How many motion sensors do I need?

Usually one per route an intruder would have to use: entrance hall, living room, bedroom corridor and stairs. You don’t need one in every room if entry points are covered by contacts.

Do motion sensors work with pets?

Pet-immune models exist up to a certain weight and height. Follow the manufacturer’s mounting height and orientation, and consider whether the pet climbs onto furniture.

Can a flood sensor shut off the water by itself?

Yes, when paired with a motorised valve on the water supply. The automation system or alarm closes it automatically and sends an alert.

Where shouldn’t I put a smoke detector?

Avoid kitchens (cooking fumes cause false alarms), next to windows or air-conditioning vents, and in the dead-air corner where the ceiling meets the wall.

Motion, Door, Flood and Smoke Sensors: Which You Need

Quick answerA well-protected home combines intruder detection (contacts on doors and windows, motion sensors in circulation areas) with technical alarms (smoke in hallways and living areas, flood sensors in the kitchen, bathrooms and utility room, and gas/CO where there are gas appliances). The key is placing them where they provide useful information and having them alert your phone or act automatically — for example, shutting off the water.

A security system is only as good as its sensors. Buying “a kit” usually leaves gaps and puts sensors where they aren’t needed. This guide explains what each type of sensor does, where it works best and the minimum combination that makes sense for a home.

Intruder sensors

Door and window contacts (magnetic)

They detect when a door or window opens. They’re the foundation of perimeter protection because they alert you before an intruder is inside.

Where: front door, garage doors, terrace doors and windows reachable from outside.
Advantage: almost no false alarms; they let you arm just the shell of the house at night.
Variants: with built-in shock or tilt sensing for shutters and large windows.

Motion sensors (PIR)

They detect body heat moving across their field of view and cover circulation areas.

Where: entrance hall, living room, corridors and stairs — aimed along the route, not at windows or heat sources.
Variants: pet-immune, with a camera to verify alarms, and dual-technology for outdoors.

Glass-break sensors

They recognise the sound or vibration of breaking glass — useful on large glazing where someone could get in without opening the window.

Technical alarms

Smoke

Early warning of fire. In a home, typical spots are bedroom corridors, the living room and areas with lots of electrical equipment. A connected detector alerts your phone even when nobody is home.

Flood

A floor probe detects water. Paired with a motorised valve on the supply, it shuts off the water automatically.

Where: under the sink, by the dishwasher and washing machine, in bathrooms, next to the water heater or boiler.

Gas and carbon monoxide

With gas supply, boilers or fireplaces, detecting leaks or CO is worthwhile. Some detectors can close a compatible gas valve.

Temperature and humidity

Useful to warn of frost, overheating in a plant room, or a freezer that stops working (see our temperature monitoring service).

Recommended minimum combination

Area	Intruder	Technical
Front door	Door contact	—
Living room	Motion	Smoke
Bedroom corridor	Motion	Smoke
Kitchen	Contact on reachable window	Flood, gas if applicable
Bathrooms	—	Flood
Utility / boiler room	Contact if it opens outside	Flood, CO with combustion appliances
Garage	Door contact	Smoke or temperature
Terrace / large glazing	Contact + glass-break or shock	—

Getting more from sensors with automation

Sensors aren’t only there to trigger a siren:

An open-window contact switches off the air conditioning in that room.
A motion sensor turns on hallway lights at low level at night.
A flood alarm shuts off the water and sends an alert.
Arming the alarm switches off lights and lowers shutters.

Worked example: a 90 m² three-bedroom apartment

An effective combination without overkill:

1 contact on the front door.
2 contacts on windows facing an accessible light well.
2 motion detectors: hall–living room and bedroom corridor (one with a camera).
2 smoke detectors: living room and hallway.
3 flood probes: under the sink, by the washing machine and in the main bathroom, with a shut-off valve on the supply.

Ten devices covering intrusion, fire and water. With automation, the kitchen window contact also switches off the air conditioning when it’s opened.

Common mistakes

Pointing motion sensors at a sunny window or an air vent.
Smoke detectors in the kitchen that end up disconnected after false alarms.
Flood probes without a shut-off valve: you get the alert, but the water keeps flowing.
Leaving the garage unprotected, even though it often connects to the house.
Never testing sensors or checking batteries.

Conclusion

The ideal mix combines contacts to detect early, motion sensors to cover interior routes and technical alarms to protect the property from fire and water. Placed thoughtfully and integrated with the alarm and automation, they alert you only when it matters and act before a problem grows. To choose the system technology, see wired vs wireless alarm systems.

Domolife plans sensor layouts for each home as part of our alarm service. Request your assessment.

Motion, door, flood and smoke sensors: which do you need?