The COP of any Air to Water Heat Pump is based on two variables.

1. Outdoor air temperature - the lower the temperature the less amount of energy is in the air.  A heat pump works as a outdoor "energy" pump moving the energy from the air to the hydronic heating fluid.  Energy is not temperature, there is only about 30% less energy in the air at -15 F versus 70 F.  Needless to say there is less energy so the heat pumps performance COP will drop as the outdoor temperature drops.

2. Design temperature of the water/glycol in the hydronic heating loop - The energy pulled out of the air is transferred to the water/glycol via an internal heat exchanger in the Arctic Heat Pump.  The higher the design temperature of this fluid the higher the operating temperature of the freon on the heat pump side.   To get the freon to a higher temperature requires more HP from the compressor.  This increases the operating cost and reduces the COP.

Ideally we want to design a hydronic system to operate at the lowest design water temperatures.  This is why hydronic design and heat load calculations are important.