Manual J Load Calculations 3D

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In this 3D video, we show how to calculate heat losses and heat gains in a residential structure in accordance with ACCA Manual J, particularly a block load for an existing structure. We owe special thanks to Russ King, Ed Janowiak, Alex Meaney, and Adam Mufich for their contributions to this video.

Heat and Comfort Basics 3D:

HVAC School Manual J Block Load Worksheet:

Kwik Model 3D:

Air Conditioning Contractors of America (ACCA):

Load calculations require us to use math, and we can use Manual J software to do that work for us. However, we still need to obtain the data for the math in the first place; Manual J tells us which measurements we need to enter into the software for the load calculation. We can use Manual J to figure out block loads of an entire building or room-by-room calculations. Manual J may also be used in existing buildings or based on the plans of a new construction.

When gathering data from walls, we need to know the area and the R-values of each material making up the wall, including drywall, insulation, and exterior building material. Higher R-values indicate better resistance to conduction; we can control the material thickness and insulation to minimize heat conduction into or out of a structure. You’ll need to deduct the studs, windows, and doors from the R-value of a wall. Partition walls between other indoor spaces, like a neighbor’s home or an unconditioned garage, have different temperature differences (delta T) than an exterior wall.

To deduct the window area from the wall area, you’ll need to multiply the length and width of the wall and the length and width of the window(s) and subtract the product(s) of the window area from the wall area. The window will have its own calculated heat losses and gains due to its ability to allow for heat transfer via radiation. Heat gains via solar radiation will vary depending on the window placement and the time of day. We need to know the window size, material, framing, transparency number of panes, and placement of the windows to understand the U-factor, which is the inverse of the R-value; higher U-factors indicate higher rates of heat transfer.

Roof overhangs can reduce incoming solar radiation, so we need to account for the height and depth of roof overhangs in our calculations. We should also know the roof type, pitch, and color.

When accounting for exterior doors, we need to know the door size, material, and placement. We also need to know the elevation of the home and how many stories it is. The ceiling height and insulation (material and R-value) are also important for our Manual J calculations. Conduction also happens through the floors, so we must know about the material and possibly even the R-value of insulation (if applicable).

When considering the impact of ductwork on heat gains and losses, we need to factor in duct location, insulation R-value (often R4, R6, or R8), and leakage (due to heat gains and losses via convection into or out of the ductwork).

We also need to know if the air handler is in a conditioned or unconditioned space and how exhaust-only ventilation may be responsible for seasonal BTU gains and losses.

Household appliances add heat, and Manual J has a default appliance heat load of 1200 BTUs. Electronics also add sensible BTUs, and dishwashers or laundry appliances may also add latent BTUs. We can also expect higher latent BTU gains in homes where occupants frequently run faucets, do laundry, or bathe/shower. Occupants add heat loads when they breathe or their bodies give off heat, usually about 200 latent BTUs per hour and 230 sensible BTUs per hour. Account for occupants by adding up the number of bedrooms and adding one (e.g., we would calculate a heat load for four occupants in a three-bedroom home).

Appliances will not run all the time, and heat loads based on weather will vary throughout the year (and even throughout each day). There may also be more or fewer occupants than the load calculation accounts for at any given time, so we want to design homes to account for the most probable and common conditions, including the occupants’ lifestyle choices (see Table 6A).

Once we have the data, we can finalize our load calculations with ACCA-approved Manual J software, like Kwik Model 3D as shown. You can also use HVAC School’s Manual J worksheet. Then, we can move on to equipment selection in accordance with ACCA Manual S.

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