Lessons at the Millennium (March 2000)
“Lots of growth coupled with the trend toward smaller class size meant we needed more rooms,” said Bob Prost, supervisor of operations and maintenance for District 140. The new school, Millennium Elementary, is designed to hold 800 students in grades K-5. It opened last fall and already serves more than 500 students.
The design of Millennium School combines a classic dual-temp loop with a sophisticated control system. The approach was to install sound, yet not extravagant, hvac equipment while investing in a high-end control system that would foster energy savings down the road.
“It’s economics,” said Prost, referring to the primary factor in the district’s design decisions.
Consulting engineer R.L. Millies & Associates Inc. of Munster, IN, designed the four-wing Millennium Elementary School to suit the district’s budget without sacrificing air conditioning capacity. Millennium is constructed with unit ventilators in the classrooms and multizone air-handling units (AHUs) for the common areas. Construction took place from August 1998 to August 1999. The fast schedule was a challenge, but it enabled District 140 to take advantage of available state money, approximately 70% of the facility’s $8 million budget. Team Mechanical Inc. (Wheeling, IL) installed the hvac system, and Johnson Controls (Milwaukee) provided the building automation system. CMS Inc. (Lockport, IL) was the general contractor for the project, and the architect was Healy, Snyder, Bender and Associates (Naperville, IL).
A Two-Pipe System For SavingsA two-pipe dual temperature system was selected for Millennium School instead of the more expensive, and more conventional four-pipe system. Some might argue that the two-pipe hvac system, in which the same supply and return pipes are used for both hot and chilled water, is a thing of the past. The installation at Millennium School, however, demonstrates that a two-pipe system may be a perfectly suitable option when construction and operating costs are a concern.
“When paired with sophisticated controls, Millennium’s two-pipe system comes close to doing the work of a four-pipe system and with substantially lower costs,” said Wayne Orlowski, Team Mechanical Inc.’s project manager.
With a two-pipe system, Millennium School does sacrifice some flexibility in switching between heating and cooling when unseasonably warm or cold days strike. The upside, however, is cheaper construction. Installation costs alone for a two-pipe system are approximately 30% less than for a four-pipe system of the same size. Plus, the district saves on annual energy costs, since a two-pipe arrangement requires less fuel. To further promote efficiency and cost savings, the system at Millennium School contains no glycol. Glycol, although it prevents pipes from freezing in the winter, reduces the efficiency of a system. Because Millennium’s chiller is distributing pure water rather than a glycol solution, it operates at peak efficiency but does need to be drained every fall. A set of isolation valves closes off the outside chilled water pipes, and the outside piping is heat-traced as well for further freeze protection.
In another dollar-conscious decision, a single 225-ton chiller was chosen over two smaller chillers. The selected Carrier (Syracuse, NY) chiller provided the desired redundancy as it has two separate refrigeration circuits and multiple compressors.
Primary-Secondary SystemMillennium School is equipped with a primary-secondary pumping system. Small pumps circulate water through either the chiller or the boilers on a constant-flow basis. These small pumps inject tempered water into the larger secondary loop that extends to all of the hvac equipment in the building. Large pumps circulate water in the secondary loop, but only the amount required to meet the needs of the areas served. The speed of these large secondary pumps is controlled by variable frequency drives (vfd) that slow the pumps down when their full capacity is not needed to heat or cool the building.
The building automation system monitors the conditions throughout the facility and commands the secondary pumps, through their vfd’s, to deliver only the amount of heated or chilled water necessary to meet the needs of the building.
The primary-secondary system did require that the school spend extra money at the time of installation, but with the expectation that the money will be made up in future energy savings since primary-secondary systems require less energy to operate.
In fact, the system at Millennium School has tertiary pumping as well. Booster pumps were positioned at the coils in each of the six AHUs to maintain constant circulation, thereby protecting against freeze-ups.
Unit Ventilators: Smart Choice for Strict CodeWhile VAV units might seem an obvious way to condition a school, Millennium School is designed with unit ventilators in each classroom. The 24 unit ventilators provide for very tight control of outside air intake. Because of the high fresh air requirement for classrooms as specified in the Chicago Building Code, unit ventilators are something of a tradition for schools in Chicago and in its neighboring suburbs. The Chicago Code has a higher than average fresh air requirement for schools: 1.5 cfm per/ft. Most other buildings require just .8 cfm.
