Manual S Equipment Selection
Manual S Equipment Selection for Heat Pumps
Manual S equipment selection for heat pumps evaluates heating capacity, cooling capacity, and design airflow at the project's design conditions. Where heating capacity at design conditions is insufficient, supplemental heating requirements are also addressed.
Heat Pump Capacity at Design Conditions
A heat pump provides both heating and cooling, so equipment selection must consider performance in both operating modes.
Cooling performance is evaluated against the Manual J cooling loads using applicable manufacturer capacity data.
Heating performance must be evaluated at the outdoor heating design temperature, not simply at a standard published rating temperature.
Available heating capacity can decrease as outdoor temperatures fall, although the extent of the change depends on the equipment's design and operating characteristics.
Manual S applies the relevant sizing requirements for the particular heat pump configuration.
Supplemental Heating Considerations
When a heat pump cannot provide the full design heating load under the applicable conditions, supplemental heating may be needed.
Supplemental heat can include electric resistance heat or another approved heating source, depending on the system configuration.
The required supplemental capacity depends on the calculated heating load, the heat pump's available output, the system's control strategy, and the applicable equipment-selection requirements.
Outdoor temperature, defrost operation, and manufacturer performance information should be considered when evaluating the heating system.
Supplemental heating should be selected and documented as part of the overall equipment design rather than assumed to be unnecessary because the heat pump has a nominal heating rating.
Cooling and Heating Sizing Tradeoffs
A heat pump must satisfy applicable requirements for both heating and cooling.
In some climates, the equipment size needed to address the heating load may differ from the size that would otherwise be selected for cooling.
Fixed-capacity and variable-capacity heat pumps can have different operating characteristics and sizing provisions under Manual S.
The designer must evaluate the appropriate equipment category and manufacturer data instead of assuming that selecting a larger nominal heat pump automatically produces a suitable system.
Airflow, Indoor Equipment, and Documentation
Heat pump performance depends on the selected indoor and outdoor equipment combination, airflow, and operating conditions.
The designer should verify that the proposed indoor unit or air handler is compatible with the outdoor unit and that the required airflow is supported by the manufacturer's data.
For ducted systems, the selected equipment's airflow information is also used in the Manual D duct design process.
The Manual S documentation should identify the proposed equipment, heating and cooling performance at the relevant conditions, and any supplemental heating included in the selection.
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