Pet-Immune Motion Detectors and Where They Still Fail

Why this matters

"Pet immune" on a box sells the idea that a dog or cat can roam an armed house. What the rating actually promises is narrower: a pet under the stated weight, at floor level, in a room where the detector is mounted exactly as the maker specifies, should not trip it. Put the same detector in a room with a sofa, a cat tree or a staircase in its view and the cat climbs into the zones meant for a person. Pet false alarms are among the most common reasons a new customer starts leaving the system off, and almost all of them were decided at the survey, when nobody asked where the animals actually go.

How a pet-immune detector tells a pet from a person

Two mechanisms, used alone or together, per the maker:

• Lens geometry. The lens puts the detector's sensing zones where a standing person's body will be, and keeps zones away from the floor at a distance. A low-slung animal walking across the room passes under the upper zones and past the point where the lowest zones meet the floor. Installers call that band near the floor the pet alley.
• Signal processing. The detector judges the size and shape of the infrared change, often with a four-element (quad) sensor that compares the top and bottom of the target. A person spans several zones vertically at once; a small animal near the floor does not. Below a threshold, the detector discounts the signal.

Both depend on where the zones fall, which depends on mounting height and the detector being plumb on a flat wall. Mount it higher or lower than the maker's specified height, or tilt it with a swivel bracket the maker did not intend for pet use, and the zones land somewhere else, and so does the pet alley.

The rating's assumptions, written out

The weight figure on the package (per the maker; ratings differ across product lines) carries conditions that are in the installation instructions, not on the front of the box:

• Floor level. The pet is walking on the floor, not on furniture, stairs, counters or sills.
• The specified mounting height and orientation, with any pet-mode setting or jumper selected.
• A stated number of animals, often one, or a combined weight. Two dogs moving together can look like one larger target.
• A typical body. Coat matters: a short-haired animal shows more infrared than a heavily furred one of the same weight, because fur insulates. Some makers note this.

Read those conditions to the customer in plain language. It is the honest version of "pet immune", and it heads off the first argument after the first dispatch.

Where the pet ends up in the person zones

Side view of a living room with a pet-immune detector at the maker's height:

 [PIR] on wall, at the maker's height
 |`--..__
 | `-.   ``--..__          upper zones, person height
 |    `-.        ``--..__
 |       `-.             ``--..__     CAT on the
 |          `-.                [=]``--..__  sofa back
 |             `-.             [SOFA    ]
 |    lower zone  `-.   DOG    [        ]
 |    near the wall  `-. ~~~   [        ]
 +---------------------`-.--------------------- floor
   DOG: below the upper zones and past where the
   lower zone meets the floor, in the pet alley

The dog, on the floor in the open, sits in the gap the lens was designed to leave. The cat weighs a fraction of what the dog does and is the one that trips it, because the sofa back lifts it into the upper zones, where its body looks like part of a person moving through. The same thing happens with:

• Stairs in view. A dog halfway up a staircase is at chest height to the detector.
• Beds, cat trees, window sills, counters, the top of a bookcase. Any perch inside the pattern.
• Animals that jump or stand. A large dog jumping at a window, or standing with front paws on the sill to bark at the mail.
• Climbers and fliers. Cats in general, ferrets, and any bird let out of its cage. A free-flying bird is in the person zones by definition.

The look-down or creep zone directly under the detector matters too: some pet-immune models drop it or let it be masked, per the maker, because a cat jumping onto something right below the detector is close enough to look large.

The questions to ask at the survey

Ask these with the customer walking you through the rooms, not from the kitchen table:

• "When you are out, where can each animal go? Which doors stay shut?"
• "Do they get on the furniture? Which pieces? The windowsills? The stairs?"
• "Is there a cat tree, a dog bed on a chair, a bird that comes out of its cage?"
• "Are they crated or shut in a room when you arm Away?"
• "Any new animal coming, a puppy that will grow?"

Write the answers on the survey, room by room. They decide where motion detection goes before any detector is chosen. See related: How to Survey a House for an Intrusion System.

