Table

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Environmental Challenge PIR Risk Solution
South-facing window with direct sun Heat source causing false alarm Relocate or use dual-tech
Radiator or convector heater Thermal disturbance Mount away from, above, or to side of heat source
Air conditioning outlet Rapid temperature change, draughts Avoid siting in airflow; relocate
Large window area Solar gain thermal drift Dual-tech or curtain detector at window
Roof space / loft conversion Temperature extremes Class II environmental rating required
Detached garage Mice, birds, cats Pet-immune or dual-tech
Retail storage with racking Blocked detection, narrow lanes Curtain detectors or wide-angle with analysis
Industrial unit with forklifts Vibration Vibration-resistant mounting, dual-tech

Detailed Guidance

Detection Pattern Fundamentals

Understanding how a PIR generates its pattern is essential for correct siting. The detector uses a segmented Fresnel lens that focuses IR energy from different zones of the room onto the pyroelectric sensor. Movement is detected when the sensor sees a rapid change in IR level as a body moves from one lens segment to the next.

This creates discrete "fingers" or "curtains" of detection, not a solid detection zone. The gaps between curtains are dead zones. This is why:

  1. Cross-axis detection is superior — the intruder crosses multiple curtain boundaries, generating multiple pulses
  2. End-on detection is poor — walking directly at the detector, the body remains in one curtain for longer and the signal is weaker
  3. Corner mounting maximises cross-axis coverage for the most likely approach routes

Practical layout:

Pet-Immune Siting Requirements

Pet-immune detectors work by masking out the lower detection zones so that small animals moving below approximately 50–60 cm are not detected. This creates two constraints:

  1. Mounting height is critical — the detector must be mounted at the correct height stated in the data sheet (typically 2.0–2.1 m); too low and the pet immune zone drops too close to the floor; too high and the zone rises above medium-sized pets
  2. Climbing is the enemy — a 15 kg springer spaniel that can climb onto a sofa, counter, or staircase landing will be above the masking zone and will trigger the detector

Practical guidance for pet-immune installations:

The BS EN 50131-2-2:2017 test methodology for pet immunity uses a 25 kg dog-shaped thermal mass moved across the detector field — detectors that pass this test must not generate an alarm for the test mass. This does not guarantee immunity to all real-world pets; manufacturer field testing data is more informative.

Anti-Masking Technology

Anti-masking detects attempts to defeat the PIR by placing an obstruction over the lens. Common attack methods include:

Anti-masking is implemented in one of three ways:

  1. IR proximity sensor — a secondary IR emitter/receiver in the detector detects close proximity of an object; effective against foam and tape
  2. Optical masking detection — monitors the received IR signal pattern for a sudden drop (blockage)
  3. Combination — both methods simultaneously for higher grade compliance

Under BS EN 50131-2-2:2017:

Installer note: anti-masking will be triggered by installation errors such as:

Always test anti-masking function during commissioning — the system must generate the correct response (fault or alarm) when the lens is covered.

Common Causes of False Alarms and Prevention

FALSE ALARM CAUSE FINDER
═══════════════════════════════════════════════════════
Alarm activation in empty property?
├── Has anyone entered recently? → Check log; may be genuine
└── No entry → Continue:
    │
    ├─ IS IT DAY OR NIGHT?
    │   ├── Day → Likely thermal: check for direct sunlight
    │   │         on lens or nearby heat source
    │   └── Night → More likely: draught, vibration, RF
    │
    ├─ CHECK ZONE TIME:
    │   ├── Short burst (< 5 sec) → Draught or vibration
    │   └── Sustained → Thermal soak or genuine intruder
    │
    ├─ THERMAL CAUSES:
    │   ├── South-facing window? → Relocate or add blind/curtain
    │   ├── Radiator directly below? → Raise bracket, move detector
    │   └── Air con duct nearby? → Move to avoid airflow
    │
    ├─ VIBRATION CAUSES:
    │   ├── Railway/road traffic? → Add anti-vibration mounts
    │   ├── Washing machine/dishwasher in room? → Dual-tech
    │   └── Loose fitting/conduit vibrating? → Secure mechanically
    │
    └─ RF INTERFERENCE:
        ├── Near DECT phone base? → Move detector > 1m
        ├── Near WiFi router? → Move; check EN 50130-4 compliance
        └── Near neon light starter? → Replace lamp or relocate

Corridor and Staircase Siting

Long corridors need curtain-pattern detectors, not standard wide-angle units. A wide-angle PIR at the end of a 20 m corridor will have poor detection in the first 10 m due to end-on approach.

Use corridor (curtain) detectors — typically 15° horizontal × 80° vertical — positioned at ceiling height (2.4–3.0 m) and aimed lengthways. The intruder crosses multiple vertical curtains as they walk along the corridor.

For staircases:

Frequently Asked Questions

How close to the ceiling should a PIR be?

Leave at least 150–200 mm between the top of the detector and the ceiling. The thin warm air layer at the ceiling can cause thermal drift. Mounting flush to the ceiling also restricts the downward detection angle, leaving a potential "creep zone" immediately below the detector.

Can a PIR detect movement through glass?

No. Standard PIR detectors cannot detect movement through glass — glass is opaque to the mid-wave infrared that PIRs operate at (8–14 μm). A person outside cannot trigger an interior PIR. However, if a window is open, warm body heat can reach the detector and cause spurious activations — ensure detection zones do not point at open windows.

What is the difference between Grade 2 and Grade 3 in terms of PIR specification?

Grade 2 requires anti-masking but allows it to generate a fault rather than an alarm. Grade 3 requires anti-masking to generate an alarm condition reported to the ARC. Grade 3 detectors also have more stringent environmental testing and may require a higher tamper protection level. Always verify the CE/UKCA marking on the detector shows the grade and environmental class.

How many PIR detectors should be fitted per room?

Typically one per room is sufficient if sited correctly (corner mounting, cross-axis coverage). Large rooms (over 15 m × 10 m), L-shaped rooms, or rooms with areas that cannot be covered from a single corner may require two detectors. Avoid overlapping coverage zones at close range — dual-detection with immediate alarm can mask test problems.

Should I use a dual-technology detector?

Dual-tech (PIR + microwave) detectors require both technologies to trigger simultaneously before generating an alarm. This dramatically reduces false alarms in challenging environments (retail stores, industrial units, garages). The trade-off is slightly reduced detection probability — if the microwave element is in a dead zone, the detector won't alarm even if PIR sees movement. Use dual-tech where false alarms are the primary concern; use standard PIR where detection reliability is paramount.

Regulations & Standards