Bigger Is Not Better:
Sizing Air Conditioners Properly

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Article published in Home Energy Magazine
Issue Dated: May/June 1995
Written by: John Proctor, Zinoviy Katsnelson, and Brad Wilson.

Key Cooling Terms

Dry-bulb Temperature P
The temperature measured by a standard thermometer.

Latent Cooling Load P The net amount of moisture added to the inside air by plants, people, cooking, infiltration, and any other moisture source. The amount of moisture in the air can be calculated from a combination of dry-bulb and wet-bulb temperature measurements.

Wet-bulb Temperature P
When a wet wick is placed over a standard thermometer and air is blown across the surface, the water evaporates and cools the thermometer below the dry-bulb temperature. This cooler temperature (called the wet-bulb temperature) depends on how much moisture is in the air.

Design Conditions P
Cooling loads vary with inside and outside conditions. A set of conditions specific to the local climate are necessary to calculate the expected cooling load for a home. Inside conditions of 75 degrees F and 50% relative humidity are usually recommended as a guideline. Outside conditions are selected for the 2.5% design point.

2.5% Design
Outside summer temperatures and coincident air moisture content that will be exceeded only 2.5% of the hours from June to September. In other words, 2.5% design conditions are outdoor temperatures historically exceeded 73 out of the 2,928 hours in these summer months.

Capacity P
The capacity of an air conditioner is measured by the amount of cooling it can do when running continuously. The total capacity is the sum of the latent capacity (ability to remove moisture from the air) and sensible capacity (ability to reduce the dry-bulb temperature). Each of these capacities is rated in BTUs per hour (Btu/h). The capacity depends on the outside and inside conditions. As it gets hotter outside (or cooler inside) the capacity drops. The capacity at a standard set of conditions is often referred to as "tons of cooling."

Tons of Cooling P Air conditioner capacity is rated at 95 degrees F outside with an inside temperature of 80 degrees F and 50% relative humidity. Each ton of air conditioning is nominally 12,000 Btu/h (this comes from the fact that it takes 12,000 Btu to melt a ton of ice). While an air conditioner may be called a three ton unit, it may not produce 36,000 Btu/h in cooling. There is a wide variety of actual capacities that are called "three tons."

EER P The Energy Efficiency Ratio is the efficiency of the air conditioner. It is capacity in BTU per hour divided by the electrical input in watts. EER changes with the inside and outside conditions, falling as the temperature difference between inside and outside gets larger. EER should not be confused with SEER.

SEER P
The Seasonal Energy Efficiency Ratio is a standard method of rating air conditioners based on three tests. All three tests are run at 80 degrees F inside and 82 degrees F outside. The first test is run with humid indoor conditions, the second with dry indoor conditions, and the third with dry conditions cycling the air conditioner on for 6 minutes and off for 24 minutes. The published SEER may not represent the actual seasonal energy efficiency of an air conditioner in your climate.

Manual J
Manual J is a widely accepted method of calculating the sensible and latent cooling (and heating) loads under design conditions. It was jointly developed by the Air Conditioning Contractors of America (ACCA) and the Air-Conditioning and Refrigeration Institute (ARI).

Manual S
Manual S is the ACCA method of selecting air conditioning equipment to meet the design loads. It ensures that both the sensible capacity and the latent capacity of the selected equipment will be adequate to meet the cooling load.

Manual D
Manual D is the ACCA method for designing duct systems. Contractors often find it a laborious process and most duct systems are just installed, not designed. The amount of time necessary to design a duct system is certainly warranted in tract construction where the design is used repeatedly and for custom homes where the total cost of the home warrants a proper design. In short, designing a duct system is essential for proper equipment performance and customer comfort.

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Calculating Cooling Loads with Manual J

Manual J procedures are based on a number of sources including the ASHRAE Handbook of Fundamentals. The basic structure of Manual J is:

where HTM is a Heat Transfer Multiplier (in Btu/h/square feet). Area is the area of the building component (such as a wall). The HTMs take into account orientation, shading, temperature difference, solar gain, thermal storage, diurnal temperature swing, construction, R-Values, and roof color.

Manual J is a simplified adaptation of more complex modeling, yet it does not make many of the gross assumptions that some other load calculation methods use. It estimates both the latent and sensible cooling loads (both are necessary to properly size a system).

It is the result of a process that involved a large part of the HVAC industry and is widely accepted. It is in fact the basis for many of the other methods that are used, including many of the computer programs. For the contractor it is "safe." Because contractors helped develop it, any compromises that were made were not in the direction of undersizing units.

Manual J bases the infiltration rate on floor area and Best, Average, or Poor construction, but far better models of infiltration exist based on blower door testing. Manual J also does not have a method of considering duct leakage (although the new Manual D discusses duct leakage at some length and recommends that duct leakage be eliminated, not calculated).

The existing duct leakage in homes consumes some (but usually not all) of the safety margin built into Manual J. If duct leakage were brought under control, units could be sized smaller than Manual J.

While Manual J is simplified, it is still not simple. Because of the many values and tables, it is easy to make an error when using it. In most cases, either a set of tables specific to the contractor's service area, or a computer program should be used to reduce the likelihood of errors. The Florida Solar Energy Center is developing a more simplified sizing methodology for Florida that compares favorably with Manual J results.

While individuals who have used Manual J extensively are convinced that it has a substantial oversizing margin, there are no field studies that have determined the size of that margin. For now, it is useful as a basic load calculating method as long as one understands and takes into consideration its limitations.

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How They Size Air Conditioning Systems In Florida

The overall response rate was a respectable 9%. An analysis of the survey results found that the following typifies residential sizing practices in that state.

