ASHRAE Journal - August 2012 - 41

engineers who I know use infiltration estimates much lower than this, even when converted to a 15 mph (7 m/s) wind; and the infiltration estimates of HVAC designers often exceed the convective/conductive heat loss of the building envelope. Standard 90.1-2010 requires that “The entire building envelope shall be designed and constructed with a continuous air barrier.” The standard goes on to require that all air barrier components must be clearly identified on the construction documents. Hopefully, this requirement will allow designers to reduce infiltration estimates and will save owners both first and operating costs. Proper installation is required to realize these savings. Standard 90.1-2010 includes the first passive solar requirements in Standard 90.1 (Figure 1). It requires that the area of south-facing glazing must exceed both the area of eastfacing glazing and the area of west-facing glazing (not added together; only exceed each individually). South is defined as ±45 degrees of due south. East and west are defined as ±30 degrees of due east or west. There are many exceptions to this requirement, including storefronts, shaded buildings, infill with a nearby building to the south, and alterations with no increase in glazing. This addition to 90.1 has wide-reaching impacts on issues such as campus and city planning, because it encourages orienting buildings to be longer in the east/west direction than the north/south direction. Consider how awkward it would be to comply with this requirement in the freestanding building with the footprint shown in Figure 2. On the other hand, if the building in Figure 2 were rotated 90-degrees, which is what the standard encourages, compliance would be easy and the building energy use would be reduced. Another new requirement was added for a small area of skylights for large rooms with tall ceilings on the top floors of buildings in certain occupancy types and climate zones. The purpose of this change is to prevent large numbers of lights from being turned on or operated at full power in spaces where skylights could provide all or some of the needed illumination under most circumstances.

±30 Degrees

±45 Degrees

Figure 1: Solar orientation requirement (north is up). ply air. This is extremely difficult to comply with in hospitals and similar facilities, and was, in my opinion, unreasonable in the 1999 through 2004 editions of 90.1. Under 90.1-2010, most hospital HVAC systems will qualify for the following bhp credits, typically resulting in a total allowance of about 2.3 bhp per 1,000 cfm (48 kW per 1000 L/s): • Fully ducted return and exhaust systems; • Return or exhaust airflow control devices (in pressurecontrolled rooms); • Filter pressure drop allowance for each bank of filters with a MERV rating of more than 8; and • Energy recovery devices (Figure 3). The fan power allowance for energy recovery devices was changed to be “(2.2 × Energy Recovery Effectiveness) — 0.5 in. w.c. (125 Pa) for each airstream.” This was intended to encourage designers to size energy recovery devices to provide high energy recovery effectiveness and low air pressure drop. The 2007 requirement for energy recovery devices was 50% effectiveness and that energy recovery was required only for systems over 5,000 cfm (2360 L/s) that use more than 70% outside air. In the 2010 edition, a table was added that bases the requirements on flow rate, climate zone, and percentage of outside air. The result is that energy recovery is required for many more commercial HVAC systems. For hospitals in humid climates, nearly all air-handling systems will require energy recovery. Reheat is one of the largest energy uses in hospitals. The same is true for laboratories and other buildings with high ventilation rate requirements. Figure 4 compares the annual energy consumption of the large hospital model used for comparing the 2004 to the 2010 editions of Standard 90.1. In 2010, many of the exceptions that permit reheat were eliminated (300 cfm [142 L/s] per air handler, 0.4 cfm/ft2, [2 L/s·m2]) and any air handler with any pressure controlled zones). The remaining exceptions are: 1. 30% of the zone design peak supply rate, or 50% if the flow rate is reduced to 20% in deadband;
ASHRAE Journal 41

HVAC Requirements (Chapter 6)
Supply air temperature reset controls are required for multiple zone HVAC systems. They shall reset at least 25% between the supply air temperature and the design room temperature based on building loads or outdoor air temperature; or zone humidity. This was one of the largest energy-saving changes in 2010 for health-care facilities, and in humid climates it requires careful consideration of appropriate control logic. The fan power limitations have long been one of the most difficult provisions of 90.1 with which to comply. The fan power requirements of 90.1-2010 and 90.1-2007 are similar, and are viable for health care because they include allowances for items such as filters with high MERV ratings and pressurecontrolled zones. For example, the basic requirement for variable air volume (VAV) systems is that they should not require more than 1.3 bhp per 1,000 cfm (27 kW per 1000 L/s) of supAugust 2012

ASHRAE Journal - August 2012

Table of Contents for the Digital Edition of ASHRAE Journal - August 2012