How an automatic sliding door works
A sliding door has one or two glass leaves running horizontally on a top track. A drive belt connects the leaves to a motor in the header (the metal box above the door). When the activation sensor detects motion, the controller drives the belt, the leaves slide open, hold for the dwell time, then slide closed under controller command — slowed in the last few centimetres to avoid impact.
Brands dominating UK installations: Dorma, Geze, Record, Tormax, Stanley. Hardware is broadly interchangeable across brands at the carrier-wheel and belt level; controllers and sensors are usually brand-specific.
The five wear points
Drive belt — toothed belt connecting motor to door leaves. Stretches over time, eventually slipping or jumping a tooth. Symptoms: door slow or inconsistent, motor heard running but door barely moving, position offset between operations. Lifespan 3–7 years depending on cycle count.
Carrier wheels — small wheels in the top track that the leaves hang from. Bearings wear, wheels develop flats or noise. Symptoms: door noisy on opening, shudder during travel, scrape at end-of-travel. Lifespan 8–15 years.
Safety beam — infrared beam across the door swing detecting people in the closing path. Drifts out of alignment from vibration, vehicle impact to frame, or accidental contact during cleaning. Symptoms: door reverses unexpectedly, refuses to close, beam LED flashing error code.
Activation sensor — microwave or motion sensor above the door detecting approach. Lens dirty, drift in sensitivity, sometimes outright failure of the unit. Symptoms: door opens for no-one, fails to open for approaching users, slow response.
Controller — the brain. PCB-based, with relays, fuses and a display. Failures are rarer than the wear items but more dramatic when they happen — door dead or stuck open. Usually replaceable as a unit; programming retained on non-volatile memory in most modern brands.
Diagnosing belt vs motor vs controller
A door that opens slowly or unevenly could be a belt or a motor. Quick diagnostic: stand inside, listen to the motor when the door is commanded to open. A motor under load (sounds like it’s working hard, possibly with a whine) but minimal door movement = belt slipping. A motor making no sound at all = motor or controller fault. A motor making the normal sound but the door behaving sluggishly = drive train resistance (carrier wheels, top track contamination).
For controller faults, the display almost always shows an error code. Photograph the code before any reset — it points the engineer at the root cause without re-creating the fault on site.
The belt replacement playbook
On most brands, drive belt replacement is a 2–3 hour job. Header cover comes off, belt tension released, old belt removed, new belt installed, tension set to manufacturer spec, door cycled, controller end-of-travel positions checked and reset if necessary.
A new belt typically gives 3–5 years of service on a high-traffic supermarket door, 5–7 years on a less busy retail entrance. Treat it as a scheduled replacement item, not a breakdown item — a worn belt that slips mid-day costs more in trading impact than the belt itself.
Safety beam maintenance
Safety beams are the most maintenance-sensitive part of an automatic sliding door. A monthly visual check (lens clean, no obvious damage) catches most issues; a quarterly engineer service includes alignment verification with a force test.
After any vehicle impact to the frame (common with sliding doors at vehicle interfaces — drive-throughs, loading bays adjacent to entrances), the safety beam should be checked even if no visible damage. The beam alignment is precise to a few millimetres; a frame nudge of 2 mm puts the beam out of spec.
When sliding doors need replacing rather than repairing
Full replacement is rare. Most sliding door installations last 15–20 years with regular component replacement (belts, carriers, sensors, eventually the controller).
Replacement becomes the right call when: the frame is corroded or structurally compromised; the controller is end-of-life with no replacement available; the original installation predates BS EN 16005 and cannot economically be brought to compliance; or the door type no longer matches the building use (e.g. retail doors retrofitted onto what is now a hospital entrance with different traffic patterns).
Even then, the replacement usually keeps the frame and replaces the working parts as a “package upgrade” — full frame replacement is the exception.