Manual J Load Calculation

Create a new project and enter the building location by ZIP code. The system automatically loads ASHRAE design conditions for the nearest weather station, including 1% cooling dry-bulb, 99% heating dry-bulb, outdoor humidity ratio, and daily temperature range classification.

Set indoor design conditions (typically 75F cooling, 70F heating) and the number of occupants. Then add rooms one at a time, entering dimensions, wall orientations, and construction details for each.

For each room, define the walls, ceiling, floor, windows, and doors that separate conditioned space from unconditioned space. Select from the pre-built assembly library (2x4 frame with R-13, 2x6 frame with R-19, SIP, ICF, etc.) or create custom assemblies using the layer editor.

Windows require NFRC-rated U-factor and SHGC values — check the window sticker or manufacturer spec sheet. If values are unknown, select from common glazing types in the reference library. Specify window orientation and any overhangs or shading.

Every input is tagged as 'measured' or 'assumed.' Measured values come from field observations or product specs. Assumed values use library defaults. The sensitivity analysis downstream shows which assumed values have the highest impact.

Choose an infiltration input method: blower door test result (enter CFM50, select conversion method), construction quality estimation (tight to very leaky per Manual J Table 5), or direct ACH entry. Blower door results are the most accurate and are required by some programs.

Mechanical ventilation is calculated automatically per ASHRAE 62.2 based on floor area and bedroom count. The ventilation load accounts for conditioning the outdoor air to indoor conditions.

Results show per-room heating load (BTU/hr), sensible cooling load, latent cooling load, and total cooling load. Building totals sum all rooms. The component breakdown shows how much of the total load comes from the envelope, infiltration, ventilation, internal gains, and solar gains.

The Sensible Heat Ratio (SHR) is displayed for equipment selection — it tells you the proportion of cooling that is temperature reduction vs. moisture removal. A low SHR (below 0.75) in humid climates means the equipment must be selected for adequate dehumidification.

Room-by-room CFM values are calculated for duct design. These flow directly into Manual D if you continue to duct sizing.