Because hibays, a.k.a. high bays, are the most common lighting fixtures for industrial and warehouse buildings, getting them to be energy efficient can improve your bottom line. If you still have standard or probe-start metal halide (MH), yellow-color high-pressure sodium (HPS) or inefficient mercury vapor (MV) hibay lighting, especially if equipped with the typical spun-aluminum domes that waste 25% of the light, you could replace them cost-effectively, reduce wattage by at least 50% and often reduce KWH by 80% if you install appropriate lighting control devices. You’ll also slash maintenance parts and labor costs.
Replace them with what?
Although many lighting retrofit contractors, hibay manufacturers and others push high-output, 5/8-inch diameter fluorescent T5HO lamps, you can see in Table 1 that high-performance, 1-inch diameter T8s are much more efficacious, the lighting industry term for efficient. The green columns are the most important.
If you want to get technical, end-of-life (EOL) lumens are much more important than initial or mean lumens, because EOL is the worst case. The higher the scotopic/photopic (S/P) ratio, the more blue content the light has and the brighter the light to the human eye. Visually-effective lumens include effects of S/P ratios. The listed fluorescent lamps are 850s, which are rated at 5,000° Kelvin, a value high enough to provide a substantial blue content without making the light look bluish.
Table 1. Focusing on the data in the green columns is the key to choosing the most efficient plant lighting systems.
Because T5HOs have peak light output at an ambient temperature of 95°F and T8s have peak light output at 77°F, and because output decreases significantly with deviations from optimum ambient temperatures, it’s good to have a cushion. Of the three T8 options shown, option G isn’t recommended unless the space is heated and air-conditioned or excessively overlit. Just because T5HOs have peak light output at a higher temperature than T8s doesn’t necessarily mean T5HOs are better in hot conditions, because T8s can often dissipate heat better in well thermally designed fixtures.
Six high-performance replacement T8 lamps cost less than four T5HO lamps, especially the 49-watt to 51-watt models. T8 lamps also last longer, especially the extra-long life versions. Plus, these are the same T8s that can be used in offices and other parts of the building, which can help minimize lamp and ballast inventories. If you decide to go with T8 hibays, it’s important to get the correct lamps, ballasts and fixtures.
There are two good choices for 5,000°K T8 lamps. One is lamps with the highest lumen output included in the Consortium of Energy Efficiency’s 32-watt high-performance listing at www.cee1.org. Many rebate programs mandate using these approved lamps. The other good choice is extra-long-life lamps, which produce fewer lumens, but last longer. Sometimes an extra 6,000-hr rating is more important than more lumens. The big three lamp manufacturers offer extra-long-life T8s. Some rebate programs might not provide rebates with extra-long-life T8s. The 4,100°K T8s can be used, but they’re not as bright as 5,000°K equivalents.
Extra-efficient, parallel-wired, program-start electronic ballasts are highly recommended, especially when used with occupancy sensors. Parallel wiring is important because the remaining lamps keep operating normally when one or more lamps burn out. The Consortium of Energy Efficiency also has a list of approved ballasts, which many rebate programs also require.
Several hibay manufacturers design reflectors for the smaller-diameter T5HO lamps and then use those same reflectors for larger-diameter T8s, a practice that hurts performance. Reflectors for T8s should have a nominal width of at least 4 inches per lamp, and the bottom of the reflector should be below the bottom of the lamp. So, if you see 4-foot long T8 hibay with 6 T8 lamps that is less than 24 inches wide or the bottom of lamps extends below the bottom of reflectors, don’t buy it.
Some hibay manufacturers have different reflector designs for low, medium and high mountings and for open or rack-aisle distributions. With proper reflectors, T8 hibays can be mounted as high as 50 feet.
Another key variable with T8 hibays is thermal design for both lamps and ballasts. In hot environments, make sure the heat from the lamps is directed away from both lamps and ballasts. Ballast compartments should be vented, and ballasts should be connected to an effective heat sink. In cold applications, the lens and housing design must allow the lamps to get warm.
What about other wattage MH, HPS and MV hibays and lowbays?
- 1,000W MH and HPS lamps often can be replaced with 12 T8s and high-ballast factor ballasts.
- 250W MH and HPS and 400W MV can be replaced with 4 T8s and high-BF ballast.
- 175W MH, 150W HPS and 250W MV can be replaced with 3 T8s and high-BF ballast.
In general, good T8 hibays are probably the best solution for about 80% of hibay applications. Other technologies, listed below, can be best for the remaining 20%.