Fine-Tuning Systems to Meet Energy Goals (and Save Money)

By Justin Iovenitti, Senior Energy Engineer

There are several factors that can contribute to high performance buildings, perhaps none more important than operations. A well-designed system that is poorly configured or controlled will not work as expected, and in some cases can end up costing much more than bargained for. This is especially true for sophisticated systems which can deliver energy efficient operations but also rely on more complex building controls. One such example that New Ecology experienced firsthand was at T.C. Martin Elementary School in Bryantown, Maryland. The newly constructed school for Charles County Public Schools utilizes a large ground source heat exchanger with 250 vertical wells. The heat exchanger is tied to two lead/lag hydronic pumps, each with a 75 horsepower motor. The pumps are configured to circulate water through the ground loop as well as to all the water source equipment in the building.

Hydronic pumps

The connected equipment included water source heat pumps for each classroom, water source VRF for administrative areas, water cooled rooftop units for the gym and cafeteria and water cooled DOAS units for ventilation throughout. The ground loop is expected to serve the full load of the building minus domestic hot water, although there is a propane-fired boiler for backup.

Due to their size and run-time (a pump runs nearly 24 hours a day throughout the year), pump power has a major effect on the overall energy performance of the school. If the pumps are not well controlled, it would be impossible to meet the client’s goal of an energy use intensity (EUI) in the low 30s (the median school in the US has a site EUI of 48.5). As the commissioning (Cx) provider, New Ecology conducted functional performance testing on all major equipment in the school.

Excavation to connect the piping loop with the school

During functional testing, New Ecology worked with the controls contractor to enable a differential pressure (DP) setpoint for the ground source loop as intended by the design engineer. This reduced the annual energy consumption of the pumps from 364,000 kWh to 148,000 kWh, a decrease of 59%. Upon reviewing the balancer’s report some weeks later, New Ecology reconvened the Cx team on-site after the school had opened to rebalance the loop. This activity decreased the DP setpoint from 47 psi to 37 psi. Pump energy was reduced even further from 148,000 kWh annually to 63,000 kWh, a decrease of another 57% percent.

Annually, the reduced DP setpoint alone will save the school $7,600.