Manual J 8th Edition

Many industry professionals that were familiar with Manual J7 find Manual J8 hard to navigate. The increased amount of information, detail, and forms discourages even seasoned pros who are used to the 7^th edition. Similarly, I have found that students are often intimidated and confused by the level of detail and the wide variety of forms they must learn in Manual J8. I try to make it more manageable by only discussing the averaging method and keeping the first calculations simple. Stay away from peak loads and zoning until the students have got a good handle on a straight forward load study. Also, I discuss the difference between the way heat loss and heat gain are handled. Heat loss HTMs (heat transfer multipliers) are all calculated using the temperature difference between the outside and inside design temperatures. Heat gain HTMs must also take heat storage and climate variation into account.

The most obvious difference between the 7^th and 8^th editions is how HTMs (heat transfer multipliers) are determined. The seventh edition provides tables that have basically taken the U value and multiplied it time a range of temperature differentials to produce a table of HTM factors. This saves the user from calculating the HTM themselves. The 8^th edition just gives the U values and leaves it to the user to multiply the temperature difference times the U value to get the HTM. For heat loss this is a very straight forward process. For heat gain it becomes more complicated because the temperature difference is usually not simply the difference between the outside and inside temperatures. Thermal storage of different materials and climate variations both affect the cooling temperature difference. Tables in the 8^th edition show both the U value of the material and the effective cooling temperature difference, abbreviated CTD. These can become confusing because they are arranged differently for different constructions and different building components. I find that if people understand some of the underlying reasoning they can do a better job managing the details. Explain to your students why the cooling temperature difference is different for each construction and material. Attic space and ceilings are a good example. Everyone has experienced how hot an attic can get in the summer. Clearly, the temperature difference across the ceiling is greater than the difference between the inside and outside temperatures. This is due to thermal storage in the attic. But this effect is not the same all across the country. Areas that do not have a wide variation in temperature store more heat, while areas with a wide temperature swing through the day store less and have an opportunity to release some of the stored heat. The low, medium, and high daily ranges found in the weather data are used to help determine the cooling temperature difference for most materials. Low daily range climates have higher CTDs because there is not much difference between the high temperature and the low temperature of the day: they get hot and stay hot. High daily range climates have lower CTDs because there is a big difference between the high temperature and the low temperature of the day: they get hot during the day and cool off at night. The combination of material heat storage and daily temperature variations makes cooling temperature differences complex. Once students understand the why, it is time to dig in and learn the how. There is really no substitute for working through several examples on each table. You can do worksheets for students to calculate heat transfer multipliers until they are comfortable. Then let them try an entire house, starting with a basic house.

ACCA has an excel file on their website that helps you perform a Manual J8 calculation. It does not replace Mnual J, but it does make using it much easier. The file is called a Manual J Speed-Sheet and is available at http://www.acca.org/speedsheet/ Another resource is my book.

Fundamentals of HVAC/R walks students through a simple house, filling in the Manual J 8^th edition form one step at a time in Unit 64 Residential Load Calculations. Remember, teaching your students how to do a proper heat load study is the first step in correct system sizing and application. Correctly sized and applied systems operate more efficiently saving energy and money. They are green two ways.

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.