Stairwells account for about 2% of multistory commercial building floorspace, with an average of one light fixture for each 58 sq.ft. of stairwell, according to the International Facility Management Association. It is trafficked 3-5% of the average day.
This application has emerged as a strong potential opportunity for energy-saving controls. In the near future, in fact, using energy-saving controls in this space will become standard practice due to commercial building energy code requirements.
Although the average stairwell is occupied infrequently and for short periods of time, many building codes require constant illumination for safety.
The Life Safety Code (NFPA 101) requires at least 10 footcandles of light on the stair tread while in use (Section 126.96.36.199). The use of automatic motion sensors is recognized as long as they provide fail-safe operation, turn the lights on upon occupancy, and keep the lights ON for at least 15 minutes after the space becomes unoccupied (Section 188.8.131.52.2).
Lutron Electronics, Lithonia, Philips Day-Brite, Philips Lightolier, LaMar Lighting and Columbia Lighting and other manufacturers now offer stairwell light fixtures with a dimming or switching controller, allowing energy savings to be captured in this application. The fixture operates at a constant low light level (energy-saving mode)—e.g., about 1 footcandle. When an integral or separately mounted occupancy sensor detects that a person has entered the space, it signals the controller to raise light level to code-compliant full brightness (occupied mode)—e.g., 10 footcandles. Some products provide complete shutoff capability for when codes allow it.
The result is up to 70-80% energy savings, say manufacturers. The energy savings may have two components: first, the existing fixture may be T12 and replaced with a more-efficient electronic-ballasted T8, T5 or T5HO (or LED) fixture. And second, the occupancy sensor ensures the lights maintain a lower light level during the majority of the time the stairwell is unoccupied.
A 2003 Lighting Research Center study in two new York City buildings demonstrated 53-60% energy savings using this approach. A later Lawrence Berkeley National Laboratory study in four California buildings demonstrated 40-60% energy savings. And a Pacific Gas & Electric study at The Fillmore Center in San Francisco demonstrated 66% energy savings.
Stairwell fixtures are available in various lamp lengths and wattages; white or clear prismatic lens; and wall or ceiling mounting. The controller may offer step dimming (single ballast), continuous dimming (single ballast) or bilevel switching (two ballasts) capability, with a choice of low-end light level. The occupancy sensor is either mounted as a part of the fixture or separately with wireless communication between the sensor and the controller, and detects occupancy via passive-infrared (PIR) or ultrasonic technology. Adjustable time delay and emergency battery backup options are typically available. Some products contain a light sensor that maintains the low light level setting during occupancy if there is a high enough light level on the stairs due to daylight contribution from windows and skylights.
The challenge is to ensure that the lights raise to full output during occupancy, a function of avoiding sensor “blind spots” and ensuring the sensor is sensitive enough to raise light output immediately upon occupancy. If using a PIR sensor, note that the sensor must have a line of sight between the sensor and the occupant, and is most sensitive to people moving laterally in front of the sensor. Ultrasonic sensors are more sensitive, do not require a line of sight, and are most sensitive to people moving to and from the sensor. Wireless sensors enable more flexibility in placement, as they are not tied to a specific fixture location.
The Lutron PowPak Stairwell Fixture, for example, uses a Lutron digital continuous dimming ballast preprogrammed to occupied and unoccupied levels, while offering field programming. The fixture receives signals from Lutron’s Radio Powr Savr wireless occupancy sensors via the company’s Clear Connect radio-frequency technology. In this solution, the wireless sensor provides flexibility in placement, ensuring adequate coverage. It raises light level not only for the fixture in the immediate area, but also the floor above and the floor below, providing a relatively seamless experience for the occupant.
Energy code standards are beginning to mandate this approach. Section 184.108.40.206(g) of the ASHRAE/IES 90.1-2010 energy standard—the minimum standard for all commercial building energy codes by October 18, 2013 per Department of Energy ruling—lighting in stairwells must “have one or more control devices to automatically reduce lighting power in any one controlled zone by at least 50% within 30 minutes of all occupants leaving that controlled zone.”
Bilevel stairwell lighting offers a simple method of saving energy in new construction and retrofit applications.