School officials at the Okeechobee (FL) School District became concerned about poor air quality and humidity in the classrooms after high humidity levels started causing structural damage, threatening costly computer equipment, and making classroom work practically unbearable for students and teachers.

“I couldn’t keep up with the heat loads in classrooms,” says Dale Barrett, director of operations. “The humidity levels were intolerable and the ceiling tiles were sagging to the point of falling down. We also were concerned about possible unwanted growth of mildew and mold spores.”

High humidity levels can have a profound effect on building occupants, woodwork, machinery, and other property. It also can lead to “Sick Building Syndrome,” which often increases sick time for building occupants and lowers their productivity.

More than 1,600 students and teachers occupy the high school. Improving comfort levels for them was essential, as was protecting delicate computer equipment in the computer science and business classrooms in Building 6.

“I had to make a move because the humidity was ruining some very expensive computer equipment,” said Barrett. Moreover, the existing, outdated air conditioning equipment was so noisy, teachers had to choose between not being heard, or turning the units off to be heard.

“I’d cut off the air conditioning so students could hear me, and they’d begin fidgeting because they would get so warm — they would get sleepy,” said business teacher Carey Pung. “The humidity would get so high, paper would stick together in the printer.”

Working with dealer

They decided on Carrier 50HJ rooftop units, because they “had a factory-installed dehumidification package in the capacities we needed,” Barrett said.

The package consists of a subcooling coil located on the leaving-air side of the evaporator coil. This cools the hot liquid refrigerant leaving the condenser coil. The location of this coil in the indoor airstream is said to enhance the latent capacity (the ability to remove moisture from the air) of the air conditioning unit by as much as 40%.

In a recent comparison study conducted in three Florida fast food restaurants, the system maintained moisture levels 15.6% lower than standard rooftop air conditioning equipment, while reducing utility bills by 7%.

For the 33-ton installation, Blair’s crew installed four 6-ton units, one 5-ton unit, and one 4-ton unit.

The package also eliminates the need to oversize equipment and add reheat devices in order to dehumidify buildings. For example, instead of incurring additional costs to oversize equipment to 7.5 tons from 5 tons, contractors can purchase a 5-ton unit with the MoistureMiser dehumidification package for less than half of the extra cost, eliminating the need for reheat devices.

Barrett said he would be more inclined to replace units than add reheat and dehumidification packages.

Financing, payback

Barrett reported that after two full years of service, the units were still providing excellent results. “The units just dried the building out,” said Barrett.

The lower humidity levels also have protected the computer equipment. “We’ve had no maintenance worry whatsoever,” said Barrett.

Students and teachers appreciate the low sound levels. “You can’t even hear these units running,” said Barrett. “Now teachers don’t have to choose between comfort and being heard.”

The air conditioning system also contains programmable direct digital control thermostats for more precise temperature control. The teachers can set their own classroom temperatures.

Importantly, the 13-SEER units are paying for themselves through energy savings. “The original units were rated at 9 SEER when they were installed more than a decade ago and were much less efficient than that when they were taken out of service,” says Barrett.

“The new units can maintain temperature and humidity at comfortable levels even though they run less than the old equipment,” Barrett said. He estimates that the new units will pay for themselves in a relatively short period of time.

Meanwhile, the school system has applied the FP&L rebate to other improvements in county schools.

“We’ve upgraded other air conditioning units, provided exit light retrofits, and installed 25 additional [rooftop] units at Central Elementary, which in turn captured their own FP&L rebates,” he said. “Those dollars go right back into upgrades throughout the county school system, benefiting students and faculty.”