Sunday, April 18, 2010

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 7th 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 7th and 8th 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 8th 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 8th 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 Another resource is my book.
Fundamentals of HVAC/R walks students through a simple house, filling in the Manual J 8th 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.

1 comment:

  1. were can i get a manual j heat gain, loss form? I've looked for days on the internet and can't find one to download or print off.