Manual S Equipment Selection
Manual S Equipment Sizing Mistakes: Common Errors to Avoid
Equipment sizing errors often result from using nominal ratings instead of manufacturer performance data, skipping latent capacity evaluation, or applying sizing requirements incorrectly. Manual S provides the procedures to avoid these errors.
Using Nominal Ratings Instead of Performance Data
Equipment marketed as a particular tonnage does not necessarily provide that exact cooling capacity at the home's design conditions.
Published ratings are established under specified test conditions. Actual equipment performance can vary with outdoor temperature, indoor conditions, airflow, and equipment combination.
Manual S evaluates the proposed equipment using applicable manufacturer performance information rather than assuming nominal capacity equals available capacity.
Selecting equipment by tonnage alone can lead to an incorrect assessment of its ability to satisfy the calculated loads.
Overlooking Sensible and Latent Cooling Loads
Manual J separates the cooling load into sensible and latent components.
An air-conditioning system may have sufficient total cooling capacity but insufficient sensible capacity under the applicable operating conditions.
Conversely, evaluating only sensible capacity can overlook the system's moisture-removal requirements.
Manual S addresses the relevant cooling capacities and equipment-sizing requirements. The designer must use performance data appropriate to the selected equipment and design conditions.
Using Incorrect Equipment Combinations or Design Conditions
Manufacturer performance data may apply only to particular indoor and outdoor equipment combinations.
Changing an indoor coil, air handler, or airflow setting can affect the available cooling capacity and other operating characteristics.
Another common mistake is evaluating equipment at a standard rating temperature instead of the project's actual heating or cooling design conditions.
The designer should verify the equipment combination, applicable indoor conditions, outdoor design temperatures, and airflow before documenting the selection.
Applying Sizing Rules Incorrectly
Manual S does not use one universal sizing percentage for every type of residential HVAC equipment.
The applicable criteria depend on the equipment category, operating characteristics, and relevant provisions of the standard.
A sizing allowance for one equipment type should not automatically be applied to another.
Equipment should also be reviewed when the Manual J loads change or when a contractor proposes a different model.
Correct equipment selection requires both reliable load calculations and appropriate performance data—not simply choosing the closest nominal size.
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