ASHRAE Journal - June 2011 - 79

For whom is right-sizing considered better? It is surely better for the builder/contractor because it cuts his cost of construction. It is better for the utility because the peak load is capped by right-sized capacity. But, is it better for the occupant who is in the space?
notion is that it would be better if the equipment were smaller (i.e., “right”), but it turns out there are a lot of myths about right-sizing that are not actually true. For whom is right-sizing considered better? It is surely better for the builder/contractor because it cuts his cost of construction. It is better for the utility because the peak load is capped by right-sized capacity. But, is it better for the occupant who is in the space? Often not. The so-called “oversized” units are the only ones that can provide what users want. Simply put, right-sizing means the load will not always be met. For example, temperatures are sometimes outside of the desired band. Consider a typical example of right-sizing a home. The size of the equipment will be determined, ostensibly, by the 99th percentile of the steady-state load. That does not sound so bad. However, it might mean for 88 hours4 per year the occupants will be uncomfortable unless they reduce clothing levels,5 turn on ceiling fans, or take some other action. Most residential load programs for sizing consider steadystate loads. But, few occupants operate their home by leaving the thermostat at a single value all the time. Set-back, setup, and off periods are quite common operating strategies. They save energy by reducing operation of the system, but extra capacity is required to recover from such periods. A “right-sized” system is not sized to provide this recovery in a reasonable time and will lead to more hours of occupant discomfort.6 Recovery is not the only reason the system may need extra capacity. Systems are often sized for typical occupancy and activities, but there are easily foreseeable situations where that is not the case. Special events happen in all buildings and it might be unacceptable to be sweating at Grandpa’s birthday party in July. It is said that the part-load efficiency and moisture control benefits of right-sizing are so significant that the penalties to occupant comfort and flexibility are warranted. This myth is often repeated, but bears further examination. It is generally true that most single-speed equipment work best at full load and loses performance at part load. This fact is the basis for saying that right-sizing improves efficiency. While true, the largest savings comes not from part-load efficiency improvements, but from insufficient capacity to meet load, that is, from occupant discomfort. A variety of ways exist to improve the part-load performance without limiting capacity. Multiple smaller units can be staged. In a residential environment, this can be done by using two smaller systems rather than one large one. Another approach is to use two-stage or variable capacity units, many of which allow systems to run continuously for a larger fraction of the year at lower loads while retaining the higher capacity required to meet requirements for hot days, recovery from setback and occupant directed loads. Intentional oversizing of such system can even improve the efficiency of the overall system. Another myth is that systems running at part load take significantly less moisture out of the air. All things being equal, more capacity means more moisture removal, just over a shorter time. But not all things are equal. Cooling systems dehumidify less effectively during the first minute or two of each cycle, as the coil goes from being warm to being cold. Since an oversized system cycles on for shorter periods, latent performance degradation at start up will have a greater impact on the oversized system. It is also true that a larger system can experience more reevaporation of the condensate, depending on the design, operations and maintenance of the system. After each cycle, moisture remains on the coil and in the drain pan. Oversized systems have bigger coils and drain pans with more moisture available for reevaporation between cycles. This is made worse if the drain pain does not drain well or the air-handling unit (AHU) is in an attic or other hot environment. If there is any flow of air through the duct system from thermosiphon effects, then this moisture is brought back into the home at the end of a cycle. The impact of this evaporation of end-of-cycle moisture also depends a lot on how hot the system gets at the end of a cycle. Well-insulated ducts in a crawlspace tend to stay cool and have little thermosiphon effect as the cold air in the ducts stays in the ducts. However, ducts in a hot attic present a worst case for setting up thermosiphon flows and allowing the air in the ducts, the coil and the drain pan to get hot. This increases the ability of the air to hold moisture (and, therefore, transport it into the house) and increases evaporation rates into the air. These issues are minimized with proper drain pan installation, good duct insulation and good duct and coil/pan placement. The biggest problem, however, is fan overrun. If the AHU shuts off when the compressor shuts off, there

4. These are not 88 continuous hours. They may be spread over many days in the peak season or, all the days in the peak season. 5. Woo hoo! 6. The amazingly backward-sounding recommendation to overcome this is to run the system all the time—even during times when no one is home—so that it can meet comfort better during occupied periods.

June 2011

ASHRAE Journal

ASHRAE Journal - June 2011

Table of Contents for the Digital Edition of ASHRAE Journal - June 2011