Impact of Oversizing Air-Conditioning Systems

The effects of proper sizing on the overall and disaggregated air-conditioning systems are evaluated in terms of energy consumption, peak demand, equipment runtime, and indoor thermal comfort. This section addresses the impact of inadequate sizing on the energy consumption of AC systems that serve office buildings, including packaged rooftop units as well as central cooling plants. Indeed, the penalties associated to AC equipment oversizing can be significant on the annual energy consumption as well as capital costs for the retrofit HVAC systems. First, the sizing approaches for HVAC systems are outlined. Then, the results of AC inadequate sizing on annual energy use for office buildings are summarized (Chui and Krarti, 2020).

HVAC Systems Design Approach

Oversizing HVAC systems is not well defined in the literature since it remains unclear when a heating and/or a cooling equipment is considered adequately sized and when its capacity exceeds the required level based on design heating and cooling loads. The calculations of design heating and cooling loads follow procedures often outlined in HVAC design guidelines while meeting thermal comfort, air quality, and energy efficiency standards. For US buildings, the calculation procedures outlined in the ASHRAE Handbook of Fundamentals are generally used to determine design thermal heating and cooling loads under specific set of climatic conditions (ASHRAE, 2017). In particular, ASHRAE design climatic conditions are defined using percentages of time these conditions exceed given threshold levels determined based on statistical analysis of historical weather data over several years (ASHRAE, 2017). Specifically, the design temperature and humidity conditions for cooling systems are based on percentiles of 0.4 or 1.0 percent of historical hourly weather data recorded for the building location during a period of 10-30 years. Similarly, the design temperatures for heating systems are based on percentiles of 99.6 or 99 percent. For instance, the hourly outdoor temperatures remain above the 99.6 percent DB temperature level during all the hours based on a 30-year period. Table 8.12 summarizes annual heating and cooling design temperatures for select US cities, including 99 or 99.6 percent heating DB temperatures and 0.4, 1, and 2 percent cooling DB and main coincident wet-bulb (MCWB) temperatures. As indicated in Table 8.12, the design conditions and consequently the heating and cooling loads vary with the threshold level considered. For instance, the design heating loads in Denver,

TABLE 8.12

Annual ASHRAE Heating and Cooling Design Temperatures for Select US Cities

New York, and Seattle increase by about 10 percent when 99.6 percent instead of 99 percent heating DB temperature is considered in the design calculations.

Some professionals believe that HVAC systems are oversized when their capacity exceeds by 25 percent of the design heating and/or cooling loads. In fact, several HVAC engineers consider oversizing by 25 percent as a safe and acceptable practice to account for uncertainties in estimating design thermal loads (Woradechjumroen et al. 2016; Dominguez-Munoz et al., 2010; and Djunaedy et al., 2011). These uncertainties are often associated with unknown specifications and operation conditions, including occupancy levels and building functions. Moreover, oversizing HVAC systems allows designers to avoid any potential liabilities and occupant complaints due to inadequate capacity selection for the heating and cooling equipment. In some cases, oversizing of HVAC systems is specified knowingly as a solution to maintain comfort when the occupancy level varies intermittently such as in auditoriums and religious places of worship (Budaiwi and Abdou, 2013). Moreover, oversizing may occur after retrofitting buildings by keeping the existing HVAC systems or replacing them with equipment having the same capacity (Booten et al., 2014). Another reason for oversizing is the reliance on basic rules of thumb to estimate the capacity required for the HVAC systems as those listed in Table 8.13 used regardless of

TABLE 8.13

Basic Rules of Thumb for Sizing HVAC Systems for Various Building Types

the building location and type (Djunaedy et al., 2011). Indeed, some HVAC designers often use these rules of thumb to avoid the time and effort required to perform the detailed calculations needed to determine more accurately design heating and cooling thermal loads. Even when using detailed simulation tools, engineers may overestimate thermal loads by ignoring or overlooking some important features of buildings such as shading devices and adjacent buildings resulting in oversized air- conditioning equipment (Djunaedy et al., 2011).

Impact of AC Sizing on Energy Use

A recent comprehensive analysis has evaluated the impact of inadequate design of AC systems for US office buildings (Chui and Krarti, 2020). In the analysis, the sizing ratio is applied to the peak cooling load to oversize or undersize the cooling capacity of the HVAC equipment. ASHRAE Standard 90.1 recommends an oversizing factor for cooling to be 1.15 (ASHRAE, 2017). In this study, the baseline sizing ratio is set to be 1.33 to account for any uncertainties in weather data and building construction details as well as to account for duct losses (DOE, 2019). The baseline cooling capacity of the HVAC system sizing ratio value is set to 1.33. It should be noted that the results outlined in this study are valid even when other baseline cooling capacities are considered. Indeed, the sizing ratios are conveniently utilized to normalize the cooling capacities for the AC systems. Table 8.14 summarizes the various sizing ratios considered in this study for each building type and location and their associated cooling capacities for the AC system. When the sizing ratio is above the baseline value, the cooling capacity for the HVAC system is considered to be oversized. To model undersizing of the AC system, the sizing ratio is set below the baseline value (i.e., 1.33) as noted in Table 8.14.

TABLE 8.14

Sizing Ratio and Cooling Capacity Used in the Analysis

Using the sizing ratios and cooling capacity fractions listed in Table 8.14, the impact of sizing the AC system on the energy consumption for both medium and large office buildings are evaluated for some US locations. Figure 8.11 illustrates the variation of the normalized annual AC energy consumption relative to the baseline performance as a function of the cooling capacity fraction. As expected, the energy penalty increases with the oversizing level for all the locations and office building type. Indeed, oversizing leads to the fact that AC systems operate at lower PLR values and thus at lower energy efficiencies for both the DX rooftop unit (medium office) and the chiller (large office). Specifically, when the AC cooling capacity is increased by 25 percent, the annual energy consumption associated with AC equipment increases relative to the baseline design configuration by a percentage ranging from a low of 4 percent experienced in Las Vegas, NV, to a high of 20 percent set in Seattle, WA, for the medium office building as shown in Figure 8.11(a). In particular, 50 percent oversizing of the AC system in Miami result in 10 percent increase of the annual cooling energy consumption, similar to the average percentage found for ACs serving buildings in Florida (Rhodes et al., 2011; Gorter, 2012).

In general, the impact of the AC system oversizing for the medium office building is more pronounced for locations with mild climates such as those of Seattle and Boulder than for hot climates such that of Phoenix and Las Vegas. Similar levels of AC oversizing result in even higher annual energy penalties for the large office building. In particular, a 25 percent AC system oversizing for the large office building leads to an increase in annual energy consumption that ranges from 14 percent for Boulder, Colorado, to 50 percent for Houston, Texas, as indicated in Figure 8.11(b). Due to the use of cooling tower to reject heat for a water-cooled chiller, the penalty

FIGURE 8.11 Normalized HVAC system annual electrical energy consumption as a function of a capacity fraction for (a) medium office building (b) large office building located at various US sites.

is higher for hot and humid locations (i.e., Houston and Miami) rather than for mild and dry locations (i.e., Boulder and Seattle).