Why bother With load Calculations

Determining the proper air-conditioning unit for any application is the key to attaining comfort, efficiency and reliability for the homeowner and for you.

BY DENNIS KALCHUK



Many technicians view load calculations as a bother, a necessary evil or even a waste of time. After all, experienced technicians know the homes in their area, they understand the local climate and they feel they know how much air-conditioning capacity is required for almost any job.

So, why bother? Why waste time poring over the blueprints on a new construction job or measuring and checking the house for a replacement job? The answer can be stated in just three words: Comfort, efficiency and reliability.

· Comfort. An undersized air conditioner will run excessively long cycles, and not keep the house cool enough on the hottest days. An oversized air conditioner will not run long enough to sufficiently remove humidity from the air. The result will be air that feels cool but perhaps unpleasantly moist and sticky.

· Efficiency. If you install an air conditioner that has either too much or too little cooling capacity, it will waste electricity. The most efficient choice is the optimum capacity. You can determine that optimum capacity only by performing an accurate load calculation.

· Reliability. An undersized air conditioner may run for hours at a stretch as it struggles to keep up with the cooling load on hot days. An oversized air conditioner will cycle on and off too frequently. Either condition tends to stress the equipment more, possibly resulting in premature equipment failure. When you replace an existing air conditioner, always ask yourself, "Could this air conditioner have failed because it was improperly sized?"

You might also ask yourself, "Why can't I just ask the homeowner if the old unit cooled the house, and if it did, replace it with a new one of the same capacity?" Again, there are three answers:

1. Homeowners aren't receiving complete comfort. While today's consumers are generally more knowledgeable than in the past, many homeowners still don't realize that their homes could be more comfortable. If they feel cool air at the registers, they assume the air conditioner is doing its job. Many think it's normal for the upstairs to be hotter in the summer and do not realize that the air would feel more comfortable with lower humidity and/or better air distribution.

2. The situation may have changed. The old system was installed 10 to 20 years ago, maybe longer. The homeowner may have added a room addition, insulation or storm windows. Old insulation may have settled or shifted, leaving gaps. Trees may have grown and added shade. If a 3-ton air conditioner was just right 15 years ago, it may be too much or too little today.

3. It's the professional approach. When homeowners see you take an extra few minutes to do the job right, they will understand that you are a professional - someone they can depend on.

Load Calculation Tips

In the past, determining the load calculation was both tedious and time consuming. With the entry of computers into the heating and cooling business, a/c sizing is easier than ever. Just choose one of the many software packages currently on the market, load it onto your computer and it will guide you through the sizing process.

During our training program, we recommend Right-J for Windows. We have found it to be accurate, simple to use and easily linked to related programs that compare projected energy costs for two or more air conditioners. However, a number of similar programs are available from other companies.

You say you don't have a computer? Seriously consider getting one as it can pay for itself just in the time it saves you. Choose a good, name brand computer with the hardware specifications necessary to run the software you select. Typically, programs are available that will run on most computers operating on Windows 3.1, Windows 95, Windows 98 or Windows NT.

If you are buying a new computer to use on replacement jobs, consider getting a laptop. You can take it with you to your meeting with the homeowner, run the calculations right there and even print out a sharp-looking estimate for the homeowner.

The advantages of doing load calculations on a computer include:

· Speed. Computers perform basic math calculations at the speed of light.

· Accuracy. The computer will guide you through the calculation process to help make sure you cover all the bases, gather the necessary information and ask the right questions. In addition, when the computer runs the calculation, it will not make those aggravating mistakes in simple arithmetic.

· Professionalism. A computer, especially a laptop that lets you offer while-you-wait service, is a great addition to your professional image.

More Load Calculation Tips

However, a computer alone is not the entire solution to accurate a/c sizing. The computer's calculations are only as good as the information you gather. Here are some tips for getting the best information possible.

Understand the basics of heat transfer. When you determine the "size," or cooling capacity required for a particular application, you are estimating the rate at which heat is being transferred into the home in Btu per hour. You will use that calculation to select an air conditioner with cooling capacity to transfer heat out of the home at the same rate. You need to remember these four basic points about heat transfer:

· Heat moves from warmer areas to cooler ones. Just as nature tries to fill a vacuum by drawing air into it, nature attempts to equalize uneven temperatures by transferring heat from a warmer space to a cooler one. To determine the cooling capacity for an application, you need to calculate how fast the heat is moving into the house.

· The greater the temperature difference, the faster the transfer. This is why two identical houses will require different size air conditioners if they are located in different climates.

· Different materials transfer heat at different speeds. Glass and steel transfer heat quickly. Wood transfers heat slowly. Insulation transfers heat very slowly.

· The greater the area, the faster the heat transfer. This is why larger houses require higher capacity air conditioners.

Interview the homeowner. By asking several basic questions, you can learn valuable information that will help you specify the right air conditioner to meet the homeowner's needs. Ask these questions:

What temperature setting do you use on the thermostat?

Have you made any changes in the house since the original air conditioner was installed?

Particularly, have you added any additional rooms, storm windows or insulation?

How well did the old system perform?

Did it keep the house cool on the hottest days?

Were any rooms too warm or too cold?

Collect data about the house. With a knowledge of the basic principles of heat transfer, it's easy to understand much of the basic information that the computer will ask you to collect and enter for an accurate load calculation. Here are some key points:

· Measure the house. You will need an overall exterior measurement plus measurements of each individual room.

· Determine the direction the house faces. Don't guess, use a compass. Since the south side faces into the sun for a greater portion of the day, its walls, windows and doors will allow more heat into the house.

· Check insulation. Don't just take the homeowner's word for it - examine the insulation in walls, the roof and floor. Check for thickness and type (R value), and wherever possible make sure there are no gaps.

· Count and measure windows. You will need to record the size, location and direction they face, as well as the number of layers of glass and any overhangs, awnings or other permanent shading that might block sunlight.

· Examine the old air-conditioning system. Check the refrigerant lines and the ductwork to insure that they are properly sized. Check the location of the outdoor section, electrical connections and any other area that might indicate a problem which needs to be addressed or an opportunity to better serve the customer.

Now that you've completed your load calculations, use them to show the homeowner how to save money. Use comparisons to demonstrate the financial impact of adding insulation, storm windows or awnings. For many replacement jobs and some new construction projects, these changes can pay for themselves over time in energy savings. In some cases, they may even allow you to cut your price by specifying a lower capacity air conditioner.

Even if this recommendation takes a few dollars off your profit margin for the job, that will be a small price to pay for years of important word-of-mouth advertising.





Dennis Kalchuk works at the National Training Center at International Comfort Products National Training Center, Heil Division, LaVergne, Tenn.