Manual S Equipment Sizing Check (Free, Simplified)

ACCA Manual S addresses residential equipment selection after the heating and cooling loads have been calculated. Start with your Manual J loads, including the sensible and latent cooling split, then compare a matched system's manufacturer performance data at your local design conditions.

This free tool uses commonly cited ACCA Manual S limits as a simplified screen: total cooling 95-115% of total load, with an upper limit of 125% for a heat pump in a heating-dominated climate; sensible and latent cooling must each meet their respective loads; furnace output is checked at 100-140% of heating load. It does not implement every exception or equipment class. ACCA's current Manual S guidance includes additional provisions for variable-capacity heat pumps. The current applicable edition and local code govern.

One refrigeration ton equals 12,000 BTU/h, but a nominal 3-ton unit does not guarantee 36,000 BTU/h at your design outdoor temperature. Indoor dry-bulb and wet-bulb temperatures, airflow, the coil match, and outdoor conditions change both capacity and its sensible/latent split. AHRI certification helps identify rated matched-system performance; use manufacturer expanded data for the actual design operating point.

For a furnace, use output BTU/h rather than fuel input. For a heat pump, use heating capacity at the heating design outdoor temperature, not its mild-weather rating. Check manufacturer defrost information and backup controls separately.

The default AC + furnace example has 30,000 BTU/h total cooling load, 24,000 sensible, and 40,000 heating. Equipment provides 33,000 total cooling, 26,000 sensible, and 48,000 furnace output BTU/h.

Switch to heat pump to use the default 28,000 BTU/h heating capacity: 28,000 / 40,000 = 70%, leaving 40,000 - 28,000 = 12,000 BTU/h supplemental heat needed at design. Selecting heat pump + backup does not itself prove that backup capacity is adequate.

As outdoor temperature falls, a building's heating load usually rises while an air-source heat pump's available capacity changes. Where the two curves intersect is the thermal balance point. Below it, supplemental heat may be needed. A single design-temperature comparison cannot locate that intersection or establish the economic changeover temperature for a dual-fuel system.

Use the simplified Manual J calculator for an initial load estimate, read what Manual J measures and how Manual J, S, and D fit together, then plan airflow with the duct size calculator. Final selection needs site-specific loads and verified manufacturer data.