ASHRAE/IESNA Standard 90.1
ASHRAE and the Illuminating Society of North America (IESNA) wrote ASHRAE/IESNA Standard 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings (Standard 90.1) as a joint venture. The latest printed edition is 2004, which was used for this text. There is a detailed, well-illustrated, and explanatory companion document, 90.1 User’s Manual ANSI/ASHRAE/IESNA Standard 90.1-2004 Energy Standard for Buildings Except Low-Rise Residential Building2.
The purpose of the Standard is “to provide minimum requirements for energy-efficient design of buildings except low-rise residential buildings.” It is a minimum standard and there are some energy reduction programs such as “Leadership in Energy and Environmental Design, LEED,” that encourage designs to have a lower energy cost than the Standard prescriptive cost. Note that the LEED program gives no acknowledgement unless design energy cost is at least 15% below the Standard 90.1 requirements.
The Standard 90.1 requirements can be met by either complying with all “Prescriptive and Performance Requirements” or by producing a design that has no higher energy cost in a year than a prescribed calculated “Energy Cost Budget.”
Prescriptive and Performance Requirements
The Standard is divided into sections that often fall to different designers. The first section of the Standard is the “Administration and Enforcement” section, to help designers and code officials. It then has six prescriptive sections that define the performance of the components of the building. Finally, it concludes with a calculation method, the “Energy Cost Budget Method” section.
The following is a brief introduction to the sections.
Building Envelope
The objective of the Standard is to ensure that design choices are both energy- efficient and cost-effective. Therefore, for example, the insulation requirements are more demanding in the colder climates.
The Standard divides climates according to temperature and moisture conditions. The temperature divisions range from the continuously hot, with no heating demands, through to the continuous heating with no cooling requirements. The designer chooses the temperature range relating to the building location, and, on a single page finds the thermal transmission requirements for the building fabric: roofs, walls, floors, doors and fenestration (windows). This is the section for the architect!
The Standard requires slightly higher performance for residential buildings, since they are generally in operation 24 hours of every day. In comparison, many non-residential buildings are in full operation for less than half the hours in a week.
One of the major problem areas of modern buildings is the sealing around penetrations in the building envelope. The building envelope includes the entire perimeter of the building: the windows, doors, walls, and the roof. The allow- able leakage around windows and doors is defined. All other parts of the building envelope are covered by the hope-filled request: “The following areas of the building envelope shall be sealed, caulked, gasketed, or weather stripped to minimize leakage.” In order to reduce the likelihood of future problems, it is worth the effort to ensure that the contractor fulfills this as a requirement.
The Standard allows some trade-off between the various sections of the build- ing envelope as long as the required overall envelope performance is maintained.
Heating, Ventilating, and Air conditioning
For single zone buildings of less than 25,000 ft2 and only one or two floors, there is a simplified approach, due to the limited number of choices that designers can make for equipment. As long as the building is a single zone, with one unit, the code requires that the unit will comply with a few straight- forward energy saving requirements.
For larger buildings there are numerous requirements for minimum equip- ment efficiencies in terms of Energy Efficiency Ratio, “EER,” Coefficient of Performance, “COP,” and Integrated Part-Load Value “IPLV.” The following section explains the meaning of each of these terms.
EER Energy Efficiency Ratio is the ratio of net cooling capacity in Btu/hour to electrical input in Watts. A small window air-conditioner, for example, is required to have a minimum EER of 9.7. This is the same as saying that it will provide 9.7 Btu/hour of cooling for an input of 1 watt, under specific test conditions. A watt is 3.412 Btu/hour so the EER of 9.7 requires 9.7 Btu/hour cooling for 3.412 Btu/hour energy input. This works out to about 2.84 times as much cooling energy as compressor energy.
The requirements for water chillers are given in IPLV and COP.
IPLV, Integrated Part-Load Value is a weighted average value of EER based on full and part load performance and is used instead of EER on larger electrically driven air-conditioners.
COP, Coefficient Of Performance, is the heat removal to energy input in consistent units. For air-cooled chillers, the minimum requirement is COP of 2.8. However, a water cooled centrifugal chiller over 300 tons, has a required minimum COP of 6.0, twice the cooling capacity per watt of the air-cooled machine. This is an area where judicious choice of equipment can make large differences in energy consumption.
In Chapter 10.1, we discussed the statement that “big plant is more efficient.” In the case of chillers, this is very true. Unfortunately, COP efficiency is not the only relevant consideration. Other energy inputs for the central plant include the energy for pumping the chilled water to end use and the condenser water to the cooling tower. In addition, the distribution-pipe heat gains must be deducted from the cooling capacity.
Having defined minimum equipment performance, the Standard then goes on to establish rules about controls and installation including insulation, sys- tem balancing and commissioning, to ensure minimum equipment utilization efficiency. We have already discussed some of the controls requirements in the previous chapter.
Service Water Heating
The section on service water heating covers minimum equipment performance and maximum standby loss. Also detailed are pipe insulation and recirculation requirements.
Lighting
On average, in the USA, buildings use about 35% of their total energy for lighting. This provides a big opportunity for savings. The Standard allows a specific number of Watts per square foot, W/ft2, however, the designer is given a certain amount of leeway in the calculations: The allowed W/ft2 can be calculated on the basis of type of building or on a room-by-room basis. The Standard allows trading between areas and between lighting and HVAC, as long as the net energy cost through the year is not increased above the prescribed allowance.
The Standard recognizes variation in use of the same type of space in different types of buildings. So, for example, corridors generally have an allowance of 0.5 W/ft2 but this is raised to 1.0 W/ft2 for hospitals.
Energy-Cost Budget Method
The energy-cost budget is a way to allow designers to have the flexibility to design the building according to their needs, as long as it does not cost more in energy than the Standard permits. To use the Energy-Cost Budget Method, the designer is instructed to calculate the energy-cost budget for standard plant equipment, then to compare that to the cost of the energy required by the equipment chosen.
The Energy-Cost Budget, ECB, requires the use of hour-by-hour building energy analysis software. No particular software is specified, but software performance is mandated. Local utility rates are used in the simulation. The building has to be analyzed, using the prescribed building envelope and equipment efficiencies, to obtain the ‘energy-cost budget’ and again with actual envelope and equipment. Compliance is achieved when the ‘design energy-cost’ does not exceed the ‘energy-cost budget’.
If you become involved in using Standard 90.1, remember that the User Manual provides a clear, easy-to-follow explanation of how to use and apply the Standard.