Chosen for dependability and tight control, the unit ventilators ensure the required amount of fresh air is pulled into the classroom, and they provided some cost savings during installation. With unit ventilators, Millennium School saved on the cost of distribution ductwork that would have been needed had the fresh air been delivered from one AHU to all the classrooms via ducts. Further, a system of unit ventilators doesn’t require the construction of a room to house a central air handler, and the structure’s ceilings can be a foot or more shorter since overhead space isn’t needed for the ductwork. That means fewer bricks are needed, an important consideration in an 80,000-sq-ft, single-story building.
Multizone - A Familiar StandardThe administrative and common areas (e.g., the gym, library, offices, and computer rooms) are conditioned with a multizone system supplied by six Carrier AHUs of varying sizes. Considered a classic or even old-fashioned approach, this method of installation locally centralizes all the “wear and tear” areas of the system. Any preventive maintenance or required repair work is conducted in one of the two mechanical rooms. The common areas are not disturbed during repairs as they would be with a VAV system. For better energy efficiency and sanitation, the school’s six AHUs at are over-standard double-wall units. To ensure cooling in the computer room and administrative offices one of the units is equipped with a separate DX coil.
Paying Attention to UsageAlthough the physical equipment at Millennium School is not fancy — solid, but not fancy — the district opted to install what is, for a school, a sophisticated control system. The building staff uses a Johnson Controls Metasys® system headquartered on a computer in the boiler room. Features such as night setback mode, which automatically adjusts to a higher setpoint in summer and to a lower setpoint in winter, are used by the district to temper fuel costs.
The control system accommodates the school’s ever-fluctuating occupancy schedule. Because all building use issues are handled through the school’s O&M department, Prost and his staff have up-to-the-minute knowledge to use in scheduling the school’s heating and cooling. “Regular day,” “alternate day,” and “holiday” schedules, which can modified on site or remotely, prevent energy waste.
A temporary override feature lets Prost adjust the system for a special event. For example, when parent-teacher conferences were held in November, portions of the building were kept well-heated into the evening. During an open house at the end of summer, the entire facility was air conditioned. Millennium School is regularly occupied in the evenings and weekends since the community relies on District 140’s facilities for such events as volleyball league play, park district functions, and evening education programs. One handy feature of the control system for handling such events is the optimal start/stop function option. The building engineer simply indicates what time an event will start and end, such as a Saturday basketball tournament, and the system, which takes into account factors such as the previous day’s outside air temperature, determines what time to turn the hvac equipment on or off for ideal occupant comfort.
Another feature, called equipment cycling protection, monitors the number of starts for each piece of equipment and allows no more than one start-up per piece of equipment, such as a chiller or AHU, per hour. The control system also monitors start-up demand and prevents too many pieces of equipment from starting up at once. In addition to the hvac equipment, the system directs other building features such as turning the parking lot lights on and off. All these features work together to make Millennium School a “smart” school with minimized electricity and gas costs.
The district is also experimenting with some of the control system’s non-hvac features such as the intelligent lighting settings. The hallway lights can be triggered to turn down to one-third or two-thirds their normal brightness during class periods when the children are in the classrooms. The learning center’s lights automatically shut off after school, but they are turned on manually when the custodian is ready to clean the area, often not until 9 p.m. Without the intelligent lighting, the learning center lights would be left on until the custodian arrived.
Prost, whose office is in another district building, likes the flexibility of being able to dial in through his laptop computer’s modem and change any system setting or check the temperature in the main loop. The school’s staff have occasionally called him after hours when they realized they forgot to tell him about a scheduling change. It’s no problem for Prost, who simply dials in from home and makes the change. Prost, who began working with the district six years ago as a maintenance engineer, understands the value of educating his own team of five custodians. He knows that all the snazzy engineering features in the world are worthless if he and his staff don’t utilize them.
“As a maintenance department, we have to get involved. That’s how we save on costs,” said Prost. “All of our custodians are getting up to speed on the control system.”
System Makes the GradeFor Kirby School District 140, a two-pipe system with multizone air-handling units and unit ventilators makes perfect sense. The district saved money on its building and installation costs, and the control system is helping Millennium School’s energy costs stay reasonable.
According to Prost, the only thing he might do differently in the future is to equip the boiler with valve control. As it is, the boiler at Millennium School operates at a minimum temperature of 140˚F and is regulated by its on-off switch. Valve control would allow the boiler to run at a higher temperature such as 180˚F, yet the water in the loop could be kept at 110˚F or 120˚F. All in all though, the district says it is pleased with the design of Millennium Elementary School and intends to eventually have all its schools equipped with computerized hvac controls systems. ES