Sizing Air Conditioners


Figure 2. Hourly sensible cooling load versus outdoor temperature monitored for a house in Phoenix, Arizona during an extraordinarily hot summer. Manual J overestimated the sensible cooling load for this house by at least 50%. Even during this hot summer, an air conditioner sized to two-thirds of Manual J would have been more appropriate.

An air conditioner sized to ACCA Manuals J and S is big enough. Industry specialists who design and sell air conditioners have long used Manual J as a standard method for determining the amount of cooling needed to deliver thermal comfort to single-family residences. The procedure is used to calculate room-by-room loads for duct design purposes and whole-house loads for equipment selection. It was jointly developed by ACCA and the Air-Conditioning and Refrigeration Institute (ARI), and is based on a number of sources, including the ASHRAE Handbook of Fundamentals.

Despite the widespread use of this procedure, many contractors have been reluctant to believe that Manual J can deliver adequate cooling under design conditions. One reason for this reluctance has been the lack of information about how actual cooling loads compare to Manual J estimates. Many who have used Manual J extensively have long suspected that it has an oversizing margin. Until recently, however, no field studies had been performed to verify this anecdotal evidence.

New data show that Manual J indeed overestimates the sensible cooling load in hot, dry climates. It is likely that the same holds true in hot, moist climates. Proctor Engineering Group, the Electric Power Research Institute, Nevada Power, and Arizona Public Service monitored air conditioning systems installed in new homes in Phoenix, Arizona, and Las Vegas, Nevada. By testing the actual cooling capacity required to maintain comfort under severe conditions, these tests have yielded the first measurements that confirm and quantify the overestimation by Manual J.

The studies showed that even during an extraordinarily hot summer, when almost 200 hours exceeded design conditions (design conditions are exceeded only 73 hours in a typical summer), the actual sensible cooling loads of the houses were less than Manual J estimates.

At the most intensively monitored sites in the studies, we recorded air flow, temperature drop, and moisture removed from the conditioned air. The research team calculated the actual capacity delivered by the air conditioner for every air conditioner cycle.

The systems were monitored from July 30 through September 25, 1995. Occupants were free to adjust their thermostat settings to any value, but most kept a constant thermostat setting. Most of the systems monitored were typical installations (including leaky ducts, which increased the cooling load that the equipment needed to deliver).

Figure 2 shows the hourly sensible cooling load and the outdoor temperature in one typical house. The duct system had a 12% return leak and a 6% supply leak. Outdoor temperatures at this house ranged as high as 116 oF (according to ASHRAE Fundamentals, the mean extreme temperature for Phoenix is 112.8 oF). Even though this time period was extraordinarily hot, the sensible load requirements for all but 3 (0.2%) of the 1,316 monitored hours were less than the Manual J estimated cooling load. Manual J overpredicted the design load for this house by almost 50%.

There was no need to oversize the air conditioner beyond the Manual J cooling load because Manual J already overestimated that load. The air conditioner installed in this house had a design sensible capacity 24% larger than Manual J-excess capacity that was not useful. The homeowners paid approximately $330 in additional first costs, and they will pay unnecessary additional operating costs every summer month for the life of the system.