The Hotline: 06/07/2004

Sizing Rules Of Thumb?

My question involves the sizing of commercial and residential loads. I have been told that you would use 400 square feet per ton for commercial and 650 for residential. Would these figures be accurate, and where did these figures originate? Do they take age or insulation values into consideration? How far back do these rules of thumb go?

QUESTION:
By Amy Judd Sharp
Per-Se Technologies

Is there an industry standard for the square foot per ton for home A/C units? For example, one contractor says a 4-ton unit is intended for houses 1,600 square feet and under, and another says a 4-ton unit is more than enough for a 1,900-square-foot home.

ANSWER:
From Dan Kramer, P.E.
Specialist Grade Member of RSES

There are rules of thumb, and, in most cases, equipment selected for basic residences located in moderate climates based on 450 square feet of floor area per ton will produce a satisfactory selection.

However, why risk your reputation on an unsupportable selection? For a few dollars more than you paid for your leak detector, you can buy a program based on ACCA Manual J that will provide you with an authoritative load estimate based on wall and ceiling construction, window design and area, design or maximum outdoor temperature, and desired indoor temperature.

A government Web site that provides information and sources for Manual J data and their prices can be found at www.eren.doe.gov/consumerinfo/refbriefs/cb7.html.

Also, run a search with the terms "Residential Air Conditioning Load Calculation Software" for program sources. There are many.

Charging A System

How would one correctly charge a water-cooled, R-22, 10-ton unit with a TXV?

ANSWER:
From Gene Silberstein
Consulting Engineer
Whitestone, N.Y.

In order to properly charge a water-cooled air conditioning system, there must be proper water flow through the condenser as well as proper airflow across the evaporator coil. If the system has continuous water flow through the condenser, make certain that the cooling tower is operational and that water is flowing through the water side of the coil.

There should be approximately 30 gallons per minute flowing through the condenser for your 10-ton system (3 gpm per ton). If the system is equipped with a water-regulating valve, there will be no water flow through the condenser coil when the system is in the off position.

Once proper water flow and airflow have been established and the system has been properly leak tested and evacuated, you can begin adding refrigerant. With the system off, you can have an initial charge of liquid refrigerant through the high side of the system. You might want to temporarily jump out the low-pressure control to keep the compressor operating during the charging process. After starting the system, add refrigerant slowly until the desired pressures are reached.

On a properly charged water-cooled air conditioning system, the water entering the condenser coil should be approximately 85 degrees F and the temperature of the water leaving the condenser should be around 95 degrees (a 10 degree difference between inlet and outlet temperatures). If the temperature split is much less than 10 degrees, chances are that the system requires more refrigerant. The evaporator saturation temperature should be around 40 degrees (68.5 psig) and the high pressure should be in the area of 200 psig. The head pressure will vary somewhat from system to system. If the system has a water regulating valve, it should be opening and closing in response to the changing head pressure.

Also, remember to remove the jumper on the low-pressure switch if you jumped it out during the charging process. Once the system is charged, the temperature split across the air side of the evaporator should be in the range from 17 degrees to 20 degrees, meaning that the supply air should be 17 degrees to 20 degrees cooler than the air returning to the unit.

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Publication date: 06/07/2004