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Sizing Air Conditioners

Home Energy Magazine Online September/October 1996

Sizing Air Conditioners: If Bigger Is Not Better, What Is?

by John Proctor and Peggy Albright

John Proctor is the managing partner of Proctor Engineering Group in San Rafael, California. Peggy Albright is an independent writing consultant for researchers in the electric utility industry.

In this follow-up to the original "Bigger Is Not Better" article, Proctor Engineering Group offers ways to improve comfort, reduce noise, and increase efficiency when installing home air conditioners.

Since the publication of "Bigger Is Not Better-Sizing Air Conditioners Properly" (HE May/June '95, p. 19), homeowners, builders, and contractors have questioned us about sizing and performance issues raised in that article. The purpose of this sequel is to answer frequently asked questions, explain the characteristics of a good air conditioning system, and describe how to get the most comfort and efficiency from a residential system.

Equipment sizing is not the only key to comfort-small details in register placement and design can have a great impact on air flow and overall comfort. For example, the louvers of this air-conditioning register are designed to spread air in four directions.

Bigger Still Is Not Better

Let's review why bigger is not better. Since optimum efficiency is achieved when systems run continuously, it is important that an air conditioner be sized to achieve the longest run times possible. Standard sizing calculations are based on a design temperature that is exceeded only 73 hours in a normal cooling season. An air conditioner sized to run continuously at design conditions will cost less initially and will have a lower operating cost.

Air Conditioning Contractors of America (ACCA) has published design manuals (Manuals J, S, D, and T) that produce far better results than the rough-and-tumble rules of thumb used by the vast majority of HVAC contractors. A contractor will achieve (and the customer will enjoy) a much higher-quality job if these manuals are followed in the design and installation of central air conditioning systems. A recent investigation of new houses has shown that an air conditioner delivering a capacity equal to Manual J would be adequate even during extraordinarily hot summers (see "How Big Is Big Enough?").

How Big Is Big Enough?

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.

We're not sure what manual the installer of this creative air conditioning system was working with, but we don't recommend it!

The main problems typically found in the field are improperly sized air conditioners, improperly designed duct systems, poor grille selection, and poor installation of all three components. These problems are most easily avoided in new construction, but retrofit contractors can and should follow the recommendations in this article whenever feasible.

The Disadvantages of Oversizing

In recent years Proctor Engineering Group has investigated air conditioner comfort, efficiency, and economy in a range of locations. One interview, with a homeowner in Palm Springs, California, brought out several issues that we have found repeatedly. This house was a moderate sized older home with beautiful overhangs shading the east and west windows. I was invited to sit at the kitchen table to talk with the owner, a man in his early 60s. He complained that his cooling bills were high and he was never comfortable during the cooling season (which extends over most of the year in Palm Springs).

As we talked the air conditioner came on and a strong stream of cold air moved by my shoulder. The owner went over to the supply register and closed the damper. He came back to the table explaining that with the register open he was blasted with cold air that made him uncomfortable. The noise coming from the closed register made it hard to have a conversation at the table. He stated that the system was always noisy. When I suggested that we move to another room for our conversation, he said, "That wouldn't make any difference, there are only hot places and cold places; no place is right in this house. We are looking for a new house."

The situation we found in this house exists, in various degrees, in millions of homes across the United States. The heating and cooling distribution system was not matched to the cooling loads of the individual rooms or to the needs of the occupants. On top of that, the air conditioner was not matched to the distribution system. Discomfort and expense are the inevitable results.

Table 1. Summer Comfort Zone.
Relative Humidity	Maximum Comfortable Temperature	Minimum Comfortable Temperature
60%	78.5^oF	72.5^oF
50%	79^oF	73^oF
40%	79.5^oF	73.5^oF
30%	80^oF	74^oF

Bigger Is Not Better- Comfort Is Better

In 1923, in an effort to pinpoint the indoor environment conditions that make people comfortable, F.C. Houghten and C.P. Yaglou conducted studies to determine how people feel under varying temperature and humidity conditions. The result of this research was the identification of a "comfort zone" based on temperature and humidity. The modern version of this comfort zone is shown in Table 1. Tolerance to heat is affected by the amount of humidity in the air-at higher temperatures, the humidity level must be held lower to ensure comfort.

The comfort zone was found to be acceptable to 90% of test subjects drawn from a range of age groups and genders, with work and life-styles involving varying levels of activity and clothing. An air conditioning system that establishes and maintains indoor conditions within this zone will provide thermal comfort. It will produce a neutral sensation-occupants will feel neither too hot nor too cold.

