New Hampshire School: Simply Radiant Design
Yet in a small town in New Hampshire, the commitment for a sustainable and efficient school, aided by state incentive funding and careful design decisions, led to the greenest school in the state, according to Ed Murdough of the NH Department of Education.
The Idlehurst Elementary School in Somersworth, NH, provides a green, efficient environment for 580 pre-K-5 education students. The 82,000-sq-ft school includes 24 classrooms, two speech and language rooms, a media center with computer lab, gymnasium, music and art rooms, and a cafetorium with a performance stage. Parts of the $19.9-million school can be used as a community-wide resource, with the gymnasium, cafetorium, library, and dedicated community room in a separate zone. Also, in the event of a disaster, the facility is designated a community shelter.
Among the measures engineered into the school are radiant floor heating with displacement ventilation, demand ventilation and CO2control, sun shades and light shelves, daylighting and continuous dimming control, VFDs, variable-volume kitchen exhaust, and gas-fired condensing boilers.
Preliminary projections are that these energy-saving measures will result in 1,454,000 kBtu/yr total building energy reduction and a nearly 30% economic cost improvement over the code minimum that meets the ASHRAE 90.1-2007 building energy standard.
HEATING/COOLING FOR LESS
HVAC represents one of the largest energy consumers in buildings, as expected, and is where we placed a lot of emphasis to achieve energy reductions. The combination of radiant floor slabs, displacement ventilation, demand ventilation, and low-temperature water heating systems was a key way to reduce heating costs. The use of daylighting control, sunshades, and light shelves helps as well to save on electricity while keeping students comfortable.
At Idlehurst, radiant floor heating is incorporated into all the classrooms. The radiant tubing is buried in the concrete floor, which is covered with vinyl composition floor tile. Once the heating season has started, the system is available to operate 24/7 during the school year, with slab temperature setpoints of +75°F in south-facing classrooms and +80° in classrooms with north-facing exposures.
Resetting of room temps with radiant floors is not utilized here, due to the sizable thermal lag that is present. Heating hot water from the boiler room is piped in primary pumping loops throughout the building, with smaller secondary pumped loops feeding radiant floor manifolds. The direct digital control of the secondary radiant floor pumps (in conjunction with automatic control valves) is activated to maintain the floor slab and room setpoints when the school’s heating season has commenced.
System sensors have the intelligence to know when the ambient temperature in a classroom reaches the set temperature and can reset the inflow water temperature from 110° down to 90°, lowering energy consumption.
An ideal companion system for radiant floor heating is displacement ventilation, a highly effective and efficient way to introduce ventilation air into spaces. In a typical classroom, ventilation air is delivered from two perforated faced displacement diffusers located in the two inside corners of each classroom at a maximum velocity of 40 fpm. The corner diffusers are angled towards the center of the classroom. As the ventilation air moves across the floor and reaches occupants, it is heated and rises as a plume up through the occupants’ breathing zone then up to a return grille located in the ceiling near the outside wall. The ventilation air enters the room two to four degrees cooler than the room setpoint temperature.
Delivering ventilation air in this manner provides the ventilation effectiveness rate of 1.2, which is significantly superior to a rate of 1.0 that can be expected from using conventional overhead air delivery systems. Because displacement ventilation allows for a lower volume of air introduced into a space, the source air handler size at Idlehurst could typically be reduced by 20%.
Demand ventilation (CO2) sensors also played an important role in achieving optimal energy efficiency. This provides the proper levels of outside ventilation air into a space to a level that is directly proportionate level of occupancy. Depending on whether a classroom is occupied at low levels or fully occupied at any particular time, CO2sensors can reduce or increase the amount of outside air delivered into the space.
The ventilation requirements and rates are as required by ASHRAE Standard 62, “Ventilation for Acceptable Indoor Air Quality.” Occupancy sensors are also provided that will, after a pre-set amount of vacancy time, actually close the supply air and return air off to the space completely. This supply and return air close-off increases duct static pressure, which is seen by duct differential pressure sensors. This, in turn, slows down the AHU through the respective fans’ VSDs, thus saving valuable system fan energy.
