SENSOR RELIABILITY

Door and Window Contact Sensors: Alignment, Gaps and False Alerts

A contact sensor can look correctly installed yet report an opening when the frame flexes, the adhesive moves or the magnet sits just outside the model’s permitted gap. Diagnose the physical installation before bypassing the zone.

Do not silence the symptom: repeated bypassing removes the protection you bought the sensor to provide. Correct the mounting, battery or supervision fault.

Installation checks

Check	What to verify
Magnet alignment	Use the exact model’s alignment marks and maximum gap—not a visual guess
Moving parts	Mount the two halves so hinges, sliding tracks and loose frames cannot strike them
Surface	Clean, dry and stable; replace failing adhesive with an approved fixing
Metal frames	Check manufacturer guidance for spacers or an alternative sensor type
Battery and tamper	Confirm charge, cover closure, tamper status and hub supervision

Test more than fully open and closed

Open and close the door or window slowly while watching live status.
Test the “almost closed” or latch-resting position that occupants actually use.
Gently press and release the frame to reveal flex or excess gap.
Repeat during colder, hotter or wetter conditions if the alert is seasonal.
Check double doors, patio doors and sash windows at every moving meeting point.
Run the alarm’s walk or zone test and confirm the correct sensor name appears.

Common alert patterns

At night

Temperature movement, a poorly latched door or battery voltage drop can expose a marginal gap.

During wind

Frame movement or an unlatched secondary leaf may briefly separate the magnet and sensor.

After redecorating

Paint, new seals, moved hardware or remounting can alter alignment by a few millimetres.

Professional reference: ProtectUK alarm and CCTV guidance.

Last checked: 1 September 2026. Follow the sensor maker’s stated gap and mounting requirements.