What is a Manual J calculation?

Manual J is the standardized method for calculating a building's peak heating and cooling loads, developed by the Air Conditioning Contractors of America (ACCA) and recognized as the standard by California building codes. It produces the exact BTU/hr your ADU needs at design conditions — the number the engineer uses to correctly size the HVAC equipment, replacing guesswork with engineering.

What it calculates

A Manual J models how heat enters and leaves the ADU at the worst-case summer and winter conditions for your climate zone, accounting for:

• Wall, ceiling, and floor areas and their construction.
• Window area and orientation (north/south/east/west), because solar gain varies dramatically by direction.
• U-factors — the thermal performance of each assembly and the windows.
• Air infiltration — how tightly the building is sealed.
• Internal heat gains from occupants, lighting, and appliances.
• Outdoor design conditions for the project's California climate zone.
• Duct location and leakage, for ducted systems.

Why correct sizing matters

The instinct to "go bigger to be safe" backfires with HVAC. Oversized equipment short-cycles — turning on and off too frequently — which hurts humidity control, comfort, and compressor life. Undersized equipment can't hold setpoint on the hottest or coldest design days. Manual J finds the right capacity so the system runs efficiently and comfortably, and it gives the plan reviewer the documentation needed to approve the equipment selection. Building departments increasingly require the calculation itself in the plan set, not just an equipment schedule.

What the engineer needs from you

• The floor plan with dimensions and ceiling heights.
• A window schedule — size, location, orientation, and glazing type.
• Wall, ceiling, and floor construction — insulation R-values if known.
• The ADU type (detached, attached, garage conversion) and the project address.

The climate zone is determined automatically from the project address, so you don't need to look it up, and if insulation values are unknown the engineer applies California code-minimum defaults — keeping the calculation defensible while construction details are finalized.

Manual J is the engineering behind the equipment on your plans. Including it up front prevents one of the most common ADU plan-check corrections.

Prep checklist

1. Provide a dimensioned floor plan with ceiling heights.
2. List every window: size, orientation, and glazing type.
3. Note insulation R-values, or let the engineer use code defaults.
4. Identify the ADU type and project address.
5. Flag unusual conditions — large west-facing glass, vaulted ceilings, exposed floors.

Design assumptions and code defaults vary by climate zone and code cycle — confirm specifics with your local building department. Our mechanical plans include the Manual J summary and the matching equipment schedule, so the sizing math travels with the drawings. See how it works, or start your order.