Why does my burglar alarm keep going off, and how do I silence it?

A false alarm is nearly always the system reporting something real that is not an intruder. The panel almost always records which zone went first, and that single piece of information turns an infuriating mystery into a ten-minute job.

Safety

What you'll need

• The alarm code and the manual
• A stepladder for ceiling sensors
• Replacement sensor batteries (optional)
• A torch (optional)

Step by step

1. Enter the code at the keypad to silence and reset.That is the intended route. Most external sirens also stop on a timer after fifteen to twenty minutes by design, so if you are away from home it will not run all night.
2. If you have lost the code, do not attack the bell box.The tamper circuit will re-trigger and you may damage the system. Isolate mains power at the fused spur if you can identify it, accept that the backup battery will run the siren for a while, and call the installer or an alarm engineer.
3. Read the panel log for the first zone.Nearly every panel stores an event history. The zone that triggered first is the one to investigate. Later zones are often just the person walking in to deal with it.
4. Check the obvious physical causes in that zone.A door or window not fully closed so the contact separates in wind, a loose magnet, a curtain, a helium balloon, a hanging decoration, or a pet that has gained access to a room it usually cannot reach.
5. Look for heat movement in front of a movement detector.Passive infrared sensors respond to moving heat. A radiator warming up under a sensor, a boiler flue, sunlight tracking across a floor, an air-conditioning unit or an open window blowing warm air are all classic causes.
6. Check for insects and cobwebs.A spider walking across a lens is very close to the sensor and looks like a large body moving. This is one of the most common causes of a 3am trigger. Clean the lens and the housing, and treat cobwebs around the sensor.
7. Replace low sensor batteries.On wireless systems a low battery causes intermittent, seemingly random faults before it announces itself. Change them all as a set on a date rather than chasing them one at a time.
8. Check the panel's standby battery.A sealed lead-acid panel battery lasts about three to five years. As it fails you get random faults and alarms during brief power dips. Replacing it is cheap and it is a very common cause of unexplained behaviour.
9. Fit pet-tolerant detectors if you have animals.Ordinary PIRs will trigger on a cat. Pet-tolerant units ignore mass below a threshold at floor level and must be mounted at the height the manufacturer states — they do not work if fitted low.
10. Get an engineer if it repeats with no identifiable zone.Random triggers across several zones point to the panel, the power supply, or interference. That is diagnosis with test equipment rather than guesswork.

Tips

• Tell your neighbours once you have fixed it. Repeated false alarms are how a bells-only system stops working socially, and that is most of its value.
• Keep the alarm code somewhere that is not the house, and make sure a second person in the household knows it.
• If the system is monitored, ring the monitoring centre before you start testing or they will escalate a fault as a real activation.

Common mistakes

• Turning the alarm off permanently instead of fixing the cause — The fault is almost always a five-minute physical problem — a spider, a battery, a curtain. Abandoning the system leaves you with the cost and none of the benefit.
• Mounting a pet-tolerant sensor at the wrong height — The pet tolerance is a geometry trick: the lower part of the field is masked. Fitted too low, an ordinary cat walks straight through the detection zone.

If it doesn't work

Questions people ask