About A Chair AAC 155 office chair by Hee Welling

HAY introduces the AAC 155 as combining "the high functionality of a work chair with optimal comfort and a modern aesthetic," while retaining "the high backrest and generous seat" of the series. It is the only About A model with a base that features both a gas lift and a tilt mechanism.

The upholstered shell, and what the textiles are

The shell is the same as that on the non-upholstered models, a molded recycled polypropylene—that is what HAY's product sheet states—but here it is fully covered. Foam is placed over the shell and then dressed with the chosen upholstery: the contact is softer, and noise is better dampened in high-traffic rooms. Conversely, upholstery must be cleaned by blotting rather than spraying.

The upholstery options offered here: Sense, HAY’s aniline leather, dyed through: it shows the grain and develops a patina with use; Hallingdal 65, a Kvadrat wool fabric (70% wool, 30% viscose), the company’s classic, highly durable option. The so-called quilted variants—HAY uses the term quilted—feature topstitching that compartments the padding: the filling does not shift over time, and the texture breaks up the solid surface.

The base: five-star, casters, and gas lift

The base is a cast aluminum five-star star: HAY writes cast aluminium five-legged castor base: mounted on casters. Five legs instead of four: this is the stability standard for rolling chairs, as a four-legged chair tips over too easily when leaning forward.

A gas lift adjusts the seat height. It is a pneumatic cylinder: a pressurized nitrogen cartridge, operated by the lever under the seat, which raises the chair effortlessly and locks it at the desired height. It is maintenance-free and can be replaced as a spare part if it eventually loses pressure.

This model adds a tilt mechanism: the backrest and seat tilt backward together under the weight of the body. This is what separates a work chair from an occasional chair: you can lean back without leaving the backrest. HAY designates it as a task chair.

Note on the height dimension. HAY publishes a single overall height for these chairs. Since the seat is adjustable via gas lift, this value corresponds to a specific position, not a fixed height.