Load calculations are good for you

April 1, 2010
Many municipalities are now requiring a residential load calculation for every new installation. Add to that the fact that a load calc is now easy to do with a — usually free — computer program. Load calcs are becoming just plain good business for the HVAC professional.

Not long ago a residential load calculation — also known as a heat loss calculation or a load calc — was something you had the luxury of skipping. There was a good reason for avoiding it. It was difficult and time-consuming. There wasn't much at risk in not doing it. And furthermore, no one expected much from "the furnace guy" anyway.

All of this has changed. For starters, many municipalities are now requiring one for every new installation. Add to that the fact that a load calc is now easy to do with a — usually free — computer program. Load calcs are becoming just plain good business for the HVAC professional.

A load calc is good
Consumer expectations for the HVAC industry are much higher than in days past. Today your customers can go online and quickly learn what you're supposed to be doing — proper equipment sizing and selection, plus energy conservation.

In this economy you need every edge over your competition. Presenting the client with a load calculation can make you instantly look smart and thorough. You have done your homework. You have load calc numbers — data about the specific job to back up your recommendations.

The load calc can also help you reduce equipment and duct costs — another competitive edge. The residential HVAC industry has been over-sizing for years just to be safe. No contractor wanted to be caught without enough heating or cooling. Since most were guessing, they needed the insurance of going big. But super sizing has led to many industry problems: unnecessarily high equipment cost, high energy bills, short cycling and excessive noise to name a few.

A load calc often indicates that replacement equipment can be smaller than the original. In that case, you may need to educate the consumer about the reasons why smaller is better. It will likely run with longer, less frequent cycles. This is good because over-sized equipment is known for short cycling and wasting energy. The new equipment will also likely be quieter, a possible concern for those used to how the old furnace sounds.

Finally, doing a load calc and showing it to the customer helps to reduce call backs. When you can show them the numbers behind your recommendations, they are less likely to doubt the final result.

What is a load calculation?
A load calc determines just how much heating and cooling is needed for a specific house or building. It takes the guesswork out of sizing HVAC equipment. Its methods have been refined to make sure there are sufficient built-in safety factors to cover the heating and cooling needs of any building in any climate. So long as you input reasonably correct data, you will have a sound recommendation.

"Load" is a common term in the HVAC business for how many Btus of heating or tons of air conditioning that the equipment must be able to produce in order to keep the building comfortable.

Back in the day, before user-friendly computer programs, performing a load calculation was time consuming. It required taking a lot of measurements with a tape measure and doing math. Although the math was simple addition and multiplication, there was a lot of it, even with a calculator.

You could spend a full day in class, for example in the well-known I=B=R school, learning all the types of data you would need to collect. These included the dimensions of the individual rooms, which direction the building faces, what it's made of, the amount and type of insulation, the number and size of windows and doors and which direction they face, the outdoor design temperatures, and many other factors.

Times have changed
Fortunately with the arrival of user-friendly computer programs, load calcs have become rather easy. You can probably get one at no charge from your equipment distributor. And you can likely get familiarized with it in just a couple hours, whether in a distributor-sponsored class or your distributor representative going through it with you.

One of the important things that the program does for you is ask for exactly the information that is needed. The program lets you enter data for both the entire house and individual rooms. For dimensions, you simply fill in the blanks. The math is instantly done for you. If you need to make a correction, you simply change the entry.

Another improvement over days past is the laser measuring device. You can stand in one place to take all of the room measurements instead of running a metal tape from wall to wall.

For items such as the composition of the roof, walls and floors, and type of windows and glass, a drop-down list lets you select a best answer. Exact information is preferable, but not required.

Do it right
Here are some temptations to avoid when you are doing a load calc:

  • Don't guess when you can get the correct information. Go inside the house and measure the rooms. The customer will be impressed at your thoroughness.
  • Ask the homeowner questions about things they may know but you don't, such as how much insulation is in the walls.
  • If there is a major uncertainty, such as how much insulation there is, try running the program for each of the possibilities, and go with the most conservative results. Don't be tempted to improve upon the program by making the design temperatures more extreme. The design temperature by definition covers 99%, not 100%, of the temperature range. A load calc program asks you to simply enter the location of the job. It then determines, based on factual data, the low and high temperatures that the equipment will need to accommodate. So, if the program says that your heating design temperature is 10ºF and you remember when it got down to 5ºF, don’t "correct" it.
  • Don't super-size the equipment just to be on the safe side.
  • Don't let your distributor up-size the equipment either.

When conventional wisdom says "go the next size up," remember that that advice comes from before easy-to-use load calc programs. With today's load calc, you can size equipment right on, save your client some money in equipment and energy, and win more jobs over your competition.

Carol Fey is a degreed technical trainer who has worked as a heating technician in Antarctica. She has published five books especially for the HVAC industry. Her website is www.carolfey.com.

About the Author

Carol Fey

Carol Fey is a technical trainer and writer, specializing in easy electricity, hydronics and troubleshooting books.  She also writes about HVAC work in Antarctica. 

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