Door and Window Contact Types and Gap Ratings

Why this matters

A magnetic contact is the simplest device in the system and the one that generates the most "it went off and nobody was there" calls on windy nights. Almost never is the contact broken. The door has sagged on its hinges, the slider shifts in its track, the steel frame is soaking up the magnet's field, or the pair was installed at the edge of its gap rating on day one and the door moved a fraction more. Picking the right contact type for the opening, and installing it well inside its rated gap, is the whole job.

How a contact works, and what "gap" means

A magnetic contact is two pieces: a switch (usually a reed switch, two thin contacts sealed in a glass capsule) on the fixed frame, wired to the panel, and a magnet on the moving door or sash. With the magnet close, its field holds the reed closed. Move the magnet away and the reed springs open: the loop opens and the panel sees the opening. The switch side goes on the frame because the frame does not move; wiring into a moving door needs a flexible door loop, which is a different device.

The gap rating is the distance between switch and magnet at which the reed changes state, as the maker measured it. Most makers state two figures, per the maker:

• Make: the distance at which an approaching magnet closes the reed.
• Break: the distance at which a receding magnet lets it open.

Break is larger than make; the difference is hysteresis, and it is what keeps a contact from chattering at one exact distance. The rating also assumes a mounting surface, usually wood or another non-magnetic material, and the switch and magnet aligned as the instructions show. Misalignment side to side cuts the effective gap, and steel cuts it more.

The section that matters: a recessed pair at the door head

 RECESSED PAIR, section         SURFACE PAIR, face view
 through the door head          at the latch side
 ====== header framing ======   | FRAME |   DOOR
 |   cable from the panel   |   |       |
 |          |               |   | [SW]  |  [MAG]
 |      [SWITCH]            |   | screwed  screwed
 |  bored up into head jamb |   | to frame to door,
 +--------------------------+   | aligned, gap set
         operating gap          | per the maker
 +--------------------------+   |       |
 |      [MAGNET]            |
 |  bored down into door    |
 |  top, near latch side    |
 |          DOOR            |

The recessed switch sits in a hole bored up into the head jamb, with its cable fished through the header to the panel. The magnet sits in a hole bored down into the top of the door, directly beneath it. The operating gap is the door's head clearance, the space between the top of the door and the jamb. Both pieces are hidden, which is why customers like recessed contacts and why techs who install them have to be right the first time.

Why near the latch side. As a hinged door starts to open, the latch edge moves first and furthest; the hinge edge barely moves at all. A pair near the latch side breaks within the first fraction of an inch of opening. A pair near the hinges may not break until the door is open far enough for someone to slip through.

The types, and the opening each one fits

• Surface contacts. Switch screwed to the frame, magnet to the door or sash. Easy to align and adjust after install, and generally rated for larger gaps than recessed pairs because the magnets are bigger. Visible, and the exposed cable on a surface run is an easy target unless concealed. A good choice on windows, retrofit doors and anything the customer does not care to see hidden.
• Recessed contacts. Bored into frame and door edge, concealed. Smaller magnets mean smaller gaps, so they need a tight, stable opening. Best on solid doors that close firmly to a stop. Poor on a loose door, and a wrong bore is a patched hole in a finished jamb.
• Wide-gap contacts. Stronger magnets for larger rated gaps. The fix for openings with real movement: older sliders, double doors with play between leaves, gates, barn doors. The trade is that a larger gap means the opening must move further before it is reported.
• Steel-door contacts. Steel in the door or frame draws the magnet's field into itself and shrinks the effective gap, sometimes to nothing. Use a contact the maker lists for steel, or the spacers the maker specifies, and expect a smaller rated gap on steel than on wood in the same family.
• Overhead-door contacts. For garage and roll-up doors: a heavy switch in a housing mounted on the floor, with armored cable to resist being run over, and a magnet on the bottom edge of the door; track-mounted versions exist too. Wide gap by design, because an overhead door rattles and shifts in its tracks and a standard pair would chatter.

Sagging doors and sloppy sliders

Openings move over time, and contacts are installed against the opening as it was on install day.

