ACCA Manual S — Equipment Selection — is the companion document to Manual J that guides HVAC contractors and engineers in selecting the right heating and cooling equipment based on calculated loads. It ensures the chosen equipment properly matches the building’s needs under actual operating conditions.
Why Manual S Exists
Equipment capacity listed on manufacturer data sheets is measured under standardized ARI/AHRI test conditions (95 F outdoor, 80/67 F indoor for cooling; 47 F outdoor for heating). Real-world conditions differ significantly. Manual S converts calculated loads to equipment capacity requirements, accounting for actual design conditions at the installation site.
The Selection Process
- Start with Manual J heating and cooling loads for each zone
- Determine site-specific design conditions (outdoor design temps, indoor setpoints, humidity)
- Look up manufacturer performance data at actual conditions
- Apply Manual S sizing criteria to verify the equipment falls within acceptable capacity ranges
Sizing Criteria
Manual S allows cooling equipment to be sized at 100-115% of the calculated sensible load (or up to 125% in certain cases). Oversizing beyond these limits causes short cycling and humidity problems. For heating, equipment should not exceed 140% of the calculated heating load for heat pumps (or higher limits for gas furnaces in very cold climates).
Sensible vs. Total Capacity
Cooling equipment delivers both sensible cooling (temperature reduction) and latent cooling (dehumidification). Manual S requires checking that sensible heat ratio (SHR) of the equipment matches the building’s sensible heat ratio under design conditions. Mismatched SHR leads to either overcooling (too cold without dehumidification) or undercooling (maintaining humidity but not temperature).