• Sizing is accomplished using Manual J by 33% of the respondents, software is used by 34% of the respondents, square footage is used by 24%, and "other" procedures are used by 8%. Generally, respondents who are not members of ACCA were much more likely to use square footage or "other" procedures.

• Of the respondents reporting "other" methods, 29% use a utility's short form, 26% use their own calculations, 19% use load sheets or manuals, 11% hire others, and 14% use personal experience or other methods.

• Of 127 contractors indicating the square feet they estimate per ton for AC sizing, the most common response (36.2%) was 500 square feet. The range was from 350 to 700 square feet per ton and the average was 502 square feet per ton.

• Fifty-two percent of respondents use a room-by-room method of sizing, 41% use whole house, and 6% use other methods.

• Air-flow calculations for each room are done with square footage estimation by 30%, with software by 22%, with Manual D by 20%, with CFM-per-ton by 18%, and with other methods by 10%.

• Of the 79 contractors providing a CFM per square feet estimate, 42 of them (53%) use 1.0 CFM per square feet but with a great deal of variability. A value of .8 CFM per square feet was the second most common response (10%) and a value of 1.5 CFM per square feet was the next most popular response.

• Construction drawings are used for obtaining take-off measurements by 62% of the respondents with 23% making their own measurements at the site, and 10% not using take-offs.

Inaccurate Sizing Methods?

The survey indicated that sizing is fairly evenly split between Manual J calculations, computer software, and estimation by floor area. Not surprisingly, each camp had strong opinions of the other methods. Many using Manual J or computerized methods regarded square footage as an inaccurate means of sizing.

Some of those using square footage mentioned that not accounting for vaulted ceilings or large expanses of glass could lead to low estimates. However, the square footage camp strongly derided Manual J and computerized methods for undersizing units. The most common reported reason for the perceived failure of Manual J or computerized methods was that customers desire lower temperatures than Manual J assumes.

Nearly 40% of the respondents indicated that they have at times purposely oversized units. Almost none purposefully undersized units. Many indicated that they round up predicted sizing by half a ton to allow for future expansion or to "reduce callbacks."

Of those who explained why they oversize, over 30% indicated a customer request - often a demand for low temperatures. By far the most commonly expressed reasons for oversizing were either to "provide more cooling" or to lower temperatures. "I oversize by 50%," indicated one contractor, "so customers will not complain."

The survey also shows that some contractors use sizing estimation values half again larger than others for sizing units, and twice as large for determining room air flow. The few respondents who did emphasize the need to size units small, were completely outnumbered by the "bigger-is-better" school.

See "How Contractors Really Size Air Conditioning Systems," by Robin K. Vieira, Danny S. Parker, Jon F. Klongerbo, Jeffrey K. Sonne, and Jo Ellen Cummings, Florida Solar Energy Center, 300 State Road 401, Cape Canaveral, FL 32920. Tel:(407)783-0300.

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Sizing Air Conditioners: Recommendations for Contractors

• Check all numbers for consistency. For example, in typical construction, total area of exterior walls facing north or east (including a wall to garage) is usually equal to the total area of the opposite south or west walls; ceiling area is usually equal to the building footprint area; window area is usually from 10% to 25% of the floor area; gross wall area is bigger than the window area.

• Use design outdoor conditions and daily temperature range exactly for your location per Manual J or ASHRAE Handbook of Fundamentals. Otherwise, use the data for the closest location with a similar climate.

• Use standard 75 degrees F design indoor temperature.

• Consider both location and level of insulation of ducts.

• When selecting cooling factors for roofs, floors, and walls consider their R-value and type; for example, frame wall or masonry wall. Partitions and knee walls that separate a conditioned space from an unconditioned space like an attic or garage should be treated separately from the exterior sunlit surfaces.

• Always account for the effect of the overhang shading. This is one of the most efficient load reduction measures. When calculating this effect, consider window height, overhang length and distance to the top of the window as shown on page 30 of Manual J.

• Pay great attention to window type, material, and interior shading. An error in this area can throw off the window heat gain by as much as 100%.

• Calculate infiltration rate based on blower door measurements.

• Calculate the latent load based on the number of people and the outdoor air humidity ratio. Do not use a "typical" multiplier of 1.3 or any other to calculate the total load from the sensible load. This implies that every building has a latent load that is exactly equal to 30% of its sensible load and that the quality of construction and location are not important. It also means that if a new source of the cooling load is added, for example another window, the moisture gains will also increase. This simply is not accurate.

• Consider ventilation load if appropriate.

• Select equipment based on the detailed manufacturer's performance data. Do not rely on the nominal tonnage since different units may have more than 10% capacity difference.

• Choose equipment based on the ACCA Manual S without using any safety factors. This method selects the unit that has the sensible capacity at least equal to but no more than 15% greater than the building sensible load, and the latent capacity equal to or greater than the latent load at standard indoor conditions of 75 degrees F dry bulb and 50% relative humidity. For dry climates, Manual S alone oversizes by approximately 20% compared to Manual J load.

• Test the duct leakage with a duct test rig and suggest that that the ducts be sealed before the new air conditioner is installed. Use the appropriate sealant and retest to ensure the quality of the job.

• When installing a complete system, design the duct work using Manual D. Test for proper installation.

• Properly evacuate the coil and refrigerant lines before releasing the refrigerant charge from the outside unit into the system (use a micron gauge).

• After installation, check the air flow across the coil and the refrigerant charge using the manufacturer's suggested methods. Correct any deficiencies.

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