NASHUA — Twelve years ago, improvements began on the heating, ventilation and air conditioning system at Amherst Street Elementary School, Nashua's oldest school building. Now, with advances in energy-efficient boilers, insulation and other technologies, officials are looking to conserve resources at three more schools.

But, at $3.5 million a building, the payback could be a long time coming, if ever.

“We have a number of schools that were built in the late 1950s, early 1960s, and they were still using their existing HVAC systems,” said Dan Donovan, chief operating officer for the Nashua School District. “This wasn't done just for savings. The boilers were coming to the end of their useful life.”

Donovan said the schools were getting about 50 years out of cast iron boilers before failure issues began to arise. Conversations began over which technology to use. In spite of its high efficiency, geothermal was rejected because of cost. Energy-efficient, gas-powered air displacement systems were the next alternative, containing heat recovery components, full envelope insulation upgrades and dehumidification systems.

An energy-efficient boiler was used at Amherst Street School, in addition to air displacement technology. The air displacement system worked fine, Donovan said, but the energy-efficient boiler didn't work so well and was replaced with a traditional cast iron unit.

“When they did at the Amherst Street boiler, we weren't getting the bang for the buck,” he said. “Now, with the technology that's out there, you can.”

By the time bonds for work on Fairgrounds and Ledge Street elementary schools were approved by the city, energy-efficient units were up to par.

Donovan said that out of the city's 12 elementary schools, five or six were built during the same period in the 1950s. Ledge Street and Fairgrounds have been completed, and work at Charlotte Avenue is under way.

Each school was put out to bid separately, costing $3.5 million apiece.

An HVAC overhaul requires about eight months of work, beginning as soon as the heating systems can be shut down in June. Once students leave for the summer, work starts in the rest of the school. As kids return in the fall, some classes are moved temporarily into gymnasiums.

The “smart” HVAC systems cost more in electricity, Donovan said, due largely to the dehumidification component, which simulates air conditioning. “The hope is that the increase in electricity will be offset by the decrease in heating fuel.”

Though savings are expected, it's too early to say how much. By next year, officials will be able to use energy efficiency rating software to calculate by school and by month how much is being saved.

The systems are centrally controlled by Gary Connors, assistant director of buildings and ground for the school district. “Typically (the new boilers) are at least 35 percent more efficient,” Connors said.

They are also implementing state-of-the-art technology that reduces man hours. With the old systems, Connors said, a worker would have to investigate a problem before a fix would be attempted.

“On the new system, when there's an issue, I get on the computer, look at it, diagnose it, and either do an online repair, or if there's an issue, you can it on the computer and send a tech out to repair it,” Connors said.

The upgrades also include wiring replacements and the installation of motion sensor lighting. But insulation was the cheapest and most effective fix.

“The (Ledge Street, Fairgrounds and Charlotte Avenue) buildings were built in the mid 50s,” Connors said. “They didn't have very tight insulation, especially from the walls to the ceiling. Basically, heat's pouring out of the building. Now with the spray-on styrofoam insulation, it essentially tightens up the building envelope so you don't have heat or air escaping the building.”

Connors said insulating alone saves 15 to 20 percent of the energy used to the schools. It's also cheap. “My guess is it's probably a year or two payback.”

School district officials hope to include the HVAC upgrades of the next four elementary schools — Broad Street, Sunset Heights, Main Dunstable and Birch Hill — in a forthcoming five-year, $34 million plan.

Dan Donovan said he'd like to see one school finished every year.

srios@newstote.com