The lighting for warehouses, manufacturing facilities, industrial work spaces, big box retail and gymnasiums often consists of high bay fixtures equipped with an HID light source — usually metal halide lamps. The higher wattage metal halide bulbs used in the high bay ceiling fixtures provide the lumen output and general lighting distribution required for these applications.
Metal halide, which always requires connection to a ballast, offers intense, bright white illumination with relatively long lamp life — up to 20,000 hours. Compared to mercury vapor or high pressure sodium alternatives, halide compounds in the quartz tube improve color rendition which gives metal halide lamps the higher quality illumination often desired in high bay applications such as big box retail.
While metal halide has proved to be a desirable light source for high bay fixtures, the emergence of high lumen output LED replacement lamps for HID gives managers of facilities utilizing high bay and school gymnasium maintenance staff an opportunity to evaluate the benefits of a change.
Several manufacturers now offer LED replacement options for high wattage metal halide lamps. They are generally available in two versions. Some LED replacements utilize the existing metal halide ballast, so the retrofit is simply a matter of removing the metal halide bulbs and replacing them with LED in the same socket. The other option, ballast bypass, requires rewiring the fixture to take the ballast out of the circuit and supplying line voltage directly to the sockets. While the advantages of these LED retrofits for metal halide may not be as dramatic as those for LED replacements for incandescent and halogen bulbs, they are significant enough to recommend a close look.
LED Offers Long Life, Lower Energy Costs, Instant On and Control Options
The 35,000 to 50,000 hour life of LED bulbs, such a dramatic difference when compared with 1000 hour incandescent or 2000 hour halogen, is less dramatic with metal halide. Typically, the type of metal halide light bulb used in high bay applications has a rated life of around 20,000 hours. While LED replacements available today double that expected life, the benefit must be weighed against the additional initial cost of LED high wattage bulbs compared to metal halide.
In addition, a key factor in determining if the added LED bulb life makes a retrofit cost effective is the cost of labor to replace the lamps after the initial installation. Metal halide bulb replacement will require twice the labor compared to LED. Does this savings exceed the bulb cost differential?
Of course there are other factors than lamp life to consider. Energy savings could be significant. A 400 watt metal halide bulb can be replaced with a 200 watt LED. A 100 watt metal halide can be replaced with a 50 watt LED. Implementing an LED replacement program should result in a 50% reduction in annual energy use (kWh) costs. It may also lower electric demand (kW) charges. The impact of this savings depends on the number of lamps being replaced, the time of operation and the electric rates.
LED technology has other potential benefits than long life and lower energy use. It also overcomes specific drawbacks of metal halide.
At initial start up, metal halide requires a 2 to 5 minute warm up time for cold lamps before full light output is achieved. Also, if power is interrupted, even momentarily, traditional probe start metal halide systems require a 5 to 10 minute cool down before the lamps can be restarted and then an additional warm up period of 10 to 15 minutes before full brightness is achieved. For pulse start systems the hot restart time is reduced to 3 to 4 minutes. Nevertheless, any lighting down time puts a stop to normal activities. LED is an instant ON technology — no matter what the temperature in the space, when the switch is flipped, the lights come on at full light output.
LED is also an easier light source than metal halide to modulate with lighting control devices. Because it is instant ON, in some applications, occupancy sensors can be used to limit lighting ON time in a space only for times when the space is actually being used.
Finally, depending on the type of metal halide bulbs being replaced with LED, the quality of the illumination — better color rendering — can be achieved.
A Few Cautions When Considering a Change to LED
If the existing high bay fixtures utilize a lens, the LED replacement lamp will be fully enclosed. While the lens may help spread the light across the space, it may also cause less effective heat dissipation. LED bulbs have a better chance of reaching their life expectancy when they are operated in well-ventilated, open fixtures. Removing the lens of the high bay fixture may solve the heat build-up issue, but it may also result in complaints of glare or uncomfortable brightness from the workers or occupants in the space. This issue may require experimentation to find an acceptable solution.
With or without the fixture lens in place, the light distribution pattern may change when LED replaces metal halide. Building managers can determine the extent of this change by installing and monitoring one or two LED replacements. In warehouses with high storage racks, anticipating both the difference in LED lumen level and light distribution at the top of a stack compared to the bottom will help in decision-making process.
One safety recommendation is worth noting. For ballast bypass retrofits, where the wiring is disconnected from the ballast and directly connected to the bulb socket, a fast acting fuse should be installed to prevent explosions in case a conventional metal halide is later installed without reconnecting the ballast.
Ultimately the decision to transition from metal halide equipped high bay fixtures to LED requires careful consideration of these and potentially other issues. Each application will have unique requirements that may favor or discourage a change. The best advice may be to recommend a test installation where factors such as initial installation costs, light distribution changes and occupant responses can be analyzed and factored into the known savings in labor for future bulb replacement and lower energy costs.
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