• A sagging door. Hinge screws loosen in softwood jambs, the door drops at the latch side, and the head clearance at the latch corner grows. That is exactly where the recessed pair is. A door that used to close with a tight head gap now closes with a much wider one, and the pair sits at the edge of its break distance.
• Wind and pressure. A latched door still moves within the play between its latch and strike and against its weatherstrip. On a windy night, gusts push and pull it in and out. Sitting near the edge of its rating, the reed opens on a gust and closes again a moment later: a fault and restore with nobody near the door.
• A sloppy slider. A sliding panel can shift side to side and up in its track, and the lock may close well short of the jamb. A surface or recessed pair on the lock stile sees the gap change every time the panel is pushed. Many sliders also allow the panel to be lifted out of the track with the lock engaged, which no contact on the lock side catches without help; that is a job for a pin or bar and a glass-break or motion as backup.

A front door that trips on windy nights

A customer reports three overnight alarms in two weeks on zone 1, the front door, each a fault and restore within a second or two, each on a gusty night. The history shows nothing else tripping before or after. The front door has a recessed pair near the latch side of the head, installed five years ago.

Before touching it, the account goes on test with the central station (see related: How to Put an Alarm Account on Test and Take It Off), and the panel is left disarmed with chime on so every fault shows at the keypad.

What the instructions say. This recessed pair's instructions, read for this example, rate it to break at 3/4 in on wood (illustrative; read your own). The shop's install default is an operating gap no more than half the rated break distance, so this pair should run at 3/8 in or less. That margin is what absorbs a door's normal movement.

What the door measures. Latched, at the latch corner of the head: 1/2 in, against 1/4 in on the original install note. Inside the 3/4 in rating, and outside the shop's 3/8 in default by 1/8 in. Pushed hard at the top latch corner, as a gust would push it, the door top shifts within its latch play and the magnet moves off the switch, adding roughly another 1/4 in of effective separation, to about 3/4 in, which reaches the break rating, and the keypad chimes. Released, it restores. That is the event history, reproduced by hand.

Why it grew. The top hinge's screws had pulled loose in the jamb, and the door had dropped at the latch side by the 1/4 in difference between 1/2 in now and 1/4 in at install.

The fix. Re-anchor the top hinge with longer screws that reach the framing behind the jamb, which pulls the door back up; that is basic carpentry, and the customer may prefer their own carpenter. Measured again: 1/4 in latched, and the hardest push opens it only to 3/8 in, still within the shop's half-rating default and well short of the 3/4 in break. Chime test: the keypad chimes as the door opens, not before, and stays quiet under any push to the closed door. Confirm the opening at the central station under zone 1 and take the account off test.

What would have been wrong. Replacing the contact (a new pair at the same gap trips the same way), or moving zone 1 to a type that tolerates repeated trips, which hides a real opening. If the hinge could not be re-anchored, a wide-gap recessed pair sized for the measured movement would have been the right replacement.

Drilling for recessed contacts

Boring a head jamb or a door top is overhead work and dust work. Use a stepladder set on the floor and rated for the tech plus tools, wear eye protection for chips falling from the bore, and keep the drill clear of the cable once it is fished. In a house built before 1978, assume painted trim may carry lead paint: drill through a dust shroud on a HEPA-filtered vacuum, mist and wipe the dust rather than sweeping it, and keep the customer and pets out of the doorway until it is clean. Never bore the top of a fire-rated door; a label on the door's hinge edge marks one, and modifying it is a question for the door's listing and the fire marshal, not the alarm job.

How to verify you got this right

For every contact, the job record shows the contact type, the maker's rated gap for the mounting surface it is on, and the measured operating gap with the opening closed. Each measured gap is within the shop's half-rating default. Each opening has been pushed and pulled closed while the keypad is watched for a chime, then opened slowly to confirm the fault appears in the first part of travel. And every zone has been received at the central station before the account comes off test.

References

• Contact manufacturer's installation instructions (make and break gaps by mounting surface, steel derating, alignment, splice method)
• See related: End-of-Line Resistor Supervision: Open, Short and Normal; How to Install a Recessed Door Contact; How to Install Surface Contacts on Windows and Sliders; The Alarm That Tripped on Windy Nights