Thursday, April 8, 2010

Why Teach Manual J?

ACCA’s manual J has been the gold standard for residential heat load calculations for many years. The latest edition, Manual J8, is frightening to students in both its size and cost. Many people ask why we teach Manual J calculations when everyone does load studies using computer software, especially after they get a look at the size of Manual J8. Others think that the sheer volume and complexity of Manual J8 makes a good argument for 600 square feet a ton ball park guesses. However, using the same logic you might ask why anyone bothers to learn their multiplication tables when they are always going to be using calculators and spreadsheets to do their calculations. I believe that understanding the underlying process is important to intelligently using the available tools. During a sale where items were being priced at 80% of their original price a clerk insisted that 80% of $120 was $150. They simply could not grasp the obvious; 80% of something could not possibly be more than the original amount. Their proof that they were correct was the fact that the answer was arrived at on a calculator, therefore it could not possibly be in error. Of course they had divided by 0.8 instead of multiplying, arriving at $150 instead of $96. The clerk’s lack of understanding of percentages and total reliance on technology had produced a ridiculous result that they were unable to recognize. Similarly, performing load studies using computer software by just dropping in numbers into a program provides many opportunities for error. If you don’t understand the process you may not recognize a ridiculous answer. Since equipment selection and duct sizing both rely heavily on the load study, the cost of a mistake is multiplied. One way to make Manual J8 more approachable is to use Manual J8 Abridged. It is more the size of previous Manual J editions and costs about half of the full J8 edition. All versions prior to Manual J8 used the averaging method. The eighth edition added peak load calculations that are similar to commercial calculations. The eighth edition also added factors for a plethora of unusual construction types and nearly 40 pages on duct loss and gain. The abridged version achieves is size reduction by only doing averaged calculations, removing factors for some of the more esoteric constructions, and drastically reducing the duct loss section. These things are not missed when teaching people to do their first load calculations.

One way to reduce the cost of Manual J to the students is to have your school join ACCA for $250 a year. If your school joins ACCA the students can buy Manual J8Abridged for $50 instead of $72. Another way is to have the school buy several copies and keep them in the library for student use.

Fundamentals of HVAC/R devotes an entire unit for load calculations using Manual J8. It takes the student step by step through a simple house, showing how the worksheet is filled out as you go. It was written to work with the full eighth edition, but also works well with the abridged version.

Sure, some folks still use 600 square feet per ton. The same folks do the duct system using the two sixes to an eight rule so they don’t have to do any duct calculations. When they go to charge the unit, they just add gas until the suction pressure is 70 psig. Many of the systems installed this way kinda sorta work. Trouble is – kinda sorta just ain’t good enough any more.

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