ASHP system performance and occupant comfort depend on proper system design. This includes proper system sizing, equipment selection, and consideration of how the system will interact with any existing systems.^15,16

Correct sizing of the heat pump system is critical to its efficiency and performance. The overall system, including any supplementary heating, needs to be able to provide the required heating and cooling capacity at the local outdoor design conditions. If the heat pump capacity is too low, the system will likely need to defrost more frequently during heating operation, and may rely too much on the supplementary heating system during colder weather, increasing overall operating costs.

The extra running costs of an undersized ASHP system generally outweigh the additional cost of installing a larger unit. However, significant oversizing should be avoided since this can present significant issues as well, such as excessive cycling and poor performance at warmer temperatures. It is recommended that equipment that most closely meets the calculated load be selected. However, in cases where duct capacity is an issue that is not easily addressed through duct improvements, smaller capacity centrally ducted ASHPs can be coupled with mini-split (ductless or mini-duct) systems to provide an effective solution. More details are provided below.

The following four steps should be followed to properly designing an ASHP system:

Step 1: Determine requirements:

• Heating (or heating and cooling) displacement
• Full HVAC system replacement
• Isolated zone
• Other

Step 2: Determine heating and cooling capacity requirements (heating and cooling load calculations) at the indoor and outdoor design conditions.

Step 3: Select the system size and type/configuration to suit the design requirements and homeowner preferences.

Step 4: Select the location of the indoor and outdoor units.

Each of the above-mentioned steps are described in more detail in the following pages.