An air conditioner can easily bring the temperature inside a house into the comfort range. In fact, bigger air conditioners virtually ensure that the temperature at the thermostat can be as cold as we set it. Unfortunately, cold alone is not comfortable. In fact, it is distinctly uncomfortable. To maintain a general level of comfort, the moisture level must also be controlled. This is best achieved by smaller, not larger, air conditioners.

Smaller Units Remove More Moisture

Figure 1. Smaller air conditioners remove more moisture from a house. In this example, a 5-ton unit running for five minutes removes 1.4 pounds of water. A 2.5-ton air conditioner in the same house, running for ten minutes, removes 1.7 pounds of moisture-an increase in moisture removal of 21%.

An air conditioner's ability to remove moisture increases when the equipment runs for longer periods of time. At the beginning of every cycle in hot moist climates, the air conditioner actually puts moisture into the house as water evaporates off the inside coil. Once it's been running a while, it begins to remove moisture. Since a smaller air conditioner runs longer to keep the house at the temperature setpoint, it removes more moisture than a larger unit would be able to (see Figure 1).

The amount of moisture removed is a function not only of how long the air conditioner runs, but also of its Sensible Heat Ratio (SHR)-the percentage of the total capacity delivered as lower house temperature. A low SHR will result in more moisture removal. For hot, wet climates, air flow across the coil should be reduced slightly to decrease the SHR, and the air conditioner condensing unit and indoor coil combination should be chosen to have a low SHR. Typical matched units from major manufacturers have SHRs in the 68%-80% range when it is 95^oF outside and 75^oF with 50% relative humidity inside. Note that if you don't use a matching indoor coil and outdoor unit from the same manufacturer, you shouldn't expect to get their published SHR.

A place in the sun may be a good thing, but too much sun can make a house's cooling load soar. The lack of overhangs and sun protection on this new house will concentrate heat gains in certain rooms, making it difficult to properly balance cooling distribution.

Even Temperatures Are Necessary for Comfort

Our homeowner in Palm Springs didn't have a problem with moisture, but he did have a problem with uneven temperatures. When the air conditioner was on, portions of his home and even different parts of individual rooms were at significantly different temperatures. Stagnation of air in one part of a room (for example, in one corner or at head level) makes people uncomfortable. Proper mixing of the air and proper distribution to individual rooms prevents this problem.

Uneven temperatures have become more common due to the modern practice of severely reducing overhangs above the windows. Without overhangs, rooms with west-facing windows will overheat in the afternoon, since their need for cooling can easily double.

An inefficient method of attempting to get proper distribution and mixing of the air is to use a large air handler fan to circulate air all or most of the time. This is sometimes effective in mixing the air, but at a high price. There is an old rule of thumb that between four and six house volumes of air must pass through the air handler in an hour. At six air changes per hour (ACH), a 1,400 ft² home would need a continuously running fan that delivers 1,120 cubic feet per minute (CFM)-equivalent to almost 3 tons-regardless of the cooling load of the house. The common practice is to install an air conditioner (inside and outside unit) with the capacity to meet those flow requirements. There are many disadvantages to this practice. They include:

The need for a larger and more expensive duct system to handle the increased flow.
Increased duct conduction due to constant circulation and the larger surface area of the duct system.
Reduced moisture removal due to short compressor cycles, caused by the oversized outdoor unit.
Reduced moisture removal due to the constant air circulation, because water re-evaporates from the coil while the compressor is off, and is distributed back around the house.
Increased cooling load due to duct leakage and fan energy delivered as heat.

A better solution is first to design and install a delivery system that properly distributes the cooling to each room, then to select and place supply grilles that "throw" the delivered air into the right places in the room to promote mixing. ACCA's Manual D: Duct Design and Manual T: Terminal Design can lead the installing contractor through the process of selecting the proper-size duct and type of register, based on the location of the register, the size of the room, the restriction of the duct run, and the dimensions and heat gain of the room. Unfortunately, only the best contractors and builders ever pay attention to these critical details.

The problems of stagnation and overheating can be reduced by proper implementation of ACCA procedures. These problems can be further reduced by ensuring that the assumptions built into the manuals are not violated. For example, it is assumed that there is no duct leakage in the system. Any longtime reader of Home Energy will immediately note that this assumption is violated in nearly all homes (including new ones). Proper installation of the duct system and leakage testing are essential to obtain comfort.

Another assumption is that the conduction losses are the same percentage of the delivered cooling regardless of the length of the duct run. This would be an insignificant assumption in a heavily insulated system (R-4 is not heavily insulated). Long duct runs through the attic lose over 15% of their cooling capacity before the conditioned air reaches its destination. Long duct runs need additional insulation to deliver the proper amount of cooling to the distant rooms.

