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How to Perform ACCA Manual J Load Calculations

By Pete Kiser

In RSES Journal’s December 2018 issue writer’s Doug Priestly and Greg Brunts described the challenge of sizing heat pumps properly and the need for technicians to use books like Manual J Load Calculation to complete some of the more complicate variables that come with the task. But what is Load Calculation? A load calculation is a method of evaluating the total sum of all of the heating and cooling loads for the home or structure.

For a simple block load calculation (four walls, floor and the ceiling) you would use the Air Conditioning Contractors of America (ACCA) MJ8ae form. To do a whole house (room-by-room) or a full load calculation you would use the ACCA MJ8 full form. These forms are available from ACCA by contacting ACCA at (703)-575-4477 or www.acca.org. Remember, do not use ACCA Manual J Load Calculations for commercial jobs or projects.

Getting started

When you are doing a Manual J Load calculation, you need to evaluate the sources of heat gains or losses for the load on the building or structure in BTU/hour. For example, we know that heat travels from hot to cold, so in the winter if it is 68°F inside the house and 0°F outside the house, then the heat from inside the house will want to travel to the outside of the house or structure. This would be an example of a heat loss.

During the summer, if it is 95°F outside and 74°F inside of the house, the heat from the outside of the house is trying to get to the cooler inside temperature. This would be an example of a heat gain.

There are different types of building envelope loads like the total heating or the total cooling loads based on the house or structure. The building envelope load depends on what the inside and outside ambient conditions are. The building envelope load is the sum of the component loads like a wall load, ceiling load, windows or door loads. Also there is a people load to consider as well: How many occupants are living in the house or how many appliances are in the house?

A building envelope load can be sensible or a latent load. Sensible loads affect the dry bulb temperature in the house. Latent loads affect the amount of moisture in the house.

A block load is a combined space load based on the equipment that is required to heat or cool the house or space.

The load area is the square feet area that is used to determine the heating and sensible cooling loads for the house or structure using the walls, windows, doors, floor and ceiling construction materials to help determine the total load.

The equipment load is the load that the heating, cooling and air conditioning equipment needs to satisfy for the occupants to be comfortable throughout the year as the seasons change.

To determine a room-by-room load or a whole house load calculation, more specific information from the house or structure is required in order to be more accurate than a block load calculation.

Design conditions for the house or structure is based on temperature, humidity and wind conditions that are used to determine the heating and cooling load requirements for the heating and cooling equipment.

The duct load is a system of round or square ducts that is used to direct air from one point in the house or structure to another dedicated point to provide heating or cooling for occupant comfort. For more information, refer to ACCA Manual D.

The equipment load in a house or structure is the sum of the building envelope loads and the heating and cooling equipment loads. For more information, refer to ACCA Manual S Equipment Sizing book.

To perform an accurate Manual J load calculation, use ACCA form J1AE or form J1.

To complete a manual MJ8AE worksheet use:                                                  

Worksheet A: Design Conditions.                                                                

Worksheet B: Window, Glass Door Loads.                                                            

Worksheet C: Skylight Loads.                                                                              

Worksheet D: Opaque Surface Loads.                                                                         

Worksheet E: Infiltration Loads.                                                                            

Worksheet F: Internal Loads.                                                                                                         

Worksheet G: Duct Loads.                                                                                        

Worksheet H: Ventilation Loads.

ACCA has a Form J-1 that is helpful for gathering job or project information. It is available from ACCA, or by contacting them directly at the address given above.

Other considerations

The accuracy of each load estimate is a reflection of the users input information. One of the problems in doing a load calculation is that the user has to make choices or choose a value that is note completely accurate because all of the exact choices are not all available when they are making the selection for the building material(s).

The user will have to interpolate between the two closest building material choices that will affect the accuracy of the total heat loss and/or heat gain on the house or structure.

The more that the user interpolates or guesses at the building material(s) choices, the more it will affect the total load summary accuracy on their ACCA Manual J report.

Some basic HVAC knowledge and basic math skills are required to perform an ACCA Manual J Load calculation.

Duct sealing in unconditioned spaces should be sealed and insulated.

For easier calculations

There is ACCA Manual J Load application software available from various sources.

I have used the Wrightsoft Right-Draw module in the Right-Suite program. In this program you can specify the scale that you want to use in inch/foot scale or a ratio scale. You can use your mouse to draw the house or structure walls, add a room or rooms, windows, doors, floor(s), ceilings. You can name the individual room(s) or add a second floor. You can find out more information on this product by contacting them at 1-(800)-225-8697 (Option 3), www.wrightsoft.com or for training at sales@wrightsoft.com

Another Software option is Cool Calc that ACCA has recognized Cool Calc Manual J as “Powered by Manual J.” ACCA reviewed the software’s functionality, defaults and capabilities to ensure that Cool Calc Manual J complies with the procedures and requirements of ANSI/ACCA 2 Manual J – 2011 (Residential Load Calculations). For an example look at the completed Cool Calc Block Load Calculation from Trenton, MI.

ACCA reminds contractors that only the software listed as approved by ACCA at http:www.acca.org/software is considered compliant in regards to Manual J Load Calculation software.

Pete Kiser is an RSES Member and Southeast Michigan ACCA Board Member.

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