Manual S Equipment Selection
Manual S Equipment Selection for Air Conditioners and Furnaces
For split systems pairing an air conditioner with a gas furnace or air handler, Manual S evaluates cooling capacity — including sensible and latent components — and heating capacity separately at the project's design conditions.
Cooling Equipment Selection
Manual S uses the Manual J cooling loads to evaluate the proposed air-conditioning equipment at the home's design conditions.
Cooling equipment must address both sensible heat, which affects indoor temperature, and latent heat, which relates to moisture removal.
Manufacturer performance information is used to evaluate total and sensible cooling capacity for the selected outdoor unit, indoor coil, and applicable airflow. Latent capacity is the difference between total and sensible capacity.
Nominal tonnage and published efficiency ratings do not establish actual cooling performance at every design condition. The selected equipment combination must satisfy the applicable Manual S sizing requirements.
Heating Equipment Selection
For a gas furnace, Manual S evaluates available heating output against the Manual J design heating load.
Furnaces have an input rating, an Annual Fuel Utilization Efficiency (AFUE) rating, and a manufacturer-specified heating output. AFUE describes seasonal fuel utilization efficiency and can help explain the relationship between input and output, but it should not replace the manufacturer's published output capacity.
For example, a furnace with a 44,000 Btu/h input rating and 80% AFUE may have a manufacturer-listed heating output of 36,000 Btu/h.
The selected furnace must provide adequate heating capacity while meeting the applicable Manual S equipment-sizing requirements.
At higher elevations, manufacturer instructions and applicable derating requirements may reduce available heating output and must be considered when evaluating the selection.
Coordinating the Furnace, Coil, and Blower
In a conventional split system, the furnace may also contain the blower that moves air across the indoor cooling coil.
The furnace, blower, evaporator coil, and outdoor condensing unit therefore cannot always be evaluated independently.
The proposed combination must support the required cooling airflow, and the blower must be capable of delivering that airflow under the applicable operating conditions.
An adequately sized furnace does not automatically establish that the cooling equipment combination or blower performance is suitable.
Documenting the Selected Equipment
A Manual S report should identify the equipment being evaluated, including relevant indoor and outdoor model numbers.
The documentation should establish heating output and cooling performance at the applicable design conditions, along with the airflow information used for selection.
Equipment substitutions may change capacity, airflow, or compatibility even when nominal tonnage remains unchanged.
Manual S provides the equipment-selection basis; Manual D addresses duct sizing using the selected equipment's airflow requirements.
Technical References
About the Author, Tracy Savoy
Tracy Savoy has worked with residential HVAC calculations and equipment selection since 2005. Through Savoy Engineering Group, she draws on practical Manual J and Manual S experience to explain equipment sizing, the importance of complete model information, and coordination when proposed equipment changes.
View Tracy's Training & Experience