Formula
The HVAC Manual J load calculation formula is as follows:
BTUh = CFM x ΔT x 1.08
Where:
BTUh = British thermal units per hour
CFM = Cubic feet per minute
ΔT = Temperature difference
If you want to know how to use the HVAC Manual J load calculation, there are a few things that you will need to do. First, you will need to find the right software program that will help you with this process. There are many different software programs available on the market today, so finding one that is compatible with your system should not be a problem.
Once you have found the software program that you feel comfortable using, the next thing that you will need to do is to determine the size of your home. This can be done by measuring the dimensions of each room in your house. Once you have these measurements, you will then need to input them into the software program.
After you have input the dimensions of your home, the software program will then calculate the amount of heat that is required to keep your home at a comfortable temperature. This information will help you determine the size of the furnace that you need to purchase. It is important to remember that the larger the furnace, the more money you will spend on your utility bills each month.
The last thing that you will need to do when you are using the HVAC Manual J load calculation is to make sure that you have all of the necessary information before you begin. This includes the type of fuel that you use to heat your home, the size of your home, and the climate that you live in. All of this information will be needed so that the software program can accurately calculate the amount of heat that is required to keep your home at a comfortable temperature.
Now that you know how to use the HVAC Manual J load calculation, it is time to put it to use.
Similar formulas to HVAC manual J load calculation used in the construction industry are:
1. HVAC cooling load calculation formula:
BTUh = CFM x ΔT x 4.5
Where:
BTUh = British thermal units per hour
CFM = Cubic feet per minute of air movement
ΔT = Temperature difference in degrees Fahrenheit between the supply air and the return air
2. HVAC heating load calculation formula:
BTUh = CFM x ΔT x 1.08 / 24
Where:
BTUh = British thermal units per hour
CFM = Cubic feet per minute of air movement
ΔT = Temperature difference in degrees Fahrenheit between the supply air and the return air
24 = Number of hours in a day
3. HVAC cooling & heating load calculation formula:
BTUh = CFM x ΔT x 4.5 + CFM x ΔT x 1.08 / 24
Where:
BTUh = British thermal units per hour
CFM = Cubic feet per minute of air movement
ΔT = Temperature difference in degrees Fahrenheit between the supply air and the return air
4.5 = Constant for cooling load calculation
1.08 = Constant for heating load calculation
24 = Number of hours in a day
To use the formulas, simply substitute the values for your specific project. For example, if you’re working on a home with 1,000 square feet of living space and the temperature difference is 70 degrees Fahrenheit, your Cooling Load BTUh calculation would be:
1,000 x 70 x 4.5 = 315,000 BTUh
Your Heating Load BTUh calculation would be:
1,000 x 70 x 1.08 / 24 = 9,000 BTUh
And your Cooling & Heating Load BTUh calculation would be:
1,000 x 70 x 4.5 + 1,000 x 70 x 1.08 / 24 = 324,000 BTUh
Once you have your BTUh calculation, you can determine the capacity of the HVAC system you need by using this chart:
Capacity (BTUh) Size (Tons)
24,000-36,000 1 Ton
36,001-60,000 1.5 Tons
60,001-84,000 2 Tons
84,001-120,000 3 Tons
120,001-180,000 4 Tons
180,001-240,000 5 Tons
240,001 and up 6+ Tons
So using the numbers from our example above, we would need a 3-ton system to properly heat and cool our 1,000 square-foot home.
As you can see, the HVAC Manual J load calculation formula is relatively simple. However, it’s important to remember that this formula is only an estimate. There are other factors that can affect the size of the system you need, such as the R-value of your home’s insulation, the number of windows and doors, and the climate you live in. When in doubt, it’s always best to consult with a professional HVAC contractor to determine the best system for your home.
