Precooling Provides Savings in the Shoulder Season

04/30/2013 |

Optimization opportunities for chiller setpoints

Ron Marinelli

Energy manager Ron Marinelli of the Hackettstown (NJ) Public School District offers some tips on fine-tuning setpoints to save energy in the spring months.

Why do the spring months create an opportunity for saving energy?
In many parts of the country, these months have mild daytime highs and cool nights – a combination that you can maximize. Daytime outside air can be 70 degrees F. but overnight the temperature goes down to 50 degrees F. with a relative humidity of 20%. Then the next day the temperature goes back up to 70 degrees. In this situation I take action to reduce both consumption and demand. Basically, I do not want to heat my facilities if I know that it will soon be 70 degrees again.

How do you adjust your setpoints on those days?
I begin by lowering the outside-air setpoint for my boilers from the wintertime setpoint of 50 degrees F. to 40 degrees with a differential of 5 degrees. This keeps the boilers from firing until the outside air dips to 35 degrees. In order to minimize mechanical cooling, I do not want the buildings to get thermally heavy before they will be affected by the 70-degree daytime temperatures.

How do your adjust your operation schedule for these periods?
I back up two hours from facility startup time. So if my facility opens at 7:30 a.m., then I start precooling at 5:30 a.m. by opening the outside-air damper of the economizer to roughly 35 to 40% from its default of 20%. My chiller will not start operation until at least 30% of my facility is calling for cooling.

By late morning, as the outside-air temp has risen and the second floor begins to get warmer, I start to close down the economizer to its 20% default.

What savings do you see from these adjustments?
On a 10-hour workday, I have been able to cut my chiller’s operation by as many as six hours. In addition to helping with my peak load contribution, I save $30.72 for each hour the chiller does not operate. So in a six-hour span, I have saved my school district $184.32 for the day. If the humidity stays low enough through the week, I can save this amount for five days, creating a weekly savings of $921.60 while maintaining adequate ventilation and comfort.

How do you adjust your schedule at the end of the workday?
Although our school day doesn’t end until 3 p.m., the second floor is mostly unoccupied from 2 to 3 p.m. when students are in other parts of the facility. In the middle of May, outside-air temperature can be as high as 75 degrees, but I can still reduce demand significantly. I raise the chiller’s loop setpoint by 2 degrees at 1 p.m., an hour before the event. At 2 p.m. my chiller goes to its unoccupied setpoint.

However, I let my circulation pumps and energy recovery units continue to run. This approach will work just as well if you have fan coil units without energy recovery units. I have been able to float the temperature for as long as 1.5 hours. By that time, school is over and the students and staff have left for the day.

How do you stay aware of changes in occupancy?
Paying very close attention to actual occupancy is an important factor. I get a monthly calendar of all events from the school administration. In addition, I touch base with the team leaders in each grade level to see if there are any special events. If I know there is a class trip or field day, and that students will only assemble with their coats on in their homerooms before going out to the buses, I take that area off the occupied schedule. If I know that an evening event is coming, I will keep the chiller operating for that wing after the school day so that I don’t need to start it up again and incur the increased demand charge.

Later in the year, during summer school, I try to keep all activities within one wing of the school.

What other tactics do you employ to keep your demand charges low?
I am careful about adjusting to maintenance work. For example, if the chiller or rooftop units need maintenance, I try to schedule that work for first thing in the morning and I take the equipment off schedule that day. By doing this I avoid having the technicians turn off the equipment only to turn it on again when they are finished. Why pay for two demand charges?

Energy management requires monitoring each and every day. The very small items can add up to significant savings!


Ron Marinelli is an energy manager and facilities director responsible for four facilities in the Hackettstown (NJ) Public School District. His book on energy management in schools will be published by Fairmont Press. Ron can be reached at

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