We added natural gas-fired condensing boilers to the design because they can deliver efficiencies up to 95% when operating with lower return water temperatures. The condensing boilers effectively use the full heating value of fuel when the return water inlet temperature is low enough to allow substantial heat recovery of the latent heat moisture in the exhaust. The lower water temperatures returned back to the boilers equate to higher boiler/system efficiencies. Since heating system supply water often only needs to be heated to 110° primarily in the fall and spring, these boilers were an ideal solution for Idlehurst.
With the combination of these energy-saving HVAC tactics, we estimate that their additional cost over a conventional classroom heating system is about $3.51 per sq ft, or about $287,500.
A key to the optimal operation of any complex multi-tactic HVAC system such as at Idlehurst is to find the right operating parameters and make initial adjustments. Commissioning the school at the start of the school year was critical in finding the right temperature setpoints in conjunction with supply air and water temperatures to ensure the comfort of the occupants. Often, rooms adjust in different ways, as we experienced in south- and north-facing rooms. The full commissioning process also provides the owner with the assurance that the installation has been successfully completed and it meets the full intent of the design.
EFFICIENT MOVEMENT OF AIR/WATER
Two other engineering strategies also contributed to Idlehurst’s energy-efficient design. VFDs on all AHU supply and return fans, along with all hydronic system pumps, provide matched operating levels with the demand levels. The implementation of these VFDs throughout the school will produce significant savings potential for the life of Idlehurst. Paybacks often run to the two- to three-year range.
While VFDs have become quite common, the second strategy — variable volume kitchen exhaust systems — is not as widespread, but is quickly beginning to gain favor. An engineered kitchen hood exhaust air control system was implemented, which varies the amount of exhaust airflow based on exhaust air temperature and the amount of smoke generated from the cooking surfaces. This system provides the proper amount of exhaust (cfm) to adequately capture fumes and smoke based on the intensity of the cooking process, thus saving energy associated with the exhaust fan speed and only having to treat quantities of outside makeup air that essentially match the exhaust airflow. The incremental system cost is approximately $8,000 over a straight on-and-off conventional kitchen exhaust system and has a typical payback period of three to four years.
INTERPLAY OF NATURAL/ARTIFICIAL LIGHT
Attention to lighting also played an important role in creating a highly energy-efficient school. Sunshades were designed on all south-facing windows. They are effective in limiting direct sunlight into spaces, reducing glare, and cutting cooling costs, particularly in air conditioned spaces. In non air conditioned spaces with sunshades, occupants remain more comfortable as temperatures rise more slowly.
Daylighting control strategies include the use of internal light shelves and automatic control of the electrical lighting in spaces. Sunlight entering through windows and reflected off a light shelf allows powered lighting levels to be automatically turned off, while maintaining proper light levels in the space. The combination of light shelves, sunshades, and continuous-dimming controls in classrooms provides significant electrical savings and a payback period of approximately five years.
While the school board was strongly behind these efforts, several financial factors helped its argument. The payback on most of these measures ranges between two to five years, which will result in significant savings over the life of the project. But more important to this effort were incentives and rewards that they would receive. The school is expecting to receive $400,000 from the Collaborative for High Performance Schools Program and has already received $75,000 from Public Service of New Hampshire.
We have estimated that on an annual basis, Idlehurst will save the following:
• 277,861 kWh, resulting in approximately $29,000 in savings;
• 5,000 therms (natural gas) resulting in approximately $12,750 in savings;
• 169,451 lbs CO2emissions (this number includes the CO2emissions associated with the direct gas savings in the buildings actual energy use and at the electrical utility power generation source).
Extrapolate these figures over a 20-yr or 30-yr cycle and the savings become significant.
In these economic times, the argument for spending additional money to create a highly efficient school can be challenging. Providing careful analyses of first-costs vs. paybacks and helping find alternative funding sources will make it easier for educators and municipal officials to choose efficient and green solutions.ES