A heat pump manufacturer's specifications are published for a reason!

Heat pumps will run under a fairly wide set of operating parameters. They will work with a wide range of entering water and air temperatures and flow rates. But just because a heat pump will run under a wide range of conditions doesn't mean they will operate efficiently. To maximize the efficiency and capacity of a heat pump, a designer should become familiar with the specification catalog supplied by the manufacturer. 

The specification catalog from a reputable manufacturer will typically show the capacity, efficiency and power draw of a heat pump at two or three air flow rates. The last two lines of the first chart shows the data of a heat pump operating at an air flow rate of 1,080 cfm (510 l/s) and 1,250 cfm (590 l/s). The second column shows the heating capacity (HC) drops from 34.8 kBtu/hr (10.2 kW) to 34.1 kBtu/hr (10.0 kW)...a drop of 2%, and the power draw increases from 2.26 kW to 2.36 kW...an increase of 2%, and that the overall COP drops from 4.33 to 4.08...a drop of almost 6%!

A reputable manufacturer will also include an "Air Flow Correction Table" in the specification catalog that shows the percentage change in heating and cooling capacity and power draw at a wide range of different air flow rates. When you are designing or installing a water to air heat pump system, care should be taken to ensure the ductwork is designed for the air flow recommended by the manufacturer. It should be emphasized to the owner that the air filter should be changed or cleaned regularly. 

And when you are installing a system in a new home or building, it must be emphasized to the homeowner and/or builder that the heat pump should not be operated until the home is completely finished and you have commissioned the heat pump. This should be done to avoid the chance of dust from construction, especially from drywall, from clogging the air coil. 

The catalog data does not show an additional, and very important reason, for ensuring the heat pump is operated at the recommended air flow rates. As the heat pump capacity drops and the power draw increases, the compression ratio of the compressor increases. The compressor has to work harder. This reduces the longevity of the compressor and it increases the potential for noise complaints from the home or building owner.