Using Manual D Speed Sheet

The free ACCA Manual D speed-sheet makes properly sizing residential duct work really easy. However, you need to complete three important steps BEFORE you are ready to use the Manual D speed-sheet. You must first do a Manual J room by room load study, select the specific equipment you will be installing, and draw out your duct system in stick form. The speed-sheet will help you size your ducts according to the required heat load in each, the unit output, the unit airflow, and the external static pressure requirements of the unit. There are three basic steps: determining the total effective length of the duct system, determining the design friction rate, and finally sizing the duct.

Effective Length
Manual D looks for the worst case duct run and bases the design friction rate on that longest run. The idea is that if the blower can move the air through the longest run, it can easily push the air through the other ducts. The assumption is made that each run will have a balancing damper, and that the balancing dampers will be used to balance the system airflow once the system is installed. The speed sheet gives you four columns to use for determining the longest effective length. You don’t have to use all of them if the worst case run is obvious. Note that you are NOT entering data for every run, just looking for the longest run.

There are three rows labeled Trunk. They are there for systems which have multiple branching trunks. Most systems will only use one.  Enter the length of the trunk duct from the plenum to the branch takeoff in one of the Trunk rows. Then enter the length of the branch run beside Runout Length. The second set of rows on this tab are for entering the equivalent length of all the fittings. They are arranged in groups of fittings with similar functions. Click on a group to go to the tab showing the different fittings. Choose a fitting that best matches the fittings you will use. You will need to remember it, or jot it down. Click return to return to the Effective Length tab. You will probably NOT have a fitting for every group. Just leave spaces blank which do not apply to your system. Repeat the process for the return. Note that the group numbers change bit because the equivalent length of return air fittings varies from supply air fittings.

Friction Rate
The second tab is for determining the design friction rate. You need to know the specific unit for this part because you will be entering the unit airflow and external static pressure. The idea is pretty simple. All things that the air moves across cause a pressure drop. You list the pressure drop in wc for all the air components. This is totaled and subtracted from the system external static pressure, and what remains is available static to be used for moving air through the ducts.

Since friction charts are based on 100 feet, the friction rate, or wc friction drop per 100 feet, needs to be determined. For example, if your available static was 0.12 and your total effective length is 200 feet, the design friction rate would be 0.06. A duct which would cause a pressure drop of 0.06 per 100 feet would create a total pressure drop of 0.12 by the time the air traveled 200 feet. The speedsheet does this for you based on the total effective length calculated on the Effective Length tab.

Duct Sizing
You need to have a Manual J calculation of the heating and cooling loads for each room before using the final tab, Duct Sizing, Simply list the room name, heating BTUs and cooling BTUs and the speed sheet calculates the duct size based on the friction rate and CFM from the Friction Rate tab. Note that it re-sizes the ducts every time you enter more data – so don’t be alarmed if it tells you the first room you enter requires a 16 inch run. The sizes are not accurate until you have all the room information in.

Trunk sizing is as easy as clicking a box for each branch duct that t trunk feeds. Note there are probably more rows for trunk ducts than you will need.  Returns are also sized the same way. You click the box of each supply run which you believe will be served by that return. This is obviously not an exact science. However,  it is important that each supply run is selected in a return. If you have return trunks, you size them by selecting the return branches which feed into the return trunk.

Creative Application of Manual D Speed Sheet
You can use the Manual D Speed Sheet as a teaching/learning tool by varying some of the entries. For example, play with different equivalent length fittings to see the effect between best case and worst case fittings. Try different external static pressures and airflows to see the effect on duct sizing. Thi is a great way to see the effect different design decisions can have on the end result.

Manual J Resources

Are you looking for materials to teach load calculations? ACCA has free excel spread sheets they call speed sheets. You can download them from their web site. They also offer a curriculum for studying Manual J8, also free from ACCA. You do have to buy either Manual J8 or Manual J8 abridged to take full advantage of these. If you are looking for load studies coverage in a text, “Fundamentals of HVACR, 2^nd ed” has a complete Unit on residential load studies. It includes examples using Manual J8 while working on a complete block load for a small house. Other Resources include Wright-Soft’s Right-J program or Elite Software’s RHVAC program. These are two very full featured commercial programs which are used a good deal in the industry. The good news is that you can download and use demo versions of them for free. Typically, there are a few limitations on the free download versions, such as not saving the files. However, you get a chance to try them out before purchase.

For teaching, I really like the ACCA Manual J Speedsheets. They are not too hard to learn, and I believe that students get a better understanding of Manual J because they do have to use the manual some to use the software. I also like the Manual D Speedsheet for ductwork. Both of these are a little limited – they are mainly for use in residential systems that are not too complex. But that is exactly where our students should be starting. We typically have our students do a block load on a small cabin and a room by room load on a 3 bedroom ranch style house. They then do a radial duct design on the cabin and an extended plenum on the ranch house. You will want to spend some time with the speedsheets before giving them to students. Although they are not difficult, there you do need to go through one a couple of times to feel completely comfortable. The instructions are in the sheets and there are ample examples on every tab. Links to the  resources mentioned are

ACCA Speedsheets

ACCA Manual J Curriculum

ACCA Educator's Page 

Wrightsoft

Elite Software  

Fundamentals of HVACR 2nd edition 

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.