Recommendations

Wherever possible, reduce the cooling load of the house. Overhangs above east and west windows are particularly effective in reducing cooling load.

Perform Manual J for all installations, and select equipment using Manual S.

Ensure that the system installed never exceeds the capacity of the equipment suggested by Manual S.

Size duct systems based on Manual D. If in doubt, size upward.

Determine the grille location and characteristics using Manual T.

Confirm proper evacuation of the line set and indoor coil with a micron gauge.

Confirm proper charge using the manufacturer's suggested method.

Confirm proper air flow by test. The flow can be determined from the coil pressure drop when pressure/flow data are available from the coil manufacturer. Or it can be determined with a duct test rig or flow hood.

Increase the duct insulation to at least R-8 (especially on long runs in the attic).

Confirm that the duct leakage is less than 3% of coil air flow for a new system and less than 6% of coil air flow for an existing system.

Water and air are being evacuated from the lines and indoor coils of this air conditioner. This process, which is often overlooked or avoided by installers, can assure that units are properly charged, and also provides an opportunity to check for leaks.

Drafts Destroy Comfort

An oversized air conditioner is a major contributor to drafts, because it is almost always married to a duct system that is too small. The ducts are unable to deliver the amount of air necessary for proper air conditioner performance (more on this later). The result is a poor compromise-air flow that is too low for the air conditioner and too high for the duct system. The "low" air flow across the oversized coil produces colder delivery temperatures, and the "high" air flow through the ducts and grilles produce high pressures, noise, and high velocities at the grilles. The grilles themselves are often too small and without proper throw or spread (particularly the cheapest ones). When low delivery temperatures are coupled with high-velocity discharge through inappropriate and poorly placed grilles, occupants experience drafts.

Bigger Is Not Better- Quiet Is Better

When an air conditioner and duct system are properly sized to meet the cooling load, they can easily distribute the cool air without being noisy. To design a quiet system, keep every supply grille below NC-25 with a face velocity below 700 feet per minute.

Grilles with dampers are invariably noisier than equivalent grilles without dampers. When the dampers are partially closed, the pressures and leaks in the ducts increase and the air flow across the coil is reduced. Occupants generally close dampers to redirect air to another room that needs more delivery. If the system is designed correctly, neither register dampers nor inline balancing dampers should be needed.

The constriction of this duct has the unfortunate result of reducing the air flow across the cooling coil.

Bigger Is Not Better- Efficient Is Better

Correct Air Flow Helps Make an Efficient System

Manual D

In a recent laboratory test of a high-efficiency air conditioner, Proctor Engineering Group found a 7% drop in efficiency when the air flow was reduced by 30%. In order to ensure that the design air flow is being achieved, the installing contractor must measure and correct the air flow across the inside coil.

Proper Charge Helps Make an Efficient System

In the summer of 1995, Proctor Engineering Group and Arizona Public Service Company monitored a group of 22 newly constructed homes. Nearly all of those homes had undercharged air conditioners. One of the worst units had 62% of the correct charge (and 79% of proper flow). The homeowner complained to the builder that the air conditioner was not working right. She was told that the wrong amount of insulation had been installed in her attic, and an insulation contractor was called in to apply additional insulation. Shortly thereafter, the true problem showed itself when the air conditioner compressor failed.

These cardboard frames are specifically designed for insulating over ducts. The walls of the boxes allow weatherizers to build up a deeper layer of insulation around long duct runs.

Eliminating Duct Leaks Helps Make an Efficient System

To ensure a tight duct system, the installing contractor must test duct integrity using specialized tools (see HE Sept/Oct '93 for more information on duct testing).

A Smaller Air Conditioner Helps Make an Efficient System

Because of the inefficiencies associated with the start-up of the air conditioner, a smaller unit will produce the same amount of cooling with lower energy consumption, under most conditions.

It is not uncommon for poor cooling performance to be attributed to insufficient equipment size, when in fact there is more than enough cooling capacity. We know designers who determine the system air flow based on floor area (this oversizes the air conditioner in energy-efficient homes), and then try to squeeze down the size of the duct system so that it can be installed in the house. They explain that they can't use a higher insulation level on the ducts because there is no room, and, when faced with poor performance, increase the size of the air conditioner.

Most household air conditioning problems will be eliminated when the capacity of the air conditioner is reduced to ACCA Manual J and Manual S standards; an appropriately designed, insulated, and leakproof distribution system is used; and the system is installed to meet the manufacturer's standards.

Resources

Transactions

Manual J, D, S, and T. Available from Air Conditioning Contractors of America, 1712 New Hampshire Ave., NW, Washington, DC 20009. Tel: (202)483-9370.

Home Energy can be reached at: contact@homeenergy.org
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