With the load calculations completed, all required information needed to select the right size of heat pump system is now available. Select the appropriate size of system following Section 5 of CAN/CSA standard C273.5-11. The following items should be considered when selecting an ASHP:

• Most manufacturers provide performance guide charts for selecting a heat pump system, similar to Exhibit 14. Use these charts to determine heating and cooling capacity at the actual design conditions for the local climate.
• Where heating capacity data is not available for the exact external design temperature relevant to the installation site, use the heating capacity information for the nearest outdoor temperature available below the external design temperature. Where cold climate air-source heat pumps are being considered, the information in NEEP's ccASHP specification tables¹⁷ (i.e. maximum heating capacities reported at -15°C or 5°F) may be used to estimate performance near design conditions and help ensure proper equipment selection.
  
• The minimum capacity of the selected system (i.e. ability to modulate) is as important as the maximum capacity. To ensure heating comfort and efficiency, select equipment with adequate turn-down to perform well during mild weather, low-load conditions. This is an important reason to not to overestimate design loads or oversize equipment and is particularly important for multi-zone systems.
• When installing multi-zone systems, consider using separate single-zone systems or increasing the number of outdoor units, each with lower capacity and with fewer zones. This will make it easier to match minimum capacity requirements and may have only a most impact on the system cost. This is mainly due to the cost associated with running long refrigerant lines to multiple indoor units, which can also have a negative impact on system performance.
• In general, avoid situations where the minimum steady-state capacity of the outdoor unit (rated at 8.3°C or 47°F) is higher than the smallest indoor unit's heating capacity. In these situations, centrally ducted and mini-ducted heat pumps should be considered as well.
• The heating capacity of heat pumps declines with lower outdoor temperatures. As such, the proposed ASHP system must be able to provide the required heating at the relevant outdoor design temperature where the system is being installed. The capacity of heat pump at the outdoor design temperature will likely be different to the rated heating output capacity. For example, Model A in Exhibit 14 has a rated capacity of 12,000 BTU/h but this model can only provide about 8,640 BTU/h (or 72% of its rated heating capacity) at Lower Mainland design temperatures, which are close to -8°C.
• The following is provided as a sizing example:
  • If a contractor were sizing an ASHP system with the performance characteristics noted in Exhibit 14 for a home with a heating load of 14,000 BTU/h at an outdoor design temperature of -11°C, the 24,000 BTU/h rated capacity heat pump (Model C) should be selected as it is able to deliver 14,400 BTU/h output at -15°C (i.e. nearest temperature below -11°C with data available). Model B would only be able to provide a 10,800 BTU/h output at -15°C, which is less than the required 14,000 BTU/h design heating load.

• In retrofit situations with existing duct work, the system size will be limited by the capacity of the existing duct work. In these cases, if the calculated load is larger than the capacity of the existing duct work and there is no scope to upgrade ductwork, select a smaller unit that will work with the existing duct system, and provide adequate supplementary heating.

• Selection of type of ASHP system should be done based on homeowner's requirements, type and condition of existing heating system, and local weather conditions. Exhibit 8 in Section 2 can be used as reference guide to help select the appropriate ASHP system for different situations.
• Supplementary heating will be required for systems with heating loads below the thermal balance point of a given home. Supplementary heating is strongly recommended on any conventional systems that are able to employ it for defrost cycles since this prevents cold air from blowing during defrost cycles, and thus improves occupant comfort. Exhibit 8 in Section 2 can be used as reference guide to select the appropriate supplementary heating system for different situations. The following table provides some high-level guidance for sizing heat pumps and any associated supplementary heating system.