When a house alarm goes off and nobody has broken in, something has usually triggered a detector or the system has a fault. Common causes are pets, spiders or insects on a motion sensor, heaters and draughts, a door not fully shut, low batteries and ageing detectors. If it sounds while the alarm isn't set, suspect a tamper fault or the siren itself. Your panel's event log will normally show which device set it off.
First, stop the noise with your code, fob or app. If you can't, see our guide on how to silence or reset a house alarm safely.
Start with the event log
Most modern panels record which detector or zone triggered the alarm, and when. You can usually see this on the keypad or in the app. Write it down.
If the same detector triggers every time, the problem is almost certainly that detector or something in its view.
Common causes of false alarms
Pets
Motion detectors (PIRs) pick up moving warm bodies, so a cat or dog in a protected room can set them off. Pet-tolerant detectors help, but a cat jumping onto furniture or climbing the stairs can still get close enough to trigger one.
Fix: keep pets out of protected rooms, use a part-set that leaves their area out, or ask about pet-tolerant detectors.
Spiders and insects
A spider walking across a sensor, a web over the lens or a fly landing on it can all trigger a motion detector, often at night.
Fix: with the alarm unset, look for webs on and around the detector and gently wipe the outside with a dry cloth. Don't open the detector.
Heaters, sunlight and draughts
PIRs react to sudden changes of heat in their view. A fan heater, a radiator coming on, strong sunlight through a window, or a draught moving curtains, balloons or decorations can be enough.
Fix: keep portable heaters out of the detector's view, close windows before setting, and move anything that sways in a draught.
Doors and windows not fully shut
Door and window contacts use a magnet to tell the panel when something opens. A door that isn't fully closed, a warped frame, or wind rattling a loose door can trip them.
Fix: shut and lock doors and windows before you set the alarm. Most keypads show an open zone when you try to set, so check the display says the system is ready.
Low batteries
A wireless detector with a low battery can behave unpredictably, and a failing backup battery in the panel or siren can cause problems during a power cut. Our guide to why a house alarm beeps covers battery warnings in more detail.
Faulty or ageing equipment
Detectors, cables and siren boxes wear out. Damp inside an outside siren, a corroded cable or a failing detector can cause random activations, often at night. This is more likely on systems that haven't been serviced for years.
Power cuts
Some alarms sound when the mains comes back on, or when the backup battery runs flat during a long power cut.
Going off when the alarm isn't set
When the system is unset, motion sensors and door contacts are normally ignored. So if the alarm goes off while it's unset, the usual suspects are a tamper fault, a personal attack (panic) button, or a fault in the outside siren itself, such as damp or a failing battery. These need an engineer.
What you can safely do
- Silence the alarm with your code, fob or app.
- Check the event log and note the detector or zone.
- Walk round the house: doors and windows shut, pets out of protected rooms, and no webs, heaters or moving objects in view of a detector.
- If your system lets you leave out (omit) a zone when setting, you can do this as a short-term fix. Remember that area won't be protected.
- Don't open detectors, the panel or the siren box, and never cut wires.
Thinking of your neighbours
An alarm that keeps going off is stressful for you and the people around you. Most outside sirens switch off automatically after a set time, and West Lothian Council's burglar alarm guidance advises having an automatic cut-off after 20 minutes. The same page explains that the council can act when an alarm keeps causing a nuisance with nobody to silence it, and may bill the owner.
If you're going away, leave a key with a trusted neighbour, friend or relative who knows how to unset the alarm.
When to call an engineer
- The same detector keeps triggering after you've checked the obvious causes.
- The alarm goes off when it isn't set.
- A tamper or fault message won't clear.
- It has gone off more than once with no clear reason.
- The system is old, you don't have the codes, or your installer can't be reached.
Get it fixed
We track down and fix false alarms across Glasgow and Central Scotland. For urgent help, call our 24/7 line on 07397 901 411 or see our emergency alarm call-out service. Call us for a price.
If your old system isn't worth repairing, get an instant price for a new alarm. Our indicative packages start from £380, and we confirm the price for your home before any work starts.