A first-week false alarm, traced and fixed

A house with a 14 ft by 18 ft family room, a short-haired dog under the detector's rated weight, and one cat. The family room has a pet-immune detector at the maker's height on the wall opposite a sofa that sits with its back to the windows. In the first week, armed Away, two dispatches came in, both from zone 6, the family room motion, at 1:12 pm and 1:47 pm on a sunny afternoon.

What the history showed. Both events were zone 6 only. No door contact, no other motion, no glass break before or after. An intruder who got into the family room without opening a door or breaking glass, then left without tripping anything else, is not a plausible story. A pet or the environment is.

What the customer showed. Asked where the cat spends sunny afternoons, the customer pointed at the sofa back, in front of the window. The cat sleeps there, wakes, stretches, jumps down, and does it again. The detector looked straight across the room at that perch. The dog had been in the room both times and had not tripped anything in a week of daily use, which is the rating doing exactly what it says.

The fix, in order of preference.

• Move the perch out of view. Not always possible; this customer would not move the sofa.
• Move the detector so the perch is behind or below it. The detector went to the window wall, above and to the side of the sofa, looking back into the room. The sofa back is now outside the pattern. The look-down zone under the new position would have covered the sofa, so it was masked per the maker's instructions.
• Drop interior motion in that room and protect the windows with contacts and a glass-break detector instead, keeping motion in the hall, where the cat does not perch and the path to the bedrooms runs. That was the fallback if the move did not hold.

Proving it without a dispatch. You cannot make a cat perform for a walk test. Where the panel can put a zone into a logged test or soak mode per its documentation, run the moved detector that way for a week: trips are recorded, not reported. Otherwise program the zone so it does not report while you observe it, and tell the customer in writing that the room is unprotected during that week. In this house the zone logged no events in seven days, including three sunny afternoons, and went back to reporting. Then walk-test the room as a person to confirm the move did not open a gap (see related: Motion Detector Coverage and the Walk-Across Rule).

A layout that passed can stop passing

A pet layout is a snapshot of one house on one day, and three things change it without anyone touching the alarm:

• The furniture moves. A sofa turned to face the television, a new cat tree, a dog bed lifted onto a chair. Ask about room changes on every service visit to a pet house, and look at the motion's view while you are there.
• The sun moves. A cat follows the sun, and the winter sun reaches deeper into a south-facing room than the summer sun does. A perch that was in shade at the June install can be the warmest spot in the room in December.
• The animal changes. A puppy grows past the rating in months. A second pet doubles the load the rating assumed. An old dog that never climbed the stairs gets replaced by a young one that does.

When a pet house that was quiet for a year starts tripping a motion zone, check those three before the detector.

When to skip pet-immune motion entirely

Some houses are not motion houses while the animals are loose. Where every downstairs room has a perch, where there is a bird that flies free, or where the dog is above the rating and not crated, the cleaner design is:

• the perimeter fully protected with contacts and glass-break detectors, armed in every mode;
• interior motions only in rooms the animals cannot reach (a closed basement, an upstairs hall behind a gate that actually holds);
• the customer arming a mode that bypasses interior zones when the animals are loose, which the zone types support directly (see related: Zone Types and What Each One Does When Tripped).

That design gives up interior detection in the pet rooms and keeps the system armed every day, which is better protection than a full motion layout the customer turns off after the second fine.

How to verify you got this right

Every motion zone on the sheet carries a note of which animals can reach that room and which perches are in its pattern, and the answer to the second is "none". Each detector is at the maker's height, plumb, with pet mode selected, and has passed a walk test as a person. Where a detector was moved for a pet, the soak or observation week is logged with its result. And the customer has heard, in plain words, what "pet immune" does and does not promise for their animals.

References

• Detector manufacturer's installation instructions (pet weight rating and its conditions, mounting height, pet-mode setting, look-down masking)
• Panel manufacturer's programming guide (zone soak or test modes where offered)
• See related: Motion Detector Coverage and the Walk-Across Rule; How to Place and Aim a Motion Detector; How to Survey a House for an Intrusion System