Automatic door bottoms

A plunger at the hinge end strikes the frame as the leaf closes, triggering a spring mechanism that drops the seal to the floor. When the leaf opens the plunger is released and the seal retracts, so it does not drag across the floor anywhere in the swing. It must therefore be fitted the right way round, with the plunger at the hinge side.

A fully mortised unit sits in a groove routed along the bottom edge of the leaf, hidden from view and suited to acoustic doors, but it requires machining the leaf. A semi-mortised unit uses a shallower groove with one face of the housing exposed. A surface applied unit screws onto the leaf face, for leaves that cannot be routed, such as thin hollow metal leaves or leaves with a fixed core.

Measure the leaf width to choose the length (most units can be cut down within a range set by the manufacturer), and the largest gap from the bottom of the leaf to the floor to choose the drop. For a mortised unit, also measure the leaf thickness to confirm there is room for the groove.

The gap beneath the leaf is usually the largest gap in the opening, and sound passes through a gap far more easily than through a solid leaf. An automatic door bottom closes that gap when the door shuts without needing a raised threshold, so in meeting rooms, test rooms or plant rooms the acoustic rating of the whole opening depends heavily on this part.

Most units have an adjusting screw on the plunger: turn it out to drop the seal further, turn it in to reduce pressure. Adjust until the seal bears evenly along its length; if one end gaps, check whether the floor slopes, as some units drop only parallel while others allow one end to drop further.

Only when that model is part of the fire door's tested configuration, because a routed slot in the bottom edge changes the leaf construction. Some products include intumescent material and are tested for fire doors, others are not. Check the door documentation before routing or fitting.

Periodically clean dust and grit from the seal and the plunger tip, since debris stops the bar dropping evenly. Check that the adjusting screw has not loosened from slam vibration, and that the strike plate on the frame where the plunger bears has not worn a recess. Do not oil the mechanism unless the manufacturer says so, because oil attracts dust and can jam the spring.

Not necessarily, since the bar drops straight onto a flat floor, which is an advantage for step-free openings. The floor must however be flat and hard at the contact line; thick carpet or tiles with deep grout joints reduce the seal. For demanding acoustic doors, a low flat threshold gives the seal an even bearing surface.