Want to learn more about HVAC design, air distribution systems, and math around it? Here are some of the books that we recommend:
1. HVAC Design Data Sourcebook by Arthur D. Anderson
2. Air Conditioning and Refrigeration, Second Edition by Edward G. Pita
3. The Heating and Cooling Load Handbook, Second Edition by William Coad and Joseph F. Hanlon
4. The ASHRAE Handbook of Fundamentals, 2013 edition
5. The Manual J – A Companion to the ASHRAE Handbook – Fundamentals
6. Air Conditioning System Design by Stanley M. Harriman
7. Heating System Design by Stanley M. Harriman
8. Commercial and Industrial Refrigeration Systems by William C. Whitman, John A. Dittman, and Albert F. Mozell
9. Industrial Ventilation: A Manual of Recommended Practice for Design, 28th edition (online version)
10. Air Distribution Basics by Harold N. Lanigan Jr.
11. The Basics of Ductwork Design by Harold N. Lanigan Jr.
12. Ductwork Design for Energy Efficient Buildings by Harold N. Lanigan Jr.
13. The Basics of HVAC Controls by John Iko and Michael Buford
14. HVAC Control Systems by John Iko and Michael Buford
15. Fundamentals of HVACR by Michael J. Moran, Howard N. Shapiro, Bruce E. Englander, and Edward G. Pita
16. HVAC Equations, Data, and Rules of Thumb, Second Edition by Arthur D. Bell and W. David Smith
17. Steam Tables: Thermodynamic Properties of Water Including Vapor, Liquid, and Solid Phases by Clifford Swenson
18. Mollier Diagram for Water and Steam: English Units by Richard R. Eckert
19. Refrigeration and Air Conditioning, Third Edition by Ralph D. Bennett and Arthur N. Stansberry
20. Principles of Heating Ventilating and Air Conditioning by Raymond D. Wright
Quick overview of each book listed below:
1. The HVAC Design Data Sourcebook is a comprehensive guide to the design data needed for HVAC systems. It includes information on load calculation, ductwork design, and equipment selection.
2. Air Conditioning and Refrigeration, Second Edition is a comprehensive guide to the design and installation of air conditioning and refrigeration systems. It includes information on load calculation, ductwork design, piping, and insulation.
3. The Heating and Cooling Load Handbook, Second Edition is a comprehensive guide to the design of heating and cooling systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
4. The ASHRAE Handbook of Fundamentals, 2013 edition is a comprehensive guide to the design and operation of heating, ventilating, and air conditioning systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
5. The Manual J – A Companion to the ASHRAE Handbook – Fundamentals is a comprehensive guide to the design of heating and cooling systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
6. Air Conditioning System Design is a comprehensive guide to the design of air conditioning systems. It includes information on load calculation, ductwork design, piping, and insulation.
7. Heating System Design is a comprehensive guide to the design of heating systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
8. Commercial and Industrial Refrigeration Systems is a comprehensive guide to the design of commercial and industrial refrigeration systems. It includes information on load calculation, ductwork design, piping, and insulation.
9. Industrial Ventilation: A Manual of Recommended Practice for Design, 28th edition (online version) is a comprehensive guide to the design of industrial ventilation systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
10. Air Distribution Basics is a comprehensive guide to the design of air distribution systems. It includes information on load calculation, ductwork design, piping, and insulation.
11. The Basics of Ductwork Design is a comprehensive guide to the design of ductwork systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
12. Ductwork Design and Installation is a comprehensive guide to the design and installation of ductwork systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
13. The Basics of HVAC Controls by John Iko and Michael Buford is a comprehensive guide to the design of HVAC controls systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
14. The Basics of Refrigeration by William C. Whitman is a comprehensive guide to the design of refrigeration systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
15. The Basics of Air Conditioning by John D. Mitchell is a comprehensive guide to the design of air conditioning systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
16. The Basics of Heating by John D. Mitchell is a comprehensive guide to the design of heating systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
17. The Basics of Ventilation by John D. Mitchell is a comprehensive guide to the design of ventilation systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
18. The Basics of Indoor Air Quality by John D. Mitchell is a comprehensive guide to the design of indoor air quality systems. It includes information on load calculation, ductwork design, equipment selection, and installation.
19. The Basics of Plumbing by John D. Mitchell is a comprehensive guide to the design of plumbing systems. It includes information on load calculation, piping, and fixtures.
20. The Basics of Electrical by John D. Mitchell is a comprehensive guide to the design of electrical systems. It includes information on load calculation, wiring